National Gallery Technical Bulletin Volume 25, 2004

Fading and Colour Change of Prussian Blue: Methods of Manufacture and the Influence of Extenders
Jo Kirby and David Saunders

Prussian blue can be described as the first of the modern, totally synthetic pigments, arising only as the result of a deliberate chemical reaction; it has no natural equivalent. Its first preparation in around 1704, by a Berlin colourmaker named Diesbach, appears to have been accidental (the intended product was a cochineal lake) (note 1), but it is clear that the pigment went into production quite soon afterwards: it was first announced anonymously in 1710 and the earliest occurrences date from a few years later (note 2). 

The benefits of an intensely coloured inexpensive blue pigment, flexible in its handling properties and not gritty or difficult to use, were obvious. It could easily be manufactured and its supply was not dependent on the vagaries of transcontinental trade or environmental conditions. The use of Prussian blue spread extremely rapidly, in easel painting, watercolour and for interior decoration and, even though doubts about its permanence were already being expressed by the mid-eighteenth century, its popularity did not diminish: effectively, there were no good alternatives until the syntheses of cobalt blue in 1802 and artificial ultramarine in 1826–7. It is the pigment of cool, white and blue decorative schemes, seen in the feathery touches of Watteau or Gainsborough, the bravura strokes of Tiepolo, or the calm, timeless skies of Canaletto. Mixed with white, its vibrancy of colour surpasses that of the insoluble natural dyestuff indigo, the only other blue pigment available at that time with a similarly high tinting strength. Prussian blue may have a greenish undertone, but this is not always so: the pigment used by Sir Joshua Reynolds for the countess’s dress in Anne, 2nd Countess of Albemarle (NG 1259, painted in 1760), and by Thomas Gainsborough in Mrs Siddons (NG 683, dated 1785; plate 1), is of very good colour. The extraordinarily high tinting strength of Prussian blue is shown in plate 2, which illustrates a sample of a modern, high-quality pigment (sample ALP in table 1, pp. 92–3) mixed with different proportions by weight of lead white. Only one part of Prussian blue mixed with 200, or even 400, parts of lead white gives light blue tints very similar to those used for painting skies. fig. 1 presents reflectance curves for seven of the mixtures illustrated in plate 2, showing the characteristic maxima for Prussian blue in the 455 to 475 nm region. 

Indigo and Prussian blue have other features in common which would have made the properties of the new pigment relatively familiar to the painter. In oil medium, both need to be ground with a large amount of oil to give a workable paint, and both can be used as glazing pigments as they have a low refractive index. Generally, however, both are used mixed with white as their colour is so deep at full strength that it is almost black. The blue pigment then benefits from the easy handling properties of lead white to allow painterly scumbles and impasto passages to be achieved. Relatively soon after the introduction of the pigment, however, it was suspected that the admixture of so much white had undesirable consequences. Indigo used as an oil paint already had a poor reputation as far as its lightfastness was concerned (note 3); when the permanence of Prussian blue also came to be questioned, the presence of white pigment or extender was recognised as a contributory factor (note 4).  

Factors that may influence colour change in Prussian blue 
An earlier article in the Technical Bulletin (1993) gave examples of eighteenth-century paintings in the National Gallery collection in which the fading of Prussian blue had been observed, and discussed eighteenth- and early nineteenth-century reports of the instability of the pigment to light and the factors that might influence this (note 5). For convenience these findings are summarised below. The present account describes the experimental investigation of these factors, particularly the method of preparation and the presence of white pigment or extender, based on the examination of samples of late eighteenth- and early nineteenth-century pigments collected from various sources and of pigments prepared in the laboratory. 

The original method for preparing Prussian blue, first published in 1724, used dried cattle blood, an alkali derived from the detonation of potassium hydrogen tartrate (potassium hydrogen 2,3-dihydroxybutanedioate) and potassium nitrate, green vitriol (iron(II) sulphate) and alum (in this case, probably the potassium salt, aluminium potassium sulphate). The resulting pale greenish-blue product was treated with hydrochloric acid to give the deep blue pigment (note 6). Neither the underlying chemical reaction producing the pigment, nor the nature of the cyano or hexacyanoferrate groups (–CN or –Fe(CN)6) present in the blood, which are essential parts of the complex, was known. The process was therefore very difficult to control. As the chemistry of the preparation became better understood, conditions such as the proportions of the starting reagents, their dilution and the temperature at which the reaction took place could be varied, with the intention of giving a product with a particular colour or range of properties, or of improving the efficiency of the process. By the end of the eighteenth century, many recipes for making the pigment from the essential starting materials of animal matter, an iron salt and alkali (usually potassium-containing) had been published (see table 2, pp. 94–5, for a selection). With increasing knowledge, the procedure could be refined and methods standardised. 

Even modern methods of manufacture of Prussian blue are not, however, without problems; the extremely fine particle size and colloidal nature of the pigment make separation of the product from adsorbed water or impurities, including excess starting reagents or contaminants produced during the reaction, particularly hard. These may influence the permanence of the pigment. Differences in working properties and texture depend very much on the treatment of the precipitated particles, which are only 0.01–0.2 μm in diameter and, after the pigment has been washed, tend to agglomerate immediately (note 7). Already in the eighteenth century, insufficient washing was known to contribute to hardness of the pigment and other undesirable properties (note 8). The tiny, colloidal particles of one variety of potassium-containing Prussian blue are particularly easily dispersed (peptised) in aqueous solutions, giving a blue suspension resembling a solution. This variety is described as ‘soluble’ and it has long been suspected of being less permanent than other varieties (note 9). The lattice structures of the two forms of the pigment (‘insoluble’ and ‘soluble’), with or without alkali metal, discussed below, are slightly different and this factor should also be considered when investigating the permanence. In fact many commercially available varieties of the pigment contain a variable amount of alkali metal, which until after the First World War was most likely to have been potassium. Sodium salts could also have been used, but they tend to give a pigment of inferior colour. After the First World War, the shortage of potassium stimulated the use of ammonium salts in the production of the pigment, and most modern methods for the manufacture of Prussian blue yield the ammonium-containing ‘soluble’ variety, which has a brighter colour and better alkali resistance (note 10). 

The deliberate addition of white pigment has been discussed above. The lightfastness of a typical modern Prussian blue pigment has been described as excellent when the proportion of blue pigment is high (90% Prussian blue, 10% titanium white), decreasing as the proportion of white pigment is increased to give very poor durability at 1% blue, 99% white (note 11). The white pigment in these experiments is different from the lead white that would have been used in the eighteenth century. By the nineteenth century, zinc white and other whites also became available, all of which have slightly different optical properties (for example, as far as the absorption or reflectance of radiation in the ultraviolet region of the spectrum is concerned) (note 12), so it is conceivable that they may affect the lightfastness of Prussian blue in different ways. 

In addition to admixed white pigment, variable amounts of different inert white extenders may be present. As well as reducing the intensity of colour of the pigment, they also influence other properties such as the softness or ease of grinding; from this point of view their inclusion may be beneficial. The original method of preparation resulted in the presence of hydrated alumina extender in the pigment, as alum (aluminium potassium sulphate) was one of the ingredients. Alumina may still be added nowadays during the manufacturing process to help the pigment settle, quite apart from its properties as an extender. Other extenders such as starch, calcite or chalk, gypsum and barytes (barium sulphate) could also be used. As with the addition of white pigment, the presence of extenders may influence the lightfastness of the Prussian blue and the addition of white pigment to an already extended blue could increase its impermanence. 

From the middle of the eighteenth century onwards it was noted that the change in colour, or fading, of Prussian blue was sometimes reversible. A reversible reduction-oxidation process during which a form of the pigment is produced is the basis of the production of blueprints and cyanotypes (note 13). It is also reported that Prussian blues tend to lose their colour on storage in a closed container by a process of reduction, due to the lack of oxygen; this is encouraged by the presence of acidity in the surrounding medium (note 14). They also tend to be reduced in the presence of oxidisable media (note 15). These changes are reversed when oxygen (from the air, for example) is supplied (note 16). The phenomenon has indeed been reported in painted samples of the pigment (note 17), but not particularly frequently, and the role of the paint medium has not been investigated previously. 

The structure of Prussian blue and the chemistry of its manufacture 

Structure 
Prussian blue is perhaps the best known of the class of materials described as type II mixed-valence transition metal complexes (note 18). It is a hydrated iron(III) hexacyanoferrate(II) complex of variable composition: depending on the method of manufacture and the conditions of precipitation, potassium, sodium or ammonium ions and a variable amount of water may be present. The formula for the alkali metalfree (‘insoluble’) pigment is FeIII 4[FeII(CN)6]3·xH2O (iron(III) hexacyanoferrate(II), where x = 14–16). X-ray diffraction and neutron scattering studies have shown that it has a face-centred cubic lattice structure, where the metal sites are occupied by iron(II) and iron(III) alternately, bridged by the cyano groups in a regular array to give a three-dimensional polymeric network. A quarter of the FeII, C and N sites are vacant. Mössbauer and other spectroscopic studies have revealed that low-spin iron(II) is coordinated octahedrally to the carbon atoms of six cyano groups, while high-spin iron(III) is co-ordinated octahedrally to the nitrogen atoms, and also to the oxygen of co-ordinated water molecules occupying the vacant N sites; the average composition of this mixed environment is FeIIIN4.5O1.5. For the ideal stoichiometry FeIII 4[FeII(CN)6]3·14H2O, a unit cell would, therefore, contain six co-ordinated and eight uncoordinated water molecules, the latter occupying zeolitic-type sites within the cell structure (note 19). 

The formula for the potassium-containing (‘soluble’) variety, which has a similar lattice structure, may be written as KFeIII[FeII(CN)6]·xH2O, where x is variable, but has been reported to be 5 in some syntheses (note 20). In this structure, as all the FeII, C and N sites are occupied, only uncoordinated water can be present. The potassium ion, K+, acts as a counter ion to complete the neutralisation of the negative charge of the [FeII(CN)6]4- group, and potassium ions occupy half the zeolitic-type sites within the unit cell taken up by water in the potassium- free form (note 21). 

The essential structural element of the pigment framework thus consists of the sequence FeIII–N–C–FeII and its intense colour arises from an electronic transition (charge transfer transition) between the two valence states of iron: from the FeII ion to the FeIII ion, which occurs when light in the red region of the visible spectrum, between 690 and 730 nm, depending on the structure, is absorbed (note 22). 

Chemistry of manufacture 
The methods of making Prussian blue fall into two types, direct and indirect. In the direct method, which is a one-step process, a solution of an iron(III) salt, such as ferric chloride, FeIIICl3, is mixed with a solution of a hexacyanoferrate(II) salt, for example K4[FeII(CN)6]. Alternatively, solutions of an iron(II) salt, such as ferrous sulphate, FeIISO4, and a hexacyanoferrate(III) salt, such as potassium ferricyanide, K3[FeIII(CN)6], are mixed. If the iron salts are present in excess, the deep blue precipitate formed is a variety of the so-called ‘insoluble’ pigment. It was originally thought that the product of the second reaction, which produces a pigment known as Turnbull’s blue, was an iron(II) hexacyanoferrate( III), but studies by X-ray diffraction and Mössbauer spectroscopy have shown that both reactions produce iron(III) hexacyanoferrate(II). The molecule contains very little, if any, potassium (although rapid precipitation may cause some to be brought down as an impurity), but does contain water. If the reagents are mixed in 1:1 molar proportions, or with the hexacyanoferrate in excess, the product is again identical in both cases, being the potassium-containing ‘soluble’ variety mentioned above, although it may be called ‘soluble’ Prussian blue or ‘soluble’ Turnbull’s blue depending on the starting materials (note 23). 

There are thus four direct preparations, producing either ‘insoluble’ or ‘soluble’ Prussian blue or Turnbull’s blue. The production of ‘insoluble’ Prussian blue can be summarised as: 

4FeIIICl3 + 3K4[FeII(CN)6] → FeIII 4[FeII(CN)6]3 + 12KCl 

and that for preparing ‘soluble’ Turnbull’s blue as: 

FeIICl2 + K3[FeIII(CN)6]→KFeIII[FeII(CN)6] + 2KCl 

The indirect method is a two-stage process, based on the reaction between an iron(II) salt and a ferrocyanide salt, typically potassium hexacyanoferrate( II), to give an insoluble white product known as Berlin white (an iron(II) hexacyanoferrate(II), M2FeII[FeII(CN)6], where M may be ammonium, sodium or potassium, depending on the reagents used). This product is then treated with a powerful oxidising agent such as a chlorate or chromate to give a ‘soluble’ Prussian blue (note 24). The production of Prussian blue from iron(II) chloride and ammonium hexacyanoferrate(II), using hydrogen peroxide as the oxidant, can be summarised as follows: 

FeIICl2 + (NH4)4[FeII(CN)6] → FeII(NH4)2[FeII(CN)6] + 2NH4Cl 

2FeII(NH4)2[FeII(CN)6] + H2O2 → 2FeIIINH4[FeII(CN)6] + 2NH3 + 2H2O 

The preparation of Prussian blue samples 
In order to study the behaviour and characteristics of Prussian blue observed in eighteenth- and nineteenth- century paintings in the National Gallery collection in the context of how the pigment was prepared at the time, it was necessary to have samples of eighteenth- and nineteenth-century pigments for comparison and to study their susceptibility to light. These are listed in table 1 and discussed below. Unfortunately, although a number of early and mid-nineteenth-century samples could be obtained, very few samples from the eighteenth century were available. The Prussian blue identified in earlier eighteenth-century pictures, such as Gainsborough’s Gainsborough’s Forest (‘Cornard Wood’) (NG 925) of about 1748, or Canaletto’s Venice: Campo San Vidal and Santa Maria della Carità (‘The Stonemason’s Yard’) (NG 127), painted between 1726 and 1728 (both discussed in the earlier Technical Bulletin article) (note 25), is likely to have been prepared by some variant of the original method, from cattle blood. However, it was not necessarily the case that any of the collected samples, detailed in table 1, had been so prepared. It was therefore necessary to prepare this type of Prussian blue in the laboratory. 

The recipe used to make the pigment from the original ingredients was that published by Robert Dossie in the first volume of his Handmaid to the Arts (London 1758). Although the original 1724 recipe, an English version of which Dossie included in the appendix of the second volume, gave an equally fine colour, he says of the 1758 recipe: ‘The proportions of the ingredients are more accurately adapted to each other’ (note 26).

 In the original 1724 recipe, four ounces (about 113 g) each of crude tartar (potassium hydrogen tartrate, potassium hydrogen 2, 3-dihydroxybutanedioate) and dried crude nitre (potassium nitrate, KNO3) were mixed and detonated with charcoal to give an alkali (essentially potassium carbonate, K2CO3). This was powdered while still hot, mixed with well-dried, powdered ox blood and heated in a covered crucible until the contents ignited. Subsequently it was heated more intensely until it glowed and there was little further flame. The contents were ground while still hot, added to two pints (about 1.1 litres) of boiling fresh water, boiled and filtered twice. Eight ounces (about 226 g) of crude alum (aluminium potassium sulphate, AlK(SO4)2·12H2O) were dissolved in two pints of boiling water; this solution was then added to the iron solution and the mixture added rapidly to the hot alkaline blood extract, giving a greenish or blue-green precipitate. The solution was well stirred until effervescence had ceased; the mixture was then left to stand and the precipitate filtered off. On the addition of two or three ounces (about 57–85 ml) of spirits of salt (hydrochloric acid, HCl), the precipitated matter turned blue. It was left overnight, then washed very thoroughly and dried in gentle heat. 

According to the recipe, the strength of calcination of the blood and alkali mixture was important as it influenced the resulting colour. In addition, as Dossie pointed out, the use of more alkali gave a lighter colour, while more acid would, by more efficient removal of the hydrated alumina and other impurities formed during the reaction, give a deeper blue. His modified version (see Dossie 1758 (ii) in table 2) used six pounds (about 2.7 kg) of dried blood to two pounds (about 900 g) of pearl ashes (potassium carbonate), four pounds (about 1.8 kg) of alum and two pounds of green vitriol, not calcined. Four pounds of acid were used to produce the deep blue colour from the greenish precipitate. These proportions and ingredients can be compared with those used in other contemporary recipes in table 2; all claimed to give satisfactory results. Blood was not the only form of animal matter: hoofs, horns and other materials were widely used, sometimes with the addition of extra iron in the recipe. For large-scale preparations, many materials, from rotten wood to chimney soot, were tried for reasons of economy, with varying degrees of success. The emphasis by Carl Hochheimer and others on the need for care in the initial calcination and, if necessary, adjustment of the proportions of ingredients during the preparation is thus entirely understandable. The effects on the final colour were also well understood. In one of Hochheimer’s recipes for so-called Paris blue, attributed to the French chemist Philippe Macquer, alum was deliberately omitted, resulting in a very deep blue pigment (note 27). By the nineteenth century many contemporary recipes for preparing Prussian blue have as their starting material potassium hexacyanoferrate(II) (cyanoferrure de potasse). However, this was still usually obtained from natural sources. These might include animal matter and iron filings; or perhaps a source of carbon, such as logs or charcoal, with nitrogen from the air and iron filings (note 28). 

The laboratory preparation based on Dossie’s second method was repeated several times, heating the dried blood/alkali mixture in a crucible in a furnace and varying the temperature and heating time. In this preparation the dried blood was a commercial product supplied for use as a garden fertiliser. Several trials were carried out using the 3:1 ratio suggested by Dossie, but better results were obtained by increasing the proportion of dried blood. The best results were achieved by heating 40–50 g of dried blood and 10 g potassium carbonate to about 350ºC. The matter burned with an orange flame, producing extremely noxious fumes, and the door of the furnace was periodically opened briefly to allow combustion to continue. The crucible was then removed, the contents stirred and the temperature of the furnace allowed to rise again; this was continued until no further combustion took place. The temperature was then raised briefly to about 450–500ºC, after which the crucible was removed, the contents ground briefly and then added to 300 ml of boiling distilled water. The mixture was boiled for 45 minutes then filtered, giving a clear, usually pale yellow, filtrate. 

Iron(II) sulphate (10 g) and aluminium potassium sulphate (20 g) were dissolved separately in distilled water, mixed and then added rapidly to the hot yellow solution, resulting in the immediate production of effervescence, an unpleasant sulphurous smell and a greenish-blue precipitate. Any prolonged delay between the production of the pale yellow dried blood extract and the addition of the alum and iron sulphate solutions tended to result in the formation of brownish iron(III) compounds, spoiling the pigment. 

The greenish-blue precipitate was left to settle before filtering, and it was observed that the material became noticeably bluer even on the filter paper. After thorough washing it was treated with hydrochloric acid to remove the hydrated alumina also formed during the reaction, resulting in a fine suspension of slightly purplish Prussian blue. The identity of the pigment was confirmed by Fourier-transform infrared analysis (note 29). In one sample (K12a in table 1), the hydrated alumina was allowed to remain as an extender, giving a pale, slightly greenish-blue product.

 It is not entirely clear which species are produced by the initial calcination, although the cyano groups in the final Prussian blue must derive from the extract. If a small amount of the filtrate from the calcinations was acidified with 2M sulphuric acid and applied to dipyridyl paper (supplied by Macherey Nagel) a red colour was produced, indicating the presence of iron(II) ions. However, when dried blood is used in the preparation, it seems unlikely that free cyanide and free iron(II) are both present in the filtrate, as in the prevailing alkaline conditions these would react to give Prussian blue prior to the addition of iron(II) sulphate (note 30). The second possibility is that the filtrate contains hexacyanoferrate( II), but that the positive dipyridyl test result is due to dissociation of this ion to give free iron(II) in the acidified solution applied to the paper (note 31). Finally, both free cyanide and hexacyanoferrate may be present; in alkaline conditions little free iron(II) is present so no Prussian blue will be formed, whereas on acidification for the dipyridyl test free iron(II) is produced, giving the positive result. Certain recipes published by Hochheimer and Weber (see table 2) use calcined hoofs, a starting material which presumably contained little or no iron, although like others Hochheimer used iron vessels at this stage of the process. Attempts to mimic this method in the laboratory, using collagen-based matter as a starting material, produced no Prussian blue, although a silica rather than iron vessel was used, but this line of investigation was not pursued exhaustively (note 32). 

The pale colour of the initial product is partly due to the alumina formed on addition of the solution of iron(II) sulphate and aluminium potassium sulphate. However, the main reason for the slow development of the colour is that at least part of the initial product formed by reaction of the iron(II) sulphate and hexacyanoferrate(II) solutions is the ‘reduced’ form of Prussian blue, Berlin white K2FeII[FeII(CN)6], which gradually oxidises on contact with air and on acidification of the solution to give ‘soluble’ Prussian blue KFeIII[FeII(CN)6]. 

Although usable quantities of Prussian blue could be obtained by this method, the yields were small compared with the quantities of starting materials. (The small yield was also noted by eighteenth- century writers: see, for example, Hellot 1762 in table 2.) The preparation often failed entirely, particularly if the iron(II) sulphate solution was heated, as recommended by Dossie, before it was added to the filtrate. This gave rise to insoluble iron(III) species (presumably including FeIII(OH)3) which, when added to the filtrate, produced no Prussian blue. It seems likely that although the original recipe was followed as closely as possible, the resulting Prussian blue was probably purer than eighteenth-century commercial products, as great care was taken to wash the precipitate and to dissolve out the alumina (except in the case where this was deliberately retained). 

For comparison, modern samples were made by the direct method using either the Prussian blue or Turnbull’s blue routes described above, or by the indirect method. In the first preparation by the direct method for Prussian blue, 2.54 g (0.02 moles) of iron(II) chloride, FeIICl2, were dissolved in 25 ml of water. Excess hydrogen peroxide was added to convert the iron(II) to iron (III). Separately, 2.84 g (0.01 moles) of ammonium hexacyanoferrate(II), (NH4)4[FeII(CN)6], were dissolved in 25 ml of water and added slowly to the iron(III)-containing solution with stirring. The blue precipitate that formed was collected, washed with two 50 ml aliquots of distilled water, and dried in a desiccator. This material is labelled SA in table 1. The preparation was repeated, but the precipitate was washed four times rather than twice, to give sample SB. 

Sample SC was also prepared by the direct method for Turnbull’s blue, by adding a solution containing 3.29 g (0.01 moles) potassium hexacyanoferrate( III), K3[FeIII(CN)6]), in 25 ml of water to a solution of 2.54 g (0.02 moles) of iron(II) chloride, FeIICl2, in 25 ml of water. The blue precipitate was again washed twice with 50 ml aliquots of distilled water before drying. 

Finally, sample SD was prepared by the indirect method. A solution of 2.84 g (0.01 moles) of ammonium hexacyanoferrate(II), (NH4)4[FeII(CN)6], in 25 ml of water was added to a solution of 2.54 g (0.02 moles) of iron(II) chloride, FeIICl2, in 25 ml of water. A pale blue precipitate formed which became dark blue on standing, particularly at the surface where the suspension was in contact with air. An excess of hydrogen peroxide was added to complete the conversion of the first-formed Berlin white to Prussian blue. The precipitate was then washed twice with 50 ml of distilled water and dried in a desiccator. 

Description of the eighteenth- and nineteenth-century Prussian blues examined 
The particle characteristics and composition of Prussian blues prepared in the laboratory according to an eighteenth-century recipe were compared with those of genuine late eighteenth- and early nineteenth- century samples (table 1), and also with Prussian blues identified in National Gallery paintings (table 3, pp. 96–9). When examined under the optical microscope, the laboratory samples were found to consist of small, irregular fragments of low refractive index, sometimes with the appearance of angular flakes. These were occasionally quite large, flat and plate-like and sometimes a more transparent, paler blue (plate 3). Elemental analysis of several of the laboratory-prepared pigments, including K7, K12a and K12b in table 1, revealed the presence of iron and a variable amount of aluminium. Potassium was not detected in these particular samples, but was present in other preparations not included in this table. 

Thorough treatment of the pigment with acid would be expected to remove most of the hydrated alumina produced during the preparation. In the case of K7, no aluminium could be detected and rather few translucent angular flakes were present. In practice, however, complete removal of the alumina was not necessarily desired. K12b still contained some alumina and also a higher proportion of the angular flakes. In K12a, where the alumina had not been dissolved out, the particles were rounded, very translucent and a pale greenish blue, similar to those of a traditional lake pigment (plate 4). 

In the genuine Prussian blue samples (table 1) and in Prussian blue identified in National Gallery paintings (table 3), analysis revealed the presence of potassium in most cases, along with other elements that derive from the extenders where present. Some of the elements detected by Energy Dispersive X-ray analysis (EDX) may be present because the samples were not washed sufficiently thoroughly to remove unreacted starting materials; for example, chlorine from FeIICl2 was detected in samples SA and SB(note 33). 

The gum was extracted from the paints in watercolour cakes and the oil from those in paint bladders so that the particle characteristics could be examined for all the samples collected. It was found that several pigments resembled the laboratory samples: they showed translucent, paler-coloured, angular flakes, and also contained aluminium. These included watercolour cakes manufactured by Ackermann (AP, AR) and Reeves and Woodyer (MA), powder pigments from the Ozias Humphrey watercolour box and Elizabeth Carey’s pastel box (OZ, VB), and the two samples of oil paint from bladders (GA, JSM). Plates 5 and 6 show typical examples from Ozias Humphrey’s watercolour box (OZ) and Elizabeth Carey’s pastel box (VB). All date from relatively early in the nineteenth century, or possibly from the late eighteenth century. The samples from the German pharmacy, which date from 1830 or before, might be expected to share these features (and some of the characteristic angular flakes were observed in samples RB, RC and RE), but the particles tend to coat or to coagulate around the extender, which makes their own features difficult to assess. On the other hand, several samples that probably date from later in the nineteenth century had intensely coloured, rounded agglomerates similar to the modern Winsor & Newton sample (WN, plate 7). These included the samples from the Bull paintbox (BCB), the Nicoll samples (NB, NC, NP) and the sample of Turnbull’s blue (TB). Deeply coloured clumps of pigment may exhibit bronzing when examined by reflected light. The phenomenon was most obvious in the Winsor & Newton sample and other later pigments, but was also seen in one of the laboratory samples (K7) and an Ackermann sample (AP), suggesting that the method of preparation is not an important factor. 

Like the laboratory samples and the earlier pigments described above, Prussian blue identified in eighteenth-century National Gallery paintings often contains angular, flake- or plate-like particles (plate 8), which may be a lighter blue. Where instrumental analysis has been carried out, some aluminium has usually been detected. These features appear to be characteristic of the method of preparation in which dried blood or other animal matter and alum are ingredients. The connection between the singular particle shape and the method of preparation has also been observed in an American eighteenth-century interior decoration scheme(note 34). The concomitant presence of aluminium in the particles is a significant feature, however, and it is probable that these qualities, together with the irregularity and variability of the particle morphology, are indicative of the method of preparation. The Prussian blue identified in some early nineteenth- century paintings, such as A Horse frightened by Lightning (NG 4927) by Théodore Géricault, shows the same characteristics. Clearly, animal matter continued to be the source of the hexacyanoferrate( II) ion during the early decades of the nineteenth century: Prussian blue with the same features has been identified, for example, in the paintings of J.M.W. Turner (note 35). 

The alumina incorporated in the pigment as a result of the method of manufacture in effect acts as an extender. Examination of the genuine Prussian blues in table 1 showed that some other sort of extender had often been added. The materials used included starch, calcium salts, barytes, kaolin and, indeed, alumina itself. As mentioned above, alumina may be added during the preparation of the modern pigment to assist precipitation, but the principal reasons for the addition of extender would be to reduce the intensity of the colour and to influence its working properties. Particularly pale varieties of the pigment were sold under names like Antwerp blue; these contained a great deal of translucent extender. One example, a watercolour cake supplied by Ackermann (AA in table 1), is very similar in colour to the laboratory sample in which the hydrated alumina formed was deliberately retained, giving a product similar to a blue lake (K12a). 

However, the particle characteristics are different: the Antwerp blue does not resemble a ‘blue lake’, but contains a mixture of dark blue particles, larger colourless particles of extender and only a few pale blue rounded particles, similar to those seen in K12a. The tinting strength of Prussian blue is so high that it is rarely used undiluted; a relatively large amount of extender could be added by the pigment manufacturer or colourman to improve the properties of the material for decorative or printing purposes, for example, without greatly reducing the strength of the colour. Where the extender has been co-precipitated with the Prussian blue, as for example in the mid-nineteenth-century preparation of bleu de ciel and bleu Marie-Louise, in which the blue is co-precipitated with barium sulphate, the colour differs from that obtained by grinding barium sulphate with blue pigment (note 36).While painting out some of the genuine eighteenth- and nineteenth-century samples for the fading experiments described below it was noticed that, compared with the Winsor & Newton pigment or the pure iron(III) hexacyanoferrate( II) pigment used as a standard for comparison, some of the samples containing extender were very translucent even at full strength (RD, RE and RF, from the German pharmacy, are examples). 

Some of the early nineteenth-century pigments that contain starch grains (RA, RB; plate 9) may have been intended for a particular use, or as lowcost pigments. Starch has also been identified in the Prussian blue in several National Gallery paintings: an example is the Portrait of a Boy (NG 4034), by an unknown French artist, which is probably early nineteenth century (plate 10). The starch grains are clearly visible in the sample from the blue paint illustrated in plate 11. While starch-containing pigments may not have been in the highest quality range, cost was not necessarily the only factor: the artist may have liked the particular ‘feel’ of the paint. It was quite noticeable that the extenders present in the eighteenth- and nineteenth-century pigments had a marked effect on their handling properties, both during grinding and when dispersed in the medium. Some extenders gave a particularly pleasant silky feel to the pigment. Constant de Massoul, writing in 1797, noted that Prussian blue was difficult to use in watercolour ‘because in drying all the particles reunite, even after having been well ground upon Porphyry’ (note 37). This was found to be so with the laboratory samples prepared from dried blood, and with several others; the presence of any extender improved the grinding markedly. 

To offset the greenish undertone of the pigment, recipes for Prussian blue occasionally suggest the addition of red dyestuff, usually cochineal, in some form (see, for example, Cröker 1736 and Secrets 1790 (v) in table 2). Particles of red lake were observed in the Random and Sneath watercolour cake (RSR in table 1), although insufficient material was available for analysis to be possible. As these red dyestuffs are particularly unstable to light, their fading would be expected to have an effect on the hue of these Prussian blues, quite apart from any sensitivity to light the blue pigment itself might have. 

Colour change in samples of Prussian blue 

Experimental 
To examine the fading of Prussian blue a number of the samples detailed in table 1 were painted out in gum arabic solution as mid-toned watercolour washes on Silversafe paper; these were not particularly pigment-rich, but nor were they characteristic of the very thin washes of Prussian blue sometimes found in watercolours. 

As admixture with a white pigment has been implicated in the impermanence of Prussian blue, a set of samples containing different proportions of blue pigment to lead white was prepared in a linseed oil medium. Lead white was chosen because this was the standard white pigment used in the eighteenth and nineteenth centuries. However, as only tiny amounts of some of the genuine eighteenth- and nineteenth-century pigments were available, a preliminary experiment was performed to find appropriate proportions of lead white and Prussian blue. A series of mixtures of pure iron(III) hexacyanoferrate( II), FeIII 4[FeII(CN)6]3·xH2O (sample ALP in table 1), with lead white was painted out. The series, seen in plate 2, comprises samples with Prussian blue to lead white proportions (weight to weight) of 1:5, 1:10, 1:20, 1:40, 1:50, 1:100, 1:200, 1:400 painted in linseed oil. Three shades, termed deep (1:10), mid (1:50) and light (1:400), were selected and used as visual standards against which to compare mixtures of the Prussian blues with lead white. Some samples were also painted as very light or extremely light by further diluting the light mixtures with lead white. Because of their scarcity, not all the historic Prussian blues were painted out, and not every shade was prepared for each sample; see table 1 for details. In every case the mixture was ground in cold-pressed linseed oil and painted onto a gessoed panel primed with lead white in linseed oil, plate 12. To assess the effect of film depth on the fading, the very light mixture containing Prussian blue from a paint bladder of 1830–40 (JSM) was painted out at three thicknesses: one, two or three layers. 

The samples were all placed in a purpose-built light box at the National Gallery (note 38). Briefly, the samples were exposed to light from twelve 65W General Electric Artificial Daylight lamps, giving an illuminance at the sample surface of 20–22,000 lux; the temperature and relative humidity during the period of these experiments were 24.5 ±1°C and 36±4% respectively. The light was not filtered to exclude ultraviolet radiation. 

The colour of each sample was measured at the outset and periodically during light exposure using a Minolta CR221 colour meter. The standard geometry of this device ensures that the surface to be measured is illuminated at 45° and the light reflected along an axis perpendicular to the surface is measured. This so-called 45/0 geometry ensures that specular reflection (gloss) is excluded from the colour measurement process; the sample area is 3 mm in diameter. The data from the colour meter were converted to Commission Internationale de l’Eclairage (CIE) L*, a*, b* co-ordinates using the standard illuminant D65 as reference (note 39). These co-ordinates refer to the lightness (L*), red-greenness (a*), and yellow-blueness (b*) of the sample respectively. 

Although dark recovery of colour has been reported for Prussian blue in very thin washes, as found in cyanotypes (note 40), the phenomenon is less frequently observed in oil paint films. During the course of the experiments described above, several attempts were made to replicate the recovery of blue colour reported for early samples of Prussian blue when they were stored in the dark after light exposure. Duplicate sets of the samples were placed in the dark after exposure to light for 74 Mlux.h, but the changes in colour that resulted from dark storage were very small. As a result of these experiments, however, three Prussian blues were selected as promising for a further study of this phenomenon. Samples of three Prussian blues, ALP, K7 and SD (see table 1), were painted out on a roughened PTFE plate in linseed oil, a copal varnish dating from 1888, and Liquitex acrylic medium. After initial colour measurement, the plate was exposed in the light box for 6.7 Mlux.h(note 41). The colour was measured again and the plate was then placed in a dark metal drawer for two weeks. At the end of the period of dark storage, the plate was measured and exposed to a further 6.7 Mlux.h, then a second period of two weeks in dark storage and, finally, a third exposure of 6.7 Mlux.h. 

Results and discussion
All the samples painted out in watercolour faded to some extent on exposure to light, some showing markedly more fading than others. fig. 2 shows the colour change in two historic samples without extender (MA and TB) and five samples of modern manufacture, one sample of ‘insoluble’ Prussian blue, FeIII 4[FeII(CN)6]3·xH2O (ALP), and four types of ‘soluble’ Prussian blue, MFeIII[FeII(CN)6]·xH2O (where M = K+ for SC and M = NH4 + for SA, SB and SD). It is clear that the ‘insoluble’ variety fades much less markedly than the modern ‘soluble’ samples and the two representative samples of historic pigments, which from their elemental composition (table 1) can be assumed to be ‘soluble’ Prussian blues. 

The effect of extender can be seen by comparing the fading of two samples prepared in the laboratory using Dossie’s 1758 recipe (fig. 3). One half of the material from this preparation (K12) contains undissolved alumina as an extender, while the other portion was treated with acid to remove most of the alumina. The sample containing alumina (K12a) shows a greater colour change. This is at first sight unexpected as the net colour change for complete fading of the paler, alumina-containing sample would be less than that for complete fading of the darker sample. This indicates that the rate of fading of the blue pigment is very greatly increased by the presence of the alumina extender, as has been previously noted. 

Although the degree of colour change seems to depend on the type of Prussian blue and is markedly affected by the presence or absence of extender, all the Prussian blue samples showed similar behaviour in terms of colour shift. The pattern is illustrated in fig. 4: in addition to lightening (which is not shown in this figure) each of the samples becomes less blue (increase in b*) and less green (increase in a*) as it fades. 

The samples painted out in oil with lead white show less straightforward behaviour, although the same pattern emerges for many of the samples examined. fig. 5 shows the colour change for five shades of a mixture of Prussian blue prepared according to Dossie’s 1758 recipe (K7) and lead white. In each case, the colour readings for the initial samples have been excluded as there is a large change in the a* and b* co-ordinates during the first 24 hours of light-accelerated ageing. This is due to colour changes in the linseed oil medium that are particularly apparent in the samples containing a higher proportion of lead white (note 42). All five shades show a gradual increase in b*, corresponding to a decrease in blueness. The extremely light and very light samples concomitantly decrease in greenness (increase in a*), while the deep sample first increases in greenness before decreasing in greenness. The pattern for the light and mid-toned samples is even more complicated; they first decrease slightly in greenness, then increase in greenness somewhat before consistently decreasing in greenness. This pattern is seen in many of the early Prussian blues examined, for example the sample from Elizabeth Carey’s pastel box from the Victoria and Albert Museum (VB) shown in fig. 6. For all the specimens examined, the extremely light, very light and light samples show an increase in a* of approximately 3 to 7, while the increase in b* is around 14 to 20 (so they are becoming less green and less blue). The final colour is near-neutral on the blue to yellow (b*) axis and appears a pale greenish grey; the lighter the initial colour the yellower the final grey. The ‘sigmoidal’ plots for the mid-tones show small overall increases in a* accompanying larger increases of approximately 13 to 20 in b*. The deep tones show smaller changes in both a* and b*, but still sufficient to produce a noticeable loss of blueness. 

There are no differences between the behaviour shown by samples of Prussian blue mixed with lead white and samples of extended Prussian blue mixed with less lead white to give the same shade. In other words, the nature of the diluent (white pigment or extender) has no effect on the fading of the Prussian blue. This is reinforced by the finding that Prussian blues containing different extenders behave very similarly, for example the sample of Prussian blue prepared from Dossie’s 1758 recipe, which is extended with alumina (K12a), and one of the samples from the German pharmacy extended with barium sulphate (RE). A trial experiment to investigate the effect of other white pigments, including titanium white, zinc white and barium sulphate, in four different binding media – cold-pressed linseed oil, Liquitex acrylic medium, egg tempera or gum arabic – gave no indication that the nature of the white pigment or the binding medium affected the rate of fading of Prussian blue. 

There were no marked differences in the fading of the three test patches comprising a sample of Prussian blue from a paint bladder of 1830–40 (JSM) mixed with lead white and painted out in one, two or three layers. In each case the increase in a* was between 1.1 and 2.1, while the increase in b* (loss of blueness) was between 12.7 and 13.7, indicating that the loss of colour was a surface effect. This confirmed the evidence from cross-sections from paintings and test samples. 

As might have been expected, the Random and Sneath watercolour cake (RSR) that contains a red lake (probably cochineal) showed a noticeable loss in redness on exposure to light, presumably due to fading of the red lake. 

The three samples of Prussian blue that were subjected to alternate periods in light and dark showed some small changes in colour. The results are presented in fig. 7 for the samples in Liquitex acrylic medium. In each case a light exposure of 6.7 Mlux.h resulted in an increase in lightness of 7 to 8 units, a decrease in redness of 4 to 6 units and a decrease in blueness of 3 to 4 units (not shown in the figure). During a period of dark storage some ‘recovery’ of the colour was observed, again shown (as ‘First dark’) in fig. 7. A second period of light exposure produced lightening and loss of redness and blueness comparable to the first light exposure, resulting in colours that were lighter, less red and less blue than those after the initial light exposure. Subsequent dark storage caused some recovery, but on each light–dark cycle it is clear that the colour is not fully recovered and that subsequent light exposure causes further irreversible fading. 

Conclusions 

Factors affecting the degree of fading 
The principal factor in the permanence of Prussian blues of different manufactures and dates appears to be the amount of extender present in the supplied pigment. Different methods of preparation only affect permanence in as much as they lead to the incidental introduction of extender, either deliberately or due to inadequate purification of the product. For example, dissolving out the aluminium salt from Prussian blue made from animal blood is easily carried out in a small-scale laboratory experiment, but is less easily achieved in the industrial preparation of the pigment. 

Adding white pigment to Prussian blue increases the colour change observed on light exposure. This may be due to increased inter-reflection of light in the paint layer, leading to greater fading, or may simply be a visual effect, because the colour change is more noticeable in the paler blue shades where less Prussian blue is present in the first place. The nature of the white pigment does not seem to affect the fading, nor does it matter whether the white is a deliberate addition to dilute the colour, or a premixed extender added to the Prussian blue by the manufacturer or colourman. When a relatively pure and a much-extended Prussian blue are each mixed with sufficient lead white to produce shades of corresponding lightness, the fading behaviour of the two samples is broadly similar. As alumina extender is present in the light-coloured, translucent, plate- like flakes characteristic of the eighteenth- and early nineteenth-century pigment, these are particularly likely to exhibit fading, sometimes only at the top of the particle. A small pigment particle surrounded by white pigment in a typical light blue paint film would also be likely to fade (note 43). Where the particles form aggregates they are less vulnerable (note 44). 

Samples in watercolour medium showed similar colour shifts on fading to the mixtures of Prussian blue and lead white in oil medium classified as ‘very light’ or ‘extremely light’. Deeper shades in oil medium showed smaller changes in colour and more complicated colour shifts. 

The formula of the pigment has some effect on its permanence; in watercolour, the modern sample of ‘insoluble’ Prussian blue faded less than modern or historic samples of ‘soluble’ Prussian blue. However, the counter ion in the different ‘soluble’ Prussian blues (K+ or NH4 +) seems to have little effect on the fading. 

The reversibility and chemistry of fading 
The films that comprised Prussian blue and white did not regain their colour when placed in the dark after considerable exposure to light, nor did the watercolour samples. The reversible behaviour was only observed in films exposed for relatively short periods before dark storage. It may be that the short-term reversible fading is simply an early stage of the long-term loss of colour, but evidence from other sources seems to indicate that there are two processes involved in the fading of Prussian blue: a fairly rapid, reversible loss of colour, and a slower irreversible colour change. The reversible change can be induced by light in oil paint films, as demonstrated in the experiments described above, but is more usually associated with, and is more dramatic in, thin washes on paper, in cyanotypes for example (note 45). The slower irreversible fading is seen in samples in all media and results in a final colour that is best described as yellowish grey. 

The chemistry behind these types of colour loss is, however, not completely clear. As already mentioned, the strong blue colour is due to the charge transfer transition between iron(II) and iron(III) in the FeIII–N–C–FeII moiety. Anything that disrupts this configuration will lead to a loss or shift of colour. 

Three reactions of the iron(III) hexacyanoferrate( II) complex have been implicated in colour change. First, a reduction of Prussian blue to Berlin white, K2FeII[FeII(CN)6] (also called Everitt’s salt), which can be light-induced, or can be initiated by a reducing agent: 

FeIII[FeII(CN)6]- + e- ↔ FeII[FeII(CN)6]2- 

Electrochemical studies of Prussian blue films have shown that the reduction is a single step process and that the blue colour returns on oxidation (note 46). Indeed, the FeII[FeII(CN)6]2- complex is an intermediate in the synthesis of Prussian blue from iron(II) and hexacyanoferrate( II) ions. As described earlier, mixing solutions containing these two ions yields a pale blue precipitate (containing both K2FeII[FeII(CN)6] and KFeIII[FeII(CN)6]), which on exposure to air deepens in colour as the Berlin white is slowly oxidised in air, or more rapidly if an oxidising agent is introduced. In the paint film, the situation is not quite the same as in the electrochemical solution studies, where there is a ready supply of counter ions, usually potassium. However, a spectroelectrochemical study of dry films of ‘soluble’ Prussian blue using in situ Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR/ATR) demonstrated that both reduction and oxidation were possible in the absence of an electrolyte (note 47). 

This redox process is believed to be responsible for the light–dark bleaching and recovery of cyanotypes. Although the paper or size may provide the reducing agent, the fading and recovery has also been noted on cyanotypes printed on glass fibre papers, suggesting that an excess of one of the reagents used in the preparation might act as a reducing agent (note 48). In either case, the cycle gradually ‘decays’ as the reservoir of reducing agent is used up, although if a cyanotype is re-sized further reduction can be induced (note 49). 

It also seems likely that the reported bleaching of Prussian blue when it is stored in oxygen-free environments and its recovery on re-exposure to air, for example when the dry pigment is stored in an airtight bottle (note 50), results from Prussian blue to Berlin white inter-conversion. 

A second possible reaction that would lead to loss of colour in Prussian blue would be the oxidation of [FeII(CN)6]4- ion. Electrochemical studies of Prussian blue films deposited on electrodes have shown that, unlike the abrupt reduction of Prussian blue to Berlin white, this oxidation proceeds through a number of intermediate steps before yielding a yellowish product, sometimes referred to as Prussian yellow (note 51). One intermediate in this reaction is the mixed valence compound known as Berlin green. While the oxidation of Prussian blue to Berlin green is reversible, the subsequent oxidation to Prussian yellow is not fully reversible (note 52). 

Prussian blue ↔Berlin green 
[FeII(CN)6]4- ↔ {[FeIII(CN)6]0.67[FeII(CN)6]0.33}3.33- + 0.67e- 

Berlin green →Prussian yellow 
{[FeIII(CN)6]0.67[FeII(CN)6]0.33}3.33- →[FeIII(CN)6]3- + 0.33e- 

The ‘very light’ and ‘extremely light’ paint samples examined in this study all yielded faded products that were a yellowish grey, possibly a mixture of Prussian yellow and a little unreacted Prussian blue highly diluted with lead white. 

A third possible reaction of the hexacyanoferrate( II) ion is a substitution reaction. The aquapentacyanoferrate(II) ion is known to be formed by the action of light on Prussian blue: 

FeIII[FeII(CN)6]- + H2O h→FeIII[FeII(CN)5 (H2O)] + CN-

The reaction is generally studied in aqueous solutions, 53 where there is a ready source of water to co-ordinate at the vacant site on the FeII ion; in hydrophobic environments, the water molecule coordinating to the iron may be one of the unassociated water molecules occupying a zeolitic site in the Prussian blue structure. As the aquapentacyanoferrate( II) complex is green, a further reaction must occur during the fading of Prussian blue paint samples examined in this study, as their final colours are considerably less green than the starting materials. It is possible that further substitution reactions displace further cyanide ligands, causing a shift in colour towards yellow and colourless, but this has not been studied in any detail. Alternatively, it is known that FeIII[FeII(CN)5 (H2O)] can also undergo redox reactions like those of Prussian blue to yield, for example, the aquapentacyano analogue of Berlin white (note 54): 

FeIII[FeII(CN)5 (H2O)] + e- FeII[FeII(CN)5(H2O)]- 

As far as easel paintings are concerned, it seems that if the reversible fading, seen in cyanotypes and watercolour paintings on occasions (or in the absence of oxygen), takes place at all, it is at the very beginning of the deterioration process. The yellowish-grey appearance of changed Prussian blue-containing paint films suggests that the long-term colour changes result from oxidation of the pigment. Perhaps because the appropriate conditions for the regeneration of Prussian blue are not present in the paint film, these changes seem to be irreversible. There is certainly no shortage of oxygen in a drying oil paint film; the oxygen would be consumed both by the oil as it cross links and by Prussian blue as it changes colour. This would accord with eighteenth-century reports of changes in the colour of the pigment relatively soon after painting (note 55). 

Acknowledgements 
We would like to thank our colleagues in the Scientific Department at the National Gallery who have supplied us with examples of the use of Prussian blue in paintings and conducted EDX analysis of pigment samples: Ashok Roy, Marika Spring and Rachel Grout, Radcliffe Reseach Fellow, 2000–2. We would also like to thank those who provided historical samples of the pigment: Dr Andreas Burmester, Doerner Institut, Munich; Professor Dr E.-L. Richter (retired), Institut für Technologie der Malerei, Staatliche Akademie der Bildenden Künste, Stuttgart; Jane Zeuner, Museum of London; MaryAnne Stevens, Royal Academy of Arts, London. Other samples were already in the possession of the National Gallery Scientific Department. In addition, Professor Richter kindly supplied us with a copy of the 1985 Diplomarbeit on Berliner Blau by Hans Becker. After the publication of the article on the fading of Prussian blue in Volume 14 of the National Gallery Technical Bulletin (1993), Professor Richter asked us to point out that it was in fact Mr Becker who made the observation of the occurrence and fading of the pigment in Watteau’s Récreation italienne (Berlin, Charlottenburg Castle). We are happy to do this. Also as a result of this publication, we have received interesting reports and had useful discussions on colour change in Prussian blue in eighteenth-century decorative schemes and easel paintings with Chi Asai, paintings conservator, Munich, Johannes Preis (of Gebr. Preis GmbH., restorers, Parsberg), then of the University of Cologne, and Marc Giudicelli, then of the Straus Center for Conservation, Harvard University Art Museums: our thanks are due to them. Finally we are particularly grateful to Dr Mike Ware for useful and stimulating discussions on the chemistry of colour change in Prussian blue.

Notes and references
1 G.E. Stahl, Experimenta, Observationes, Animad-versiones, CCC Numero, chymicae et physicae, Berlin 1731, pp. 280–3.
2 ‘Notitia Coerulei Berolinensis nuper inventi’, Miscellanea Berolinensia, I, 1710, pp. 377–8; J. Kirby, ‘Fading and Colour Change of Prussian Blue: Occurrences and Early Reports’, National Gallery Technical Bulletin, 14, 1993, pp. 62–71, esp. pp. 63, 70–1.
3 M. van Eikema Hommes, ‘Discoloration in Renaissance and Baroque Oil Paintings: Instructions for Painters, Theoretical Concepts, and Scientific Data’, PhD Thesis, University of Amsterdam, 2002, pp. 139–48.
4 Kirby 1993 (cited in note 2), pp. 65, 67.
5 Kirby 1993 (cited in note 2).
6 J. Woodward, ‘Praeparatio caerulei Prussiaci ex Germania missa ad Johannem Woodward’, Philosophical Transactions, XXXIII, no. 381, January-February 1724, pp. 15–17.
7 J.A. Sistino, ‘Ferriferrocyanide Pigments: Iron Blue’, Pigment Handbook, ed. T.C. Patton, New York 1973, Vol. 1, pp. 401–7, esp. pp. 403–4.
8 C.F.A. Hochheimer, Chemische Farben-Lehre, oder ausführlicher Unterricht von Bereitung der Farben zu allen Arten der Malerey, 2 parts in 1 volume, Leipzig 1792–4, p. 171.
9 A.H. Church, The Chemistry of Paints and Painting, 3rd edn, London 1901, pp. 213–16. Mortimer and Rosseinsky point out that the soluble form is actually thermodynamically less soluble than the insoluble form: see R.J. Mortimer and D.R. Rosseinsky, ‘Iron Hexacyanoferrate Films: Spectroelectrochemical Distinction and Electrodeposition Sequence of “Soluble” (K+-containing) and “Insoluble” (K+-free) Prussian Blue, and Composition Changes in Polyelectrochromic Switching’, Journal of the Chemical Society Dalton Transactions, 1984, pp. 2059–61.
10 H. Holtzman, ‘Alkali Resistance of the Iron Blues’, Industrial and Engineering Chemistry, 37, 1945, pp. 855–61.
11 Sistino 1973 (cited in note 7), pp. 404–5. Barbara Berrie, citing Degussa Chemische, Vossen Blue Pigments and Blue Pigment Compounds, Teterboro, NJ, 1974, gives a change in permanence rating from eight (permanent) when pure to four when mixed with ten parts of titanium white: B.H. Berrie, ‘Prussian Blue’, Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 3, ed. E. West FitzHugh, Washington, DC 1997, pp. 191–217.
12 See, for example, W.A. Kampfer, ‘Titanium Dioxide’, in Pigment Handbook 1973 (cited in note 7), pp. 1–36, esp. p. 2; H.B. Stephenson, ‘Zinc Oxide and Leaded Zinc Oxide’, in the same volume, pp. 37–51; E.J. Dunn, ‘White Hiding Lead Pigments’, in the same volume, pp. 65–84. The three reflectance curves may also be compared in H. Kühn, ‘Zinc White’, Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 1, ed. R.L. Feller, Cambridge/Washington, DC 1985, pp. 169–86, esp. p. 173.
13 M. Ware, ‘A new blueprint for cyanotypes’, Ag+ Photographic, 7, 1995, pp. 74–81.
14 Sistino 1973 (cited in note 7), p. 405; ‘A Review of Literature in the Foreign Paint and Varnish Field’, Paint, Oil and Chemical Review, 25 June 1936, pp. 11–14, esp. p. 13; Berrie 1997 (cited in note 11), p. 197.
15 Berrie 1997 (cited in note 11), p. 197.
16 Sistino 1973 (cited in note 7), p. 405; Berrie 1997 (cited in note 11), p. 197.
17 Kirby 1993 (cited in note 2), p. 65.
18 P. Day, ‘Introduction to Mixed-Valence Chemistry’, in Mixed-Valence Compounds: Theory and Applications in Chemistry, Physics, Geology and Biology, ed. D.B. Brown, Dordrecht/ London 1980, pp. 3–24; A.G. Sharpe, The Chemistry of Cyano Complexes of the Transition Metals, London 1976, pp. 121–6.
19 F. Herren, P. Fischer, A. Ludi and W. Halg, ‘Neutron Diffraction Study of Prussian Blue, Fe4[Fe(CN)6]3·xH2O: Location of Water Molecules and Long-Range Magnetic Order’, Inorganic Chemistry, 19, 1980, pp. 956–9; A. Ludi, ‘Descriptive Chemistry of Mixed-Valence Compounds’, in Mixed-Valence Compounds 1980 (cited in note 18), pp. 25–47, esp. pp. 39–40; Berrie 1997 (cited in note 11), pp. 199–202. It is likely that the Fe3+ ions are in fact hydrolysed and present as [Fe(OH)]2+: see C.A. Lundgren and R.W. Murray, ‘Observations on the Composition of Prussian Blue Films and Their Electrochemistry’, Inorganic Chemistry, 27, 1988, pp. 933–9.
20 D.R. Rosseinsky, J.S. Tonge, J. Berthelot and J.F. Cassidy, ‘Site-transfer Conductivity in Solid Iron Hexacyanoferrates by Dielectric Relaxometry, Voltammetry and Spectroscopy’, Journal of the Chemical Society Faraday Transactions I, 83, 1987, pp. 231–43.
21 H.J. Buser, D. Schwarzenbach, W. Petter and A. Ludi, ‘The Crystal Structure of Prussian Blue: Fe4[Fe(CN)6]3·xH2O’, Inorganic Chemistry, 16, 11, 1977, pp. 2704–10.
22 The wavelength of maximum absorption, _max, will depend on the type of Prussian blue. According to Mortimer and Rosseinsky 1984 (cited in note 9), _max for ‘soluble’ Prussian blue is 690 nm (14500 cm-1), while that for ‘insoluble’ Prussian blue is 730 nm (13700 cm-1). The difference can be attributed to the different ligand field splitting in the two complexes: in the former the Fe(III) centre is surrounded by six nitrogen atoms, while in the latter it is surrounded by both nitrogen and oxygen atoms with an average environment of FeIIIN4.5O1.5. Robin gives a value of 14100 cm-1 for ‘soluble’ Prussian blue (M.B. Robin, ‘The Color and Electronic Configuration of Prussian Blue’, Inorganic Chemistry, 1, 2, 1962, pp. 337–42).
23 Sharpe 1976 (cited in note 18), pp. 121–3.
24 See, for example, Inorganic Pigments: Manufacturing Processes, ed. M.H. Gutcho, Park Ridge 1980, pp. 433–9; N. Heaton, Outlines of Paint Technology, 3rd edn, London 1948, pp. 154–8; J.S. Remington and W. Francis, Pigments: Their Manufacture, Properties and Use, London 1954, pp. 94–9.
25 Kirby 1993 (cited in note 2), p. 64.
26 R. Dossie, The Handmaid to the Arts, 2 vols, London 1758, Vol. I, pp. 77–8; Vol. 2, pp. 387–91.
27 Hochheimer 1792 (cited in note 8), pp. 133–72, esp. p. 144 onwards; 207–22: for Paris blue see p. 142. The difficulties that could arise during manufacture are described by J.A. Weber, Chemische Erfahrungen bey meinem und anderen Fabriken in Deutschland, Neuwied 1793, pp. 134–65, 189–200. It is notable that Hochheimer refers to the use of an iron vessel for the calcination and this is likely to have been a general practice: see, for example, W. Nicholson, A Dictionary of Chemistry, 2 vols, London 1795, Vol. 1, p. 226, cited by Sally Carew-Reed, ‘Lewis Berger & Sons (1766–1960): An English colour manufactory’, 2 vols, 3rd year project, Department of Conservation & Technology, Courtauld Institute of Art, 1997: ‘Prussian blue’. The annotated copy she cites is in the archives of Lewis Berger & Sons, who manufactured Prussian blue from 1766.
28 J.-F.-D. Riffault Deshêtres, A.-D. Vergnaud and C.-J. Toussaint, Nouveau manuel complet du fabricant de couleurs et de vernis, new edn revised by F. Malepeyre and E. Winckler, 2 vols (Encyclopédie Roret – Manuels Roret), Paris 1884, Vol. 1, pp. 237–307. Berger & Sons seem to have used ‘prussiate of potash’, which they prepared themselves, from about 1834: Carew-Reed 1997 (cited in note 27). The methods of making the pigment, particularly in Germany, are summarised by H.K.L. Becker, Berliner Blau, Diplomarbeit am Institut für Technologie der Malerei der Staatlichen Akademie der bildenden Künste zu Stuttgart, 1985, pp. 9–37. In 1837 a medal was awarded to Lewis Thompson by the Society of Arts for his introduction of a method of making potassium ferrocyanide by roasting charcoal with potash and iron filings at red heat: Ure’s Dictionary of Arts, Manufactures and Mines, ed. R. Hunt, London 1860, p. 545 (cited by M. Ware, Cyanotype: The History, Science and Art of Photographic Printing in Prussian Blue, London 1999, p. 22).
29 Fourier transform infrared analysis was performed by Jennifer Pilc.
30 In the laboratory, an alkaline solution containing free iron(II) and free cyanide ions rapidly produced a dark coloured precipitate containing a mixture of Prussian blue and iron(III) hydroxide.
31 When an alkaline solution of K4[FeII(CN)6] is applied to a dipyridyl paper a positive result is not obtained. However, if the solution is acidified before it is applied to the paper, a positive result is obtained; obviously some FeII is formed by acid-initiated breakdown of the hexacyanoferrate(II) ion. Indeed, if a solution of K4[FeII(CN)6] is acidified and left for some hours, a ‘soluble’ Prussian blue is formed, presumably by the reaction of the small amount of free FeII produced with the remaining hexacyanoferrate(II). No Prussian blue is formed from an alkaline solution of K4[FeII(CN)6].
32 L.J.M. Coleby, ‘A History of Prussian Blue’, Annals of Science, 4, 1939, pp. 206–11.
33 The laboratory-prepared samples and collected samples were examined by Marika Spring. Prussian blue was identified in paintings during the course of routine examination by Ashok Roy, Marika Spring and Rachel Grout, by SEM–EDX or (for earlier analyses) laser microspectral analysis (LMA), or by Catherine Higgitt using FTIR. In some cases the pigment was identified microchemically (it reacts rapidly with alkali, giving a brown product).
34 F.S. Welsh, ‘Particle Characteristics of Prussian Blue in an Historical Paint’, Journal of the American Institute for Conservation, 27, 1988, pp. 55–63.
35 J.H. Townsend, ‘The Materials of J.M.W. Turner: Pigments’, Studies in Conservation, 38, 1993, pp. 231–54.
36 Riffault Deshêtres et al. 1884 (cited in note 28), Vol. 1, p. 44. Georges Halphen describes how the director of a factory at Dugny tested the effects of extender pigments, including different forms of barium sulphate, zinc white and lead white (céruse), on the colour of Prussian blue; the differences were apparently quite marked: G. Halphen, Couleurs et vernis, Paris 1895, pp. 15–17.
37 C. de Massoul, A Treatise on the Art of Painting and the Composition of Colours, London 1797, p. 182.
38 D. Saunders and J. Kirby, ‘A comparison of light-accelerated ageing regimes in some galleries and museums’, The Conservator, 25, 2001, pp. 95–103.
39 Commission Internationale de l’Eclairage, ‘Recommendations on uniform colour spaces, colour difference equations, psychometric colour terms’, Supplement No. 2 to CIE Publication No.15 (E-2.3.1), 1971/(TC-1.3), 1978.
40 Ware 1999 (cited in note 28), pp. 120–6.
41 An exposure of 6.7 Mlux.h is equivalent to around twelve years in a museum under the lighting conditions recommended for oil paintings (200 lux during 56 viewing hours per week), or to exposure outside for one month during typical spring or autumn conditions in London.
42 The initially reversible yellowing and bleaching of a fresh drying oil film, due to conjugation of unsaturated bonds in the oil, is well-known and is particularly apparent in white or light-coloured paint. This effect will be discussed in more detail, from the point of view of colour measurement, in a forthcoming publication.
43 R.M. Johnston-Feller, R.L. Feller, C.W. Bailie and M. Curran, ‘The Kinetics of Fading: Opaque Paint Films pigmented with Alizarin Lake and Titanium Dioxide’, Journal of the American Institute for Conservation, 23, 1984, pp. 114–29.
44 O. Hafner, ‘Lightfastness of Organic Pigments as a Function of Their Particle Size Distribution’, Journal of Paint Technology, 47, 1975, pp. 64–9.
45 Ware 1999 (cited in note 28), pp. 120–6; P.M. Whitmore, C. Bailie and S.A. Connors, ‘Micro-fading tests to predict the result of exhibition: Progress and prospects’, Tradition and Innovation: Advances in Conservation; Contributions to the Melbourne IIC Congress, 10–14 October 2000, eds A. Roy and P. Smith, London 2000, pp. 200–5.
46 Mortimer and Rosseinsky 1984 (cited in note 9); P.A. Christensen, A. Hamnett and S.J. Higgins, ‘A Study of Electrochemically grown Prussian Blue Films using Fourier-transform Infra-red Spectroscopy’, Journal of the Chemical Society Dalton Transactions, 1990, pp. 2233–8; A. Dostal, B. Meyer, F. Scholz, U. Schröder, A.M. Bond, F. Marken and S.J. Shaw, ‘Electrochemical Study of Microcrystalline Solid Prussian Blue Particles Mechanically Attached to Graphite and Gold Electrodes: Electrochemically Induced Lattice Reconstruction’, Journal of Physical Chemistry, 99, 1995, pp. 2096–103.
47 P.J. Kulesza, M.A. Malik, A. Denca and J. Strojek, ‘In Situ FT-IR/ATR Spectroelectrochemistry of Prussian Blue in the Solid State’, Analytical Chemistry, 68, 14, 1996, pp. 2442–6.
48 Cyanotypes printed on glass plates coated with alumina or silica, glass fibre papers, or ceramics also show reversible fading, despite the absence of the oxidisable groups present in cellulose papers. The cyanotypes are prepared from solutions containing a citrate, which might act as a reducing agent for the reversible fading of Prussian blue. Ware 1999 (cited in note 28), p. 125, and personal communication, Dr Mike Ware, 31 January 1997.
49 When a cyanotype on paper that has been exposed to light–dark cycles until the reversible fading no longer occurs is re-sized with a solution of critic acid, ammonium citrate or ammonium oxalate, the reversible fading is again observed on light–dark cycling. Personal communication, Dr Mike Ware, 31 January 1997.
50 ‘A Review of Literature in the Foreign Paint and Varnish Field’ 1936 (cited in note 14). A study of the effect of an oxygen-free environment on the rate of fading of various pigments found that, unlike most of the other pigments tested, Prussian blue lost colour more rapidly when oxygen was excluded: J.S. Arney, A.J. Jacobs and R. Newman, ‘The influence of oxygen on the fading of organic colorants’, Journal of the American Institute for Conservation, 18, 1979, pp. 108–17. Eder (J.M. Eder, History of Photography, trans. E. Epstean, New York 1978, p. 130) gives an account of Desmortiers’ observations on fading in the absence of air in the context of dyeing fabrics. Desmortiers emphasises in his Recherches sur la décoloration spontanée du bleu de Prusse, 1801, that fabrics must have free access to the air in order to reverse the fading and restore the blue colour of the fabric: Ware 1999 (cited in note 28), p. 114.
51 Mortimer and Rosseinsky 1984 (cited in note 9); Christensen et al. 1990, Dostal et al. 1995 (both cited in note 46).
52 Ware 1999 (cited in note 28), pp. 147–8, and references therein; the reaction path for the formation of Prussian yellow from Prussian blue via Berlin green seems to differ for the insoluble and soluble forms of Prussian blue: D.R. Rosseinsky and A. Glidle, ‘EDX, Spectroscopy, and Composition Sudies of Electrochromic Iron(III) Hexacyanoferrate(II) Deposition’, Journal of The Electrochemical Scociety, 150, 9, 2003, pp. c641–5.
53 Sharpe 1976 (cited in note 18).
54 V.B. Nechitayilo, V.I. Styopkin, Z.A. Tkachenko, Y.G. Goltsov, V.P. Sherstyuk and V.V. Zhilinskaya, ‘Structure and electrochromic properties of ferric aquapentacyanoferrate – a new analogue of Prussian blue’, Electrochimica Acta, 40, 1995, pp. 2501–4.
55 Kirby 1993 (cited in note 2), pp. 64–5.

TABLE 1 Details of the early Prussian blue samples and pigments prepared in the laboratory
Supplier	Date	Extender (note a)	Ref.	Shades in oil (note b)	
John Nicoll: ‘Celestial blue’
John Nicoll: ‘Prussian blue’
John Nicoll: ‘Blue’	mid-19th century
mid-19th century
mid-19th century	CaSO4: Ca, S, Fe, (K)
hydrated alumina: Fe, K, Al, (S)
BaSO4: Ba, S, Fe, Si, (Al), (K)	NC
NP
NB	LMD
L2MD
L2MD	
Made from [(NH4)4FeII(CN)6] + FeIII (FeIICl2 + H2O2)
As SA (above), washed twice with water
Made from [K3FeIII(CN)6] + FeIICl2
Made from [(NH4)4FeII(CN)6] + FeIICl2 + H2O2	–
–
–
–	none: Fe, Cl
none: Fe, Cl
none: Fe, K, (Al), (Cl)
none: Fe, (Al), (S)	SA
SB
SC
SD	–
L2MD
L2MD
–	
Aldrich Chemicals (pure FeIII
4[FeII(CN)6]3)	n/a
	none: Fe, (S)	ALP	L3L2LMD	
E.-L. Richter: ‘Prussian blue, barytes, gypsum’
E.-L. Richter: ‘Prussian blue’
E.-L. Richter: ‘Prussian blue, gypsum’
E.-L. Richter: ‘Prussian blue, kaolin, quartz’
E.-L. Richter: ‘Prussian blue, barytes’
E.-L. Richter: ‘Prussian blue, gypsum’
E.-L. Richter: ‘Prussian blue, barytes’	1800–30
1800–30
1800–30
1800–30
1800–30
1800–30
1800–30	Starch (note c): Ba, Ca, Fe, S, (K)
Starch (note c): Fe, K, S, (Al)
Fe, K, (Al), (S)
Al, Si, K, Fe, (Ti or Ba)
Al, Ba, Ca, Fe, S, K, Si, (P)
Al, Fe, Ba, Ca, K, S, Si, (Na), (P)
Ba, Fe, S, K, Si, (Al)	RA
RB
RC
RD
RE
RF
RG	–
–
–
L2MD
LMD
L2MD
–	
A. Burmester: Doerner Institut ‘Good Berlin blue from Weimar’ (note d)
A. Burmester: Doerner Institut ‘Berlin blue’ (note d)	11.4.1855
30.4.1842	Al, Fe, K, S, Si, (P)
Fe, (Ca), (Cu), (K)	BA
BB	–
–
	
The Royal Academy: from Ozias Humphrey’s watercolour box
The Royal Academy: Ackermann cake from Reeves box
The Royal Academy: Random & Sneath cake from Reeves box
The Royal Academy: Chinese blue from Bull box	1742–1810
–
–
–	Fe, Al, K, S, (Cu), (Si)
Fe, Al, K
Al, Ca, K, Fe, S, (Cu), (P)
Fe, Al, K, Si, (S)	OZ
AR
RSR
BCB	–
–
–
L3L2LM	
Winsor & Newton alkali ferric ferrocyanide	–	Fe, K, (Al)	WN	L2MD	
Ackermann ‘Prussian blue’
Ackermann ‘Antwerp blue’	1796–1827
1796–1827	Fe, Al, K, (P), (S), (Si)
Al, K, S, Fe, Si, (K), (P)	AP
AA	L3LM
–	
The Museum of London: Reeves & Woodyer cake
The Museum of London: Reeves box cake (note e)
The Museum of London: Reeves & Inwood powder (note e)
The Museum of London: Brown of Oxford Street box	1805–17
18th century
1797?
1791	Fe, Al, K, (S)
not analysed
not analysed
not analysed	MA
MB
MC
ME	L3L
–
L2L
–	
The Victoria & Albert Museum: Elizabeth Carey’s box
The Victoria & Albert Museum: Elizabeth Carey’s box	1770–1831
1770–1831	lead white: Pb, Al, Fe (Cu)
none: Al, Fe, S (K), (Si)	VA
VB	–
L3L2M	
Gainsborough’s House: paint bladder	early 19th century	Fe, K, (Al), (S), (Si)	GA	L3L2M	
J.S. Mills: paint bladder	1830–40	Fe, K	JSM	L3L2LM	
The British Museum: ‘Turnbull’s blue’	–	Fe, K, (Al), (S)	TB	L2MD	
Made using a recipe of 1758
Made using a recipe of 1758
Made using a recipe of 1758	–
–
–	none: Fe, (S)
hydrated alumina: Al, S, Fe
none: Fe, (Al)	K7
K12a
K12b	L3L2LMD
L3LM
L3L	

Notes
a Determined by EDX analysis. Results indicate the elements detected: strong peaks underlined, minor elements in brackets. In italics = in mounting medium.
b D = deep, M = mid, L = light, L2 = very light, and L3 = extremely light.
c Detected by FTIR.
d ‘gutes Berliner Blau von Weimar am 11 Apr 1855 … gerieben’, ‘Berliner Blau vom 30.Apr 1842 …’
e Despite label, pigment is indigo.

Professor Richter’s samples are from an old German pharmacy near Darmstadt: E.L. Richter and H. Härlin, ‘A Nineteenth-Century Collection of Pigments and Painting Materials’, Studies in Conservation, 19, 1974, pp. 76–82.

TABLE 2 Ingredients in eighteenth-century Prussian blue recipes and their proportions
Recipe	Animal matter	Alkali	Iron salt	Alum	Notes	
Woodward 1724 (the original)	x dried blood	4 crude tartar, 4 crude nitre	1 green vitriol, calcined	8	Brown in his experiments on the method
found the yield of alkali from the
detonation of the tartar and nitre to be
4 parts and the final yield of pigment
about 1 part. All solutions hot.	
Shaw 1734
(i.e. Woodward)	4 dried blood	4 crude tartar, 4 crude nitre	1 green vitriol,
calcined	8	–	
Cröker 1736	x dried blood	6 tartar, 6 saltpetre	1 English vitriol,
calcined
(Englischen Vitriol)	6	To 1.5 pounds of the blood/alkali mixture
add 1.5 Loth (3/64 pounds) calcined
vitriol. A little cochineal (Concinelle)
added to the alum solution. No
mention of acid.	
Dossie’s version of the
original, 1758	8 dried blood	8 pearl ashes	1 calcined green vitriol	16	Proportion of vitriol is by weight of the
calcined material. Greenish precipitate.
6 parts acid to dissolve hydrated
alumina, etc. for every 16 parts K2CO3
to yield 3 parts product. For a deeper
colour (but lower yield) use more acid;
for a paler use more alkali.	
Dossie 1758 (ii)	3 dried blood	1 pearl ashes	1 green vitriol	2	Green precipitate. 4 parts acid to dissolve
hydrated alumina, etc. For a paler
colour use, e.g., 3 parts blood and 1
part green vitriol, but then use 5 parts
alum.	
Hellot 1762 (i)	3 dried blood	3 crude tartar, 3 potash,
1.5 saltpetre	2 (green) vitriol	8	Green precipitate. Sufficient acid.
Yield ≤0.375 parts.	
Hellot 1762 (ii)	3 dried blood	3 ashes (cendres
gravelées), 2 crude
tartar, 1.5 saltpetre	2 (green) vitriol	8	Washed, dried precipitate will be blue
without the need for acid treatment.
Yield 2.5 parts, but colour less good.	
Hellot 1762 (iii)	3 dried blood	3 quick lime, 2 crude
tartar, 1.5 saltpetre	1.5 (green) vitriol	6	Yield c.0.4 parts, but of very good colour,
‘the effect similar to the most beautiful
ultramarine’.	
Hellot 1762 (iv)	3 dried blood	3 quick lime, 2 crude
tartar, 2 calcined nitre	1 (green) vitriol	4	More product than the preceding, but less
fine colour.	
Hellot 1762 (v)	3 dried blood	4.5 quick lime, 2 crude
tartar, 1.5 saltpetre	1 (green) vitriol	4	Yield c.0.5 parts, but the best blue.	
Hellot 1762 (vi)	3 dried blood	6 quick lime, 2 crude
tartar, 1.5 calcined nitre
	1 (green) vitriol	4	Yield c.1.6 parts; good colour, similar to
(iii).	
Le Pileur d’Apligny 1779	8 dried blood	8 potash (potasse)	3 iron sulphate
(vitriol de Mars)	4	Greenish precipitate. 4 parts acid to
dissolve alumina and very thorough
washing, preferably with hard water.	
Secrets 1790 (i)	2 dried blood	2 crude tartar, 2 crude
nitre	1 iron sulphate
(vitriol de Mars)	4	Close to the original method, also
included (no. iii). Use more rather than
less acid to dissolve hydrated alumina,
etc., as colour thereby improved.	
Secrets 1790 (v)	2 dried blood	2 crude potash, 1 red
tartar, 0.75 oz saltpetre	x iron sulphate (vitriol
de Angleterre)	4	Several other recipes in this book derive
from Hellot 1762. Three others include
red dyestuffs (cochineal, safflower) to
offset the green undertone.	
Hochheimer 1792 (ii)	8 dried blood	8 potash	3 iron vitriol	4	Original method also included (no. i).
Greenish precipitate; 3–4 parts
hydrochloric acid used to dissolve
excess alumina, etc.	
Hochheimer 1792 (v)	100 animal
charcoal	100 potash	25 green vitriol
+ 6–8 pounds old
iron	100	A large-scale preparation starting with
1 Centner (100 pounds) burnt horns, hoofs
or other animal matter, or even chimney
soot; alkaline product called Blutlauge
whether derived from blood or not. Tests
made to adjust proportions of vitriol and
alum to get desired shade; addition of acid
may be unnecessary.	
Hochheimer 1792 (vi)	15 (parts) calcined
hoofs (Klauenkohlen)	20 potash	6 English vitriol	40	Method ascribed to Weber (see below);
3 x 25 pound portions of calcined hoofs
added to 100 pounds potash at half-hour
intervals. Pale blue precipitate treated
with acid to obtain a darker blue.	
Hochheimer 1792 (vii)	36 (parts) hoofs	16 French tartar, 25 potash	5 English vitriol	16	Described as the Dutch method. 1–1.6 parts
sulphuric acid used to dissolve excess
hydrated alumina, etc.	
Hochheimer 1794 (i)	32–37 (parts)
calcined hoofs	50 clean potash	12 green vitriol	80-100	Iron vessel used for calcining the hoofs.
Roughly equal proportions of calcined
hoofs added to the potash at half-hour
intervals. Treat with acid if required. In
a modification of this method, for every
2 parts potash used, 1 part acid and
4 parts alum solution are added to the
alkaline solution; sufficient vitriol is
then used to complete the reaction.	
Weber 1793 (i)	20 (parts) calcined
hoofs	18 raw potash	5 or 6 (green) vitriol	8 or 10	The alkaline solution should be greenish:
absence of colour a bad sign. 0.75 parts
sulphuric acid added to alum/iron
sulphate mixture before addition to
alkaline extract; another 3 parts iron salt
added subsequently to ensure completion
of reaction.	
Weber 1793 (ii)	14 (parts) calcined
hoofs	13 pure potash	5 (green) vitriol	15	Or use 20 parts alum, then use acid to
redissolve the excess precipitated as
hydrated alumina.	
Massoul 1797	2 dried blood	1 tartar alkali	1 iron sulphate
(Martial vitriol)	3	Yellowish-green alkaline solution. On the
addition of the alum/iron sulphate the
mixture becomes reddish brown, with a
sediment that is blue on the surface,
yellow in the centre. It turns green, then
blue on exposure to the air. Acid optional.	

Sources
Woodward 1724: J. Woodward, ‘Praeparatio caerulei Prussiaci ex Germania missa ad Johannem Woodward’, Philosophical Transactions, XXXIII, no. 381, January–February 1724, pp. 15–17. See also the following paper in the same issue, commenting on the preparation: J. Brown, ‘Observations and Experiments upon the Foregoing preparation’, pp. 17–24.
Shaw 1734: P. Shaw, Chemical Lectures publickly read at London in the years 1731 and 1732 ... for the improvement of Arts, Trades and Natural Philosophy, London 1734, pp. 180–2.
Cröker 1736: J.M. Cröker, Der wohl anführende Mahler, Jena 1736, ed. U. Schiessl, Mittenwald 1982, pp. 535–6.
Dossie 1758: [R. Dossie], The Handmaid to the Arts, 2 vols, London 1758; Vol. 1. pp. 77–83, Vol. 2, pp. 387–91.
Hellot 1762: [J. Hellot], ‘Sur la préparation du Bleu de Prusse’, Histoire de l’Académie Royale des Sciences, avec les Mémoires de mathématique & de physique pour la même année, Année MDCCLVI, 1762, pp. 53–9.
Le Pileur d’Apligny 1779: M. Le Pileur d’Apligny, Traité des couleurs matérielles, Paris 1779, pp. 36–8.
Secrets 1790–1: Secrets concernant les arts et métiers, new enl. edn, 4 vols, Paris 1790–1; Vol. 2, 1790, pp. 322–43.
Hochheimer 1792, 1794: C.F.A. Hochheimer, Chemische Farben-Lehre, oder ausführliche Unterricht von Bereitung der Farben zu allen Arten der Malerey, 2 parts in 1 volume, Leipzig 1792, 1794, pp. 133–72, 207–22.
Weber 1793: J.A. Weber, Chemische Erfahrungen bey meinem und anderen Fabriken in Deutschland, Neuwied 1793, pp. 142–93.
Massoul 1797: C. de Massoul, A Treatise on the Art of Painting and the Composition of Colours, London 1797, pp. 181–3. Trans. from French, but no French edn known.

Notes
Alum: aluminium potassium sulphate, AlK(SO4)2·12H2O
Green vitriol: iron(II) sulphate, FeSO4·7H2O
Tartar: potassium hydrogen tartrate, the mono potassium salt of (+)-2,3- dihydroxybutanedioic acid. Tartar alkali is potassium carbonate
Nitre, saltpetre: potassium nitrate, KNO3
Pearl ashes, ashes, cendres gravelées: potassium carbonate, K2CO3
Potash, potasse: unclear whether this is potassium hydroxide, KOH, or potassium carbonate
Quick lime: calcium oxide, CaO
The acid used was usually hydrochloric acid, HCl, spirits of salt.

TABLE 3 Examples of occurrences of Prussian blue in National Gallery paintings

British School (note 1)
Artist	Painting	Date	Occurrence of pigment	Elements detected	Notes	
Thomas
Gainsborough	Gainsborough’s Forest (‘Cornard Wood’), NG 925 
Portrait of the Artist with his Wife and Daughter, NG 6547 
Mr and Mrs Andrews, NG 6301 
John Plampin, NG 5984 
The Painter’s Daughters chasing a Butterfly, NG 1811 
Dr Ralph Schomberg, NG 684 (note 2)
The Watering Place, NG 109
Mrs Siddons, NG 683
Mr and Mrs William Hallett (‘The Morning Walk’), NG 6209	c.1748
c.1748
c.1748–9
c.1755
probably c.1756
c.1770
c.1774–7
c.1783–5
c.1785	sky (trace)
wife’s dress; sky (trace)
dress; sky (trace)
sky (with other pigments)
skirt of left-hand girl
sky
sky
dress
dress (with lead white, black)	–
Fe, Al
–
Al, Fe, K, (Ca) (Si)
–
–
––
Si, Fe, K,
(Mg)	Pale, flake-like particles; faded
Flake-like particles
Transparent, flake-like particles; possibly faded
Flake-like particles; faded
Some large, greenish-blue flakes; faded
Calcite (CaCO3) present in Prussian blue-containing paint, but unclear if as extender for blue pigment or deliberate addition to paint 
Flake-like particles; faded
Some flake-like particles
–	
William Hogarth	The Graham Children, NG 4756
Marriage à la Mode, NG 113–118 (note 3)	1742
before 1743	older girl’s dress (with ultramarine)
various contexts	–
–	–
–	
Sir Joshua Reynolds	Anne, Countess of Albemarle, NG 1259
Colonel Banastre Tarleton, NG 5985	probably 1759–60
1782	dress
coat	Fe
–	Some flake-like particles
–	
George Stubbs	The Millbanke and Melbourne Families, NG 6429	c.1769	clothing; sky; foliage	–	–	

Dutch School
Artist	Painting	Date	Occurrence of pigment	Elements detected	Notes	
Jan van Huysum	Flowers in a Terracotta Vase, NG 796 (note 4)	1736	various	–	–	
Jan van Os	Fruit and Flowers in a Terracotta Vase, NG 6520 (note 5)	1777–8	various	–	–	

French School
Artist	Painting	Date	Occurrence of pigment	Elements detected 	Notes	
Nicolas-Toussaint Charlet	Children at a Church Door, NG 4140	c.1817–45	sky	–	Starch extender (also present in black paint)	
Imitator of Thomas Couture	Caught by the Tide, NG 4613	1860–90	dress	Fe, Si, (Al)	Amorphous, stain-like appearance; quartz extender	
Jacques-Louis David	Portrait of Jacobus Blauw, NG 6495
Portrait of the Vicomtesse Vilain XIIII and her Daughter, NG 6545	1795
1816	coat
dress	–
–	–
–	
Hilaire-Germain- Edgar Degas	Princess Pauline de Metternich, NG 3337
Hélène Rouart in her Father’s Study, NG 6469	c.1865
c.1886	underlayers
dress	Fe
–	Finely divided
–	
Ferdinand-Victor Eugène Delacroix	Ovid among the Scythians, NG 6262	1859	sky	Fe, Al, K	Finely divided	
Jean-François Detroy	Time unveiling Truth, NG 6454
Jason swearing Eternal Affection to Medea, NG 6330
The Capture of the Golden Fleece, NG 6512	1733
1742–3
1742–3	sky; drapery
drapery
sky; drapery	–
–
–	Some flake-like particles
–
–	
François-Hubert Drouais	Le Comte de Vaudreuil, NG 4253	1758	coat	Al, Si, Ca, Fe, K	A deep, stain-like blue as very little white present	
French School	Portrait of a Boy, NG 4034	before 1810	jacket	Fe, Al, K, Ca	Starch extender	
French School	Portrait of a Lady, NG 2218	19th century	background	Si, Al, Fe, K	Silica-containing extender (kaolin type?)	
Jean-Louis-André- Théodore Géricault	A Horse frightened by Lightning, NG 4927	c.1813–14	greenish horizon	Al, Fe, (K), (Si), (Ca)	Large flake-like particles	
Jean-Baptiste Greuze	A Child with an Apple, NG 1020	late 18th century	cushion	–	–	
Nicolas Lancret	The Four Times of Day: Afternoon, NG 5869	1739–41	sky	Al, Fe, K	Some large, translucent flake-like particles	
Edouard Manet	Music in the Tuileries Gardens, NG 3260 (note 6)	1862	sky underpaint	–	Very finely divided	
Claude-Oscar Monet	Bathers at La Grenouillère, NG 6456 (note 7)	1869	sky; clothing	–	–	
Adolphe Monticelli	A Vase of Wild Flowers, NG 5015
Still Life: Oysters, Fish, NG 5013
Still Life: Fruit, NG 5014	Probably 1870–80
c.1878–82
c.1878–82	various
various
various	Fe, K
Fe
Ca, S, Fe, K, Al, Si	––
Calcium sulphate extender	
Adolphe Monticelli (after?)	Torchlight Procession, NG 5009	probably 1870–86	various	Ca, Fe, K (Si)	Calcite (CaCO3) extender; notably bright colour	
Jean-Baptiste Perronneau	A Girl with a Kitten, NG 3588	1745	background; dress	–	Pastel on paper; pigment mixed with varying amounts of chalk to give different shades of blue	

Italian School
Artist	Painting	Date	Occurrence of pigment	Elements detected	Notes	
Attributed to
Bernardo Bellotto	Venice: Upper Reaches of the Grand Canal facing Santa Croce, NG 2514	after 1768	sky	–	Some fading	
Canaletto	Venice: Campo S. Vidal and Santa Maria della Carità (‘The Stonemason’s Yard’), NG 127 (note 8)
Venice: The Feast Day of Saint Roch, NG 937 (note 9)
Venice: A Regatta on the Grand Canal, NG 938
Venice: The Upper Reaches of the Grand Canal with S. Simeone Piccolo, NG 163 (note 10)
Venice: The Basin of San Marco on Ascension Day, NG 4453
A Regatta on the Grand Canal, NG 4454
Eton College, NG 942	1726–30
c.1735
c.1735
c.1738
c.1740
c.1740
c.1754	sky
sky; draperies
sky; water; draperies
sky; water
sky; water
sky; water
sky; water	–
Ca, Fe, Al
–
–
–
–
–	Flake-like particles
Flake-like particles (possibly calcite or chalk present)
–
Flake-like particles; some fading
–
Flake-like particles
–	
Studio of Canaletto	Venice: Palazzo Grimani, NG 941	c.1756–68	sky	–	Faded	
Corrado Giaquinto	Apotheosis of the Spanish(?) Monarchy(?), NG 6229	c.1751	sky	Fe, (Al)	–	
Francesco Guardi	Venice: The Punta della Dogana, NG 6156
Venice: The Giudecca with the Zitelle, NG 6157	1780s
1780s	sky
sky	–
–	–
–	
Italian School	Portrait of a Woman, NG 2217	19th century	sky	Al, Fe, K	–	
Giovanni Battista Tiepolo	Saints Maximus and Oswald(?), NG 1192
An Allegory with Venus and Time, NG 6387 (note 11)	c.1745
c.1754–8
	sky
sky; drapery	–
–	–
Flake-like particles	

Spanish School
Artist	Painting	Date	Occurrence of pigment	Elements detected	Notes	
Francisco de Goya	A Picnic, NG 1471
Doña Isabel de Porcel, NG 1473 (note 12)
The Duke of Wellington, NG 6322	1785–90
before 1805
1812–14	sky; foliage
in clothing of overpainted male portrait beneath
in sitter’s decorations	Fe, Al
–
–	With ultramarine
Painted over a portrait of a man in uniform
–	

Prussian blue in retouchings and overpaint
Artist	Painting	Date	Occurrence of pigment	Elements detected	Notes	
Follower of Botticelli	Saint Francis, NG 598	1490–1500;
retouching,
probably
19th century	angel’s robe	–	Starch extender	
Master of the
Mornauer Portrait	Portrait of Alexander Mornauer,
NG 6532 (note 13)	c.1464–88;
18th-century
overpaint	blue overpaint over panelled
background	Fe, Al	Flake-like particles	
Pietro Perugino	The Virgin and Child with an Angel,
NG 288.1	c.1496–1500;
later
retouchings	extensive use on false
additions	–	–	

Notes
Note: Usually the Prussian blue has been mixed with variable amounts of lead white (thus lead is always detected, but is omitted here); other pigments may also be present. Where elemental analysis was not carried out the pigment was identified microchemically (it reacts rapidly with alkali, giving a brown product), or by FTIR. Prussian blue is also often found in mixtures to make greens.
1 J. Egerton, National Gallery Catalogues: The British School, London 1998.
2 D. Bomford, A. Roy and D. Saunders, ‘Gainsborough’s “Dr Ralph Schomberg”’, National Gallery Technical Bulletin, 12, 1988, pp. 44–57, esp. pp. 49–50.
3 D. Bomford and A. Roy, ‘Hogarth’s “Marriage à la Mode”’, National Gallery Technical Bulletin, 6, 1982, pp. 44–67, esp. pp. 62–4.
4 J. Kirby and D. Saunders, ‘Sixteenth- to eighteenth-century green colours in landscape and flower paintings: composition and deterioration’, Painting Techniques: History, Materials and Studio Practice; Contributions to the Dublin IIC Congress, 7–11 September 1998, eds A. Roy and P. Smith, London 1998, pp. 155–9, esp. pp. 155–6.
5 Kirby and Saunders 1998 (cited in note 4).
6 D. Bomford, J. Kirby, J. Leighton and A. Roy, Art in the Making: Impressionism, exh cat, London 1990, pp. 112–19, 200.
7 London 1990 (cited in note 6), pp. 120–5, 200.
8 D. Bomford and A. Roy, ‘Canaletto’s “Stonemason’s Yard” and “San Simeone Piccolo”’, National Gallery Technical Bulletin, 14, 1993, pp. 34–41, esp. pp. 38–9.
9 D. Bomford and A. Roy, ‘Canaletto’s “Venice: The Feastday of S. Roch”’, National Gallery Technical Bulletin, 6, 1982, pp. 40–3. 
10 Bomford and Roy 1993 (cited in note 8).
11 J. Kirby, ‘Fading and Colour Change of Prussian Blue: Occurrences and Early Reports’, National Gallery Technical Bulletin, 14, 1993, pp. 62–71, esp. pp. 62, 64.
12 M. Wyld, ‘Goya’s Re-use of a Canvas for “Doña Isabel”’, National Gallery Technical Bulletin, 5, 1981, pp. 38–43.
13 S. Foister, ‘The Portrait of Alexander Mornauer’, The Burlington Magazine, CXXXIII, September 1991, pp. 613–18.

Series editor Ashok Roy

© National Gallery Company Limited 2005

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First published in Great Britain in 2005 by National Gallery Company Limited St Vincent House, 30 Orange Street
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525046
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