Below you can find details for Barytes, Blanc fixe, Calcium carbonate, Silica, Diatomaceous earth, Glass, Gypsum, Lead white, Starch, Titanium white, Witherite and Zinc white.
Fluorite, Smalt, Lead tin yellow, Naples yellow and Turner&#;s yellow often appear to be colourless when seen in transmitted light though they are not white pigments.
Barytes
BaSO4 Orthorhombic. Refractive index 1.638-1.648
Ground barite, or barytes as it is generally known in the art world, can be difficult to see when mounted in Meltmount with a refractive index of 1.662 as the relief is so low. When viewed between crossed polars, the birefringence is low too, and particles generally appear as a first order grey. The artificially prepared form known as blanc fixe has smaller particles which adds to the difficulty of finding them. However, if it is realised that there is something there, the very problem of seeing the particles tends to provide an indication of their identification. Particles of blanc fixe seen by the authors have mostly been elongated, and it has been just possible to determine that some of the larger particles were length fast. If a dispersion staining objective is available and particles of barytes are immersed in a Cargille refractive index fluid n = 1.62, the particles will appear with light blue edges if an annular stop is used and brown edges with a central one. Celestine, which has a rather similar refractive index and birefringence and may therefore be confused with barytes, has yellow and green edges with an annular stop (blue and magenta with a central stop).
Barytes is prepared from the natural barite, which is found widely in Europe, America and Canada. In the U.K. there are extensive deposits in Shropshire and Cumbria. Barite is nearly always found associated with calcium sulphate or calcium carbonate. The mineral is mined, sorted, crushed and, after several grindings, levigated. Barytes usually contains some iron, and Hurst [] describes how this was &#;bleached&#; with hydrochloric acid, but Zerr and Rübencamp [], say that artificial ultramarine was added to correct any yellow cast, and this may represent continental practice.
From the eighteenth century onwards, barytes was frequently added to lead white; Andrew Ure [] reported that &#;Venice white&#; was equal parts of white lead and barytes, &#;Hamburg white&#; was one part of lead white to two parts of barytes, and that &#;Dutch white&#; was one part of lead white to three parts of barytes. Although this sounds like wholesale adulteration, it has been found that the addition of a small amount of barytes improves the weathering quality of the paint.
During the last decade of the eighteenth century and for much of the nineteenth, barytes was used in watercolour under the name of &#;constant white&#;, and Field regarded it as superior to zinc white. It probably lost its position because it had the unfortunate character of being noticeably less opaque when wet than when dry, which made it difficult for amateurs to judge the tonal effect that would be achieved. This was particularly relevant when it was used in mixtures.
In oil or varnish barytes has very little covering power, and its oil absorption is low. This made it valuable as an extender of pigments which have a high oil absorption as well as those which cover well and have a very high tinting strength. It gives body to pigments made from aniline dyes. Zerr and Rübenkamp, when discussing the advantage of adding barytes to strongly staining pigments, cite the chromes as an example, &#;&#;with their specially high covering power, and their mixed products, the chrome greens could not be used as painter&#;s colours without an addition of barytes, which facilitates distribution, brightens the shade, and favourably modifies the general workability of the paint&#;. (N.B. the authors have found examples of chrome green which did not include barytes.) From the paint manufacturer&#;s point of view, barytes has a further value in that its fine hard particles help in grinding the ordinary mineral colours. Barytes is very inert, being unaffected by acids, alkalis and hydrogen sulphide, it also has no effect on other colours.
Blanc fixe
Like barytes the artificially prepared form known as blanc fixe has a low birefringence and a very low relief, it also has much smaller particles which adds to the difficulty of finding them. However, if it is realised that there is something there, the very problem of seeing the particles tends to provide an indication of their identification. Particles of blanc fixe seen by the authors have mostly been elongated, and it has been just possible to determine that some of the larger particles were length fast. Blanc fixe is mainly used as a base for lakes, and then aggregates of the particles are easily seen because they are coloured. When viewing such particles with a microscope, it is important to distinguish between the visual contrast resulting from the colour, and relief. If there is some doubt about this, illuminate the preparation with light of approximately the same colour as the particles by introducing a coloured filter. This will reduce the colour contrast and make it easier to appreciate the relief.
Blanc fixe is prepared by adding a solution of a soluble sulphate, such as sodium sulphate, or sulphuric acid to a solution of a barium salt, it was also often prepared by treating the mineral witherite (barium carbonate) with hydrochloric acid to produce barium chloride and then treating the chloride with a soluble sulphate such as magnesium sulphate.
Chemically blanc fixe is identical to the mineral but is much more finely divided. Apart from its use as a base for lakes, it is used in paper finishing, and in cellulose lacquers. Blanc fixe is very inert, being unaffected by acids, alkalis and hydrogen sulphide, it also has no effect on other colours.
Calcium carbonate whites
Limestone, Chalk, Limewash, Eggshell, Oyster shell white
Calcite is calcium carbonate, CaCO3, trigonal, ne 1.486, no1.658
Dolomite is calcium magnesium carbonate, but part of the magnesium can be replaced by iron or manganese, CaCO3.(Mg,Fe,Mn)CO3, trigonal. Typically ne 1.501, no 1.679. However the birefringence and refractive index increase with an increase in the proportion of iron and manganese.
Aragonite calcium carbonate CaCO3, orthorhombic, n 1.530 &#; 1.675
Both calcite and dolomite were used to form limewash and mortars, and are likely to appear in paint in the form of limestone, but calcite can also appear in the form of chalk, or eggshell, and will also be present, with aragonite in oyster-shell white.
Limestone particles show low relief, and twinkle in Meltmount if viewed in plane polarised light when the stage is rotated. They also show good cleavage and one is likely to see a number, whose shape is based on a rhomb, that extinguish symmetrically. With most achromatic objectives, the particles are likely to appear pinkish, or have pinkish edges: with apochromats, and some modern achromats the particles will appear colourless. The birefringence, as with most carbonates, is high.
Limestone may be composed of pure calcite, but it frequently contains some dolomite. Coccoliths (see chalk) are rare in limestone, but very occasional examples can occur. Ground dolomitic limestone is very similar to pure calcitic limestone. Dolomite produces similar rhombs which twinkle, when the stage is rotated and the particles are mounted in Meltmount, in just the same way that calcite particles do. However, because dolomite has a slightly greater omega refractive index than Meltmount, it will be seen that particles of dolomite and calcite can be distinguished by checking, in plane polarised light, whether the refractive index at the extinction position that corresponds to the position of low relief is higher or lower than Meltmount. In both dolomite and calcite the rhombs will show the omega index and their lowest relief, when the long axis of the rhomb is parallel to the polariser. Twinning, which is much less common in dolomite than in calcite, can occur parallel to the edges of the rhomb or to the long axis in both materials. However, twinning parallel to the short axis is only found in dolomite. Large calcitic and dolomitic particles can be distinguished in Meltmount most easily if they are viewed with a dispersion staining objective. If viewed with an aperture (annular) stop, calcite will show a yellow edge and dolomite a blue one.
NB. Witherite twinkles in Meltmount, as well as showing similar dispersion staining colours and birefringence to dolomite. However, it breaks with less regularity, and any pseudo-rhombe-shaped particles are likely to be unusual and are even less likely to show symmetrical extinction. Witherite does not seem to have been used much as an extender.
Chalk is always composed of pure calcite. When mounted in Meltmount and viewed in plane polarised light, particles of chalk have low relief, and all the particles, except the coccoliths, will twinkle when the stage is rotated. With most achromats the particles are likely to appear slightly pinkish, but when viewed with an apochromatic objective or some modern achromats, the particles will appear colourless. The birefringence is high. Most of the particles are of irregular shape, but a few rhombs may be present and a number of circular and/or elliptical coccoliths will almost certainly be found. In Meltmount, both types of coccoliths are easier to find if the slide is viewed between crossed polars. Circular coccoliths show a rather irregular dark cross on a bright circular background. Elliptical ones show greyish or dark curved bands that partly follow the outline of the coccolith.
Limewash
Under the microscope, limewash takes the form of very fine interlocked particles, which have a high birefringence. Most of the crystals are elongated and fast length. If calcium hydroxide is protected from the air, it may not be converted even after several centuries, and occasional rounded isotropic particles of the hydroxide may consequently be found.
Limewash was used extensively as a surface coating in buildings. It was prepared by first heating lumps of limestone strongly to form quicklime &#; calcium oxide. This was then slaked in water to form calcium hydroxide. This starts as a violent reaction, but the resulting creamy, strongly alkaline material, called lime putty, was, ideally, not used for several months. After being passed through a sieve to remove any lumps, the lime putty was let down with more water to the consistency of milk. If a coloured limewash was required, earth pigments were added at this stage. It was then brushed over the walls as a faintly coloured watery suspension, which was very transparent and had apparently little effect. Some of the water was absorbed by the walls quickly which allowed the hydroxide to remain in position. Even when the water has evaporated the &#;paint&#; still appears somewhat transparent, but when it has reacted with the carbon dioxide in the air to form calcite it becomes a hard wearing very opaque covering. One of the advantages of limewash is that it is porous to water vapour and will not trap water in damp walls. Where limewash was used externally it was frequently mixed with tallow to reduce its permeability to the liquid phase. Limewash hold well on plaster, cob and most stonework, but tends to flake off wood.
Eggshell
Shell white made from egg shells appears as thin, colourless, broken flakes with low relief and high birefringence. All the particles show a refractive index of less than 1.66, and most will twinkle in the sense that their edges will show less relief. Some flakes are likely to appear to be covered with small black dots which may mean that the particle as a whole remains visible throughout the rotation of the stage.
The term &#;shell white&#; in European literature probably referred to a white made from egg shells (a white briefly mentioned by Field). Dossie says that it &#;is used by some in water colours; and preferred to flake or the troy white&#;. He adds that it should be prepared from egg shells that have had their inner skins removed. The &#;sand&#; in some hour glasses has been found to consist of ground eggshell.
Oyster shell white contains a mixture of calcite and aragonite. Aragonite is biaxial and the refractive index ranges from 1.53 to 1.685. Under the microscope oyster shell white (called gofun in Japan) is characterised by colourless,thin, elongated flakes and particles of low relief and high birefringence. The flakes have a fibrous quality and most will not extinguish, but they are most easily seen between crossed polars. Extinction is mostly oblique. It is variable in angle, and is most clearly seen in the needle-like particles, which are length fast. An occasional needle may extinguish parallel.
In Japan, gofun is still produced by the traditional method of crushing and grinding weathered sea shells without calcining them: the shells are mostly from oysters. The white was used in Japan as early as and it has been suggested that gofun replaced clay as a white pigment some time during the sixteenth or seventeenth centuries. It has been widely used by Japanese painters and printmakers since the seventeenth century. The white is transparent in oil, but covers well in watercolour.
Dossie mentions a &#;pearl white&#; made from &#;the powder of pearls or the finer parts of oyster-shells&#; and he goes on to explain that &#;this white is used in miniature paintings; and agree[s] much better with vegetable colours than flake, white lead, or troy white&#;. Field makes the point that pearl white &#;should be employed with as little gum as possible as it destroys [the] body, opacity, or whiteness&#;. Pearl white was apparently prepared from oyster shells dried either in the sun or by gentle heat. The pigment was mentioned in The Art of Drawing in and Field comments that in watercolour &#;it combines &#; with all other colours,without injuring the most delicate, and is itself perfectly permanent and innoxious&#;. Field goes on to point out that a preparation of bismuth was sold as a cosmetic and was also referred to as pearl white &#; this however was liable to blacken if exposed to impure air.
Silica, Quartz, Tripoli, Fullers earth, Diatomaceous earth
Quartz. Silica dioxide SiO2 Trigonal: refractive index 1.55 to 1.54
Opal. Hydrated silica SiO2 . nH2O Amorphous: refractive index 1.406-1.434
Under the microscope particles of silica can be water white, but with most achromatic objectives they show a lilac or pinkish edge. Most particles have conchoidal fractures. Quartz is slightly birefringent and pigment-sized particles are likely to show only low first order colours between crossed polars. In general it looks like an anisotropic ground glass with slightly less obvious conchoidal fractures. Similar looking material that is isotropic may be ground glass, which was sometimes added to paint as a dryer, or plant opal in the form of phytoliths, which might remain if the area had been rubbed down with Dutch rush. Diatoms, which are also made of opal, are easily recognized from their distinct outlines and regularly dotted, or punctured surfaces.
Quartz is one of the most widely distributed materials in the world. In its most perfect form it is known as rock crystal, but it is more commonly found as sand and sandstone. It is naturally present in many samples of earth pigments, particularly ochres. During the 20th century the value of silica in paint manufacture began to be appreciated and its introduction was probably only delayed because of the difficulty of preparing it in a sufficiently fine form. Historically it was prepared by heating flint pebbles and quenching them in cold water. This made the pebbles friable, and they could then be ground to a fine powder and levigated.
Diatomaceous earth is composed of the skeletons &#; technically known as &#;frustules&#; &#; of microscopic aquatic organisms. Deposits occur in most parts of the world. One of the most extensive is in California where it is about 500 metres thick. When necessary diatomaceous earth is purified by chemical means followed by washing, calcining and milling.
From the point of view of the paint manufacturer, too much silica in natural earth pigments, such as ochres, makes the paint difficult to grind and causes excessive wear on equipment. This is one of the reasons why earth pigments are now hardly used in commercial paint. However, the sticky, or greasy quality which some pigments impart to paint, can be reduced by the addition of one or two percent of quartz, while the presence of quartz also improves the resistance of paint films to wear.
Extremely fine grades of silica flour can be used as a base for lakes. Such lakes are no more expensive than blanc fixe based ones and have the advantage of being more transparent [OCCA 1]. &#;Diatomite is used as a flatting agent and for improving intercoat adhesion in multiple coat paint systems. The &#;brush pile&#; structure formed by the diatomite particles is claimed to contribute reinforcement to a paint film and enhance its flexibility &#;&#; [OCCA 6]. The presence of diatomaceous earth in paint has also been found to reduce the tendency of some paints to settle.
Ground glass
Ground glass appears as broken colourless particles, which may show slightly violet edges with some achromatic objectives. It is isotropic, and the refractive index is usually about 1.5. The particles are likely to be bounded by well defined conchoidal fractures and the edges of these fractures may show slight edge depolarisation when the preparation is viewed between crossed polars.
The constituents of glass are of two types. The glass formers, or acidic oxides which can form a glass with or without other materials. The second type of constituent is a basic oxide, and these cannot form a glass alone, but can thin out the atomic network of the glass formers and produce a glass of lower melting point. The main glass former is silica, but glasses can be produced using germanium oxide, beryllium oxide and phosphorous pentoxide. The basic oxides are potash, soda, lime, and lead. Glass can be regarded as being a mixture of these materials, but of no definite composition, which has been melted at a high temperature and allowed to cool sufficiently rapidly to avoid crystallisation taking place. Crown glass, which is used for windows, and bottles, is made from a mixture of silica, potash, soda, and lime. In flint glasses, which have more sparkle because of their greater dispersion, the lime is replaced with lead oxide. Crown glasses tend to have a refractive index in the 1.5 to 1.55 range, and a specific gravity between c. 2.45 and c. 2.65; while flint glass can have a refractive index as low as c. 1.5 and a specific gravity of about 2.6, but some specialist flint glasses can have a refractive index as high as 1.75, and a specific gravity of 5. However, it seems most unlikely that these specialist glasses will be found in paint.
Glass was believed to act as a dryer, and may be present in paint for that reason. Ground glass was also used by varnish makers at the bottom of their crucibles to stop local overheating, and some of this might have got into paint if varnish was added to it.
Gypsum and other forms of calcium sulphate
Gypsum: the dihydrate of calcium sulphate CaSO4 . 2(H2O) monoclinic, n = 1.52-1.53
The hemi-hydrate CaSO4 . 1/2(H20) uniaxial no = 1.55 ne = 1.57
Anhydrite: the anhydrous sulphate CaSO4 orthorhombic, n = 1.570-1.614
When mounted in Meltmount, the particles may appear irregular in shape or have straight parallel sides and ends; some of the particles may take the form of rhombs. The particles will have low relief and are likely to appear slightly pink. Between crossed polars the extinction is oblique. If the slide has been heated sufficiently to drive off some of the water of crystallization, anhydrite or the hemi-hydrate is likely to have been produced as a pseudomorph of gypsum, and the particles will show moving dark spots and blotches when rotated between crossed polars and it is likely to be difficult to determine just where extinction occurs. This type of extinction is known to gemmologists as &#;tabby&#; extinction. However, if the ground mineral gypsum is mounted in Meltmount, and the mounting temperature has been kept below 100° C, or is viewed in a refractive index fluid, the particles will show a normal sharp, oblique extinction, and no tabby effects will be seen. The relief will be slightly higher than the heated gypsum. The grounds of many Italian paintings consist of a mixture of anhydrate and the hemi-hydrate, and material from such grounds are likely to include lath-shaped, fibrous particles with square or ragged ends; rhombs are likely to be absent. The birefringence is usually noticeably greater than that of gypsum, and the extinction is parallel, though the particles are likely to show tabby extinction if the slide has been heated sufficiently when the particles were mounted.
If gypsum that has had its water of crystallization driven off by heat, is allowed to stand in excess water for some hours, it is completely reconverted into the dihydrate in the form of needle-like crystals, which may be either separate or in clusters. If this is used to make gesso sottile, the result is indistinguishable from plaster of Paris. Gesso sottile has not been observed in the grounds of early Italian paintings, but under the name of gesso a oro is used by modern Italian gilders [Gettens and Mrose, Studies in Conservation vol 1 no 4].
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Raw gypsum that has been ground and &#;bolted&#; (i.e. sieved through a specially woven cloth), is sometimes sold as terra alba. Gypsum can also be reduced to a fine powder easily by crushing it after it has been heated. Which product is formed depends on the temperature to which the gypsum has been raised. If the temperature is carefully controlled to between 120° C and 130° C the hemi-hydrate, called plaster of Paris, is formed. Because of the controlled temperature that was required to make it, plaster of Paris was made in small quantities and was consequently relatively expensive. If the temperature is between 130° and 300° C the anhydrous sulphate is produced, but the crystal structure of gypsum is not destroyed, and it can take up water to form gypsum once again. This is builder&#;s plaster. If the temperature exceeds 400° C the whole of the gypsum is &#;dead burnt&#;. This is a variety of the anhydrous sulphate and similar to the mineral anhydrite. Between 300° and 400° C some anhydrite is produced.
Calcium sulphate in one form or another occurs frequently in the gesso grounds of paintings and gilded objects from the Mediterranean countries, notably Italy and Spain. It also occurs on some work from the south of France, but it can occur on work from other places, if Italian workmen, or traditions, have been employed. Gettens and Mrose found that mixtures of anhydrous calcium sulphate and gypsum were present in grounds of paintings of the Florentine, Sienese and Umbrian schools of the fourteenth and fifteenth centuries. However, the grounds of the Venetian school paintings that were examined, consisted almost entirely of ground raw gypsum. The use of plaster of Paris in grounds has not been recorded. It seems likely that the grounds found on applied art objects will follow the same pattern, but little work has been done in this field to date.
Raw gypsum was used to a limited extent as an extender in cheap paints. The attitude to the use of gypsum in paper finishing appears to have varied in different countries. Hurst mentions that &#;it is used extensively by paper stainers and makers of paper hangings, who prefer it to barytes&#;; but Zerr and Rübencamp say that the wallpaper printers avoid it because it would &#;adversely affect the use of the colours&#;. The latter add that a great number of artificial mineral colours as well as earth colours and lake pigments are cheapened by the addition of gypsum, in particular, the chrome yellows, ultramarine blues, bremen blue, copper-arsenic greens, some chrome greens, red and yellow ferric oxide colours, and even lamp black. The authors have not found most of these pigments extended with gypsum. According to Remington and Francis the use of gypsum as an extender has declined, they say that its principal use is in powder distemper colours in which dextrin, glue or casein was used as the binder. When used in water paint, its slight solubility is an advantage, a minute amount of calcium sulphate is deposited throughout the distemper film when the water dries. This assists in binding the film together and so to the wall. Because gypsum is slightly soluble in water, it can migrate into a paint layer from the wall or ground layer behind if the wall is damp. The dissolved gypsum is carried by the water until it reaches the surface and the water evaporates, leaving crystals of gypsum behind.
Lead white
Basic lead carbonate, Pb3(OH)2(CO3 Trigonal. Refractive index 1.94 &#; 2.09
In plane polarised light, white lead appears as irregularly shaped particles which show good relief and vary in size from c. 0.5 to c. 10 micrometres. If an apochromatic lens is being used, these will appear almost colourless, but will be noticeably green if they are viewed with most achromatic objectives; however with a few of the latest achromats the particles may appear yellow. Lead white is highly birefringent and between crossed polars even very small particles show a good white, and larger ones can show first order yellow or red, but a few particles may show second order colours. In a number of samples hexagonal basal flakes can be seen, though in most instances only one or two of the angles will be visible, and as the light from the substage will be travelling parallel to the optic axis, these particles will remain dark between crossed polars. Lead white is a flaky material. In some samples as much as 30 percent of the irregularly shaped particles may also be lying so the light travels through them parallel to the optic axis and they consequently remain dark when viewed between crossed polars. Larger double-ended crystals (which may be normal lead carbonate) occur in some samples. Normal lead carbonate is orthorhombic n = 1.80-2.08. Lead white was often mixed with chalk to stiffen it and improve the impasto effects. During the twentieth century lead white was often mixed with barytes to improve its weathering properties when used out of doors.
There are numerous detailed accounts of the ways in which lead white has been made. The oldest process is known as the Dutch, or stack process, and it seems to be similar to the methods used in the classical and medieval periods. In essence lead strips were exposed to the fumes of vinegar in closed clay pots. The pots were buried in dung or spent tan bark which provided the necessary warmth. During the course of about three months (depending on the weather) the lead became converted into lead white. In the nineteenth century a number of attempts were made to reduce the time involved. The most important of these resulted in the &#;chamber&#; or German process. In this the strips of lead were hung in large brick chambers, instead of pots, and heated acetic acid and water vapours, together with carbon dioxide were introduced into the chamber in controlled amounts. This produced the pigment in about two thirds of the time required by the Dutch process. The third important method was the &#;French&#;, &#;Thenare&#;, or &#;Clichy&#; process which was first used in . In this a solution of normal lead acetate was boiled with litharge and the white lead precipitated by passing carbon dioxide through the resulting solution. Kremser weiss, which is variously rendered in England as &#;Kremnitz&#; or &#;Cremnitz&#; white, is a lead white which gets its name from being prepared by precipitation, a method first used in Krems in Austria. This was also a precipitation process: in it, trays containing a paste of litharge and lead acetate were placed in a chamber and exposed to carbon dioxide. The paste was raked over at intervals until it was converted to lead white. There are numerous variants of these processes, as well as several systems of preparing lead white by electrolytic methods, one of which was used on a large scale in America and involved the use of lead nitrate. In another a lead anode is suspended in an electrolyte containing sodium acetate which is immediately precipitated as basic lead carbonate.
Almost all writers say that the best lead white is produced by the Dutch method and the extent to which it was still used at the end of the nineteenth century can be gauged by Hurst&#;s statement that of the 46 plants producing lead white in Great Britain, 42 used the Dutch method, three used chamber methods, and one prepared the pigment by precipitation. The electrolytic methods also gained a good reputation. The French method was initially highly regarded, but it was found difficult to control.
It seems likely that &#;ceruse&#; was a mixture of lead white and chalk, although seventeenth century English writers seem to have believed that it was a special form of lead white. Tingry explains that ceruse, which only cost one third of the price charged for lead white, was prepared like lead white but that thicker pieces were used and the pigment was scraped of as it formed; finally it was ground with pipe clay, or sometimes with chalk which, he said, does not produce such a good pigment.
By the &#;s it was found that, because lead white particles exhibit an affinity for oil, the water in lead white paste could be replaced with oil without having to dry the pigment. The process is known as &#;flushing&#; and its use meant that the preparation of lead white paint could be a dust free process and therefore much safer for the operatives.
Lead white has a low oil absorption, and is an excellent dryer. It can be mixed with most colours without affecting them, but is darkened by hydrogen sulphide. When it is used as an external protective coating it &#;chalks&#; on weathering which means that the surface can be repainted with minimum of cleaning down. However, the dust, including that from weathering, is extremely dangerous, and the lead can build up in the human body over long periods.
In a water medium, lead white, when used alone, tends to form aggregates and brushes out poorly. The addition of a small quantity of chalk improves its handling properties, which may be why Boutet recommended that miniaturists should use ceruse instead of lead white. In oil, lead white handles well, having good flow and brushability, but Laurie, in discussing Rembrandt&#;s impasto technique points out that &#;the stiffest white lead in oil still flows, but if a little whitening is worked in with the palette knife, we get white lead of the Rembrandt quality&#;. Although lead white forms an opaque paint initially, it is frequently seen that it becomes more transparent with time. This is probably partly because of the rise in the refractive index of the oil, but it has been suggested that it is also connected with the formation of lead salts due to the reaction between the free fatty acids of the oil and the pigment.
Paint made from different batches of lead white can vary in colour and opacity, and in commercial paints small amounts of a blue such as ultramarine are sometimes added to counteract the yellowing of the oil. It has been found that the presence of large crystals in lead white lowers the opacity, these are seldom found in stack process lead white, but tend to occur when the pigment has been made by the chamber or precipitation processes. In badly prepared lead white a considerable proportion of the normal lead acetate can be present, and unless this is removed by washing, it results in an abnormally hard paint film.
Lead white is still available to artists, though its use on buildings is now restricted in most countries.
quenching them in cold water. This made the pebbles friable, and they could then be ground to a fine powder and levigated.
Starch
Refractive index c. 1.53
The two starches that are most likely to be found by conservators are wheat starch and rice starch. In plane polarised light both are transparent and colourless. Rice starch appears as small polyhedral grains which may vary in size between three and twelve micrometres in diameter. Between crossed polars the grains are likely to appear grey with a well-defined black cross. Wheat starch is larger and rounded. They may be circular, ellipsoid, or near kidney shape in outline. Between crossed polars, the grains appear with a faint black cross. Grains on edge show a cross shaped like two &#;Y&#;s joined together at the tails and this feature is peculiar to wheat starch. If the starch had been heated sufficiently in water, no crosses will be seen between crossed polars. [For details of other starches, see McCrone The Particle Atlas Vol II]
Wheat starch was used to make bookbinders paste and rice starch was also used to stick paper. Wheat starch was also used as a base for lakes, and has been found used as an extender in a watercolour tablet.
Titanium white
Titanium dioxide. Usually rutile today, but early examples are likely to be anatase. Normally sold mixed with other materials.
Anatase e2.49, w 2.56, birefringence 0.073.
Rutile e = 2.6, w 2.9, birefringence 0.3.
In transmitted plane polarised light, the pigment can be seen to be composed of 0.5 &#; 1.0 micrometre particles, which appear yellow due to the scattering of blue light &#; the yellowish colour is not affected by whether an achromatic or apochromatic lens is used. The relief in Meltmount is high. Between crossed polars, thin masses and individual particles of rutile show a first order grey or white, and it will be appreciated that with such small particles, the birefringence is very high. In a similar situation anatase is perceptively greyer. In incident light the pigment appears white.
According to Gettens and Stout, the technical problems of preparing the pigment meant that it was not produced commercially until about -. Most of the pigment is now produced from ilmenite, an ore of titanium and iron. This is digested with sulphuric acid, and the coagulated mass dissolved in water and heated to boiling point to precipitate the titanium in the form of metatitanic acid.The precipitate is neutralized with barium carbonate and then calcined.
Titanium white paints are seldom made from pure titanium dioxide, and the pigment is usually sold mixed with barium carbonate, barium sulphate, or zinc oxide. In such mixtures the titanium dioxide may form as little as 25% of the total. Much of the coloured paint used on buildings today is a titanium white paint mixed with a &#;stainer&#;. Because the white pigment appears yellow in transmitted light, a yellow &#;stained&#; titanium paint may not be recognised as yellow.
The pigment is extremely stable, it is unaffected by heat, dilute acids or alkalies, light or hydrogen sulphide. It has excellent opacity, and its main defects as a pigment are that it dries slowly in oil and forms a soft film. This last can be corrected by an admixture of zinc oxide. It was suggested as an artists pigment soon after it became available and has gradually become the most important of artists&#; white pigments. It chalks rapidly on exposure to weather when used in oil, but survives much better in coatings made with synthetic resins.
A titanated barytes was first produced between and . Anatase was used in paint from . Rutile appears in . [David A. Crown The Forensic Examination of Paints & Pigments] The point of the change seems to have been to reduce chalking and to obtain greater opacity [Mattiello Edit. Protective and Decorative Coatings Vol II p 34]. However. even in the 21st century anatase was still being specified in certain industrial applications. Winsor and Newton didn&#;t list the pigment in , but it appears in their catalogue.
Witherite
Barium carbonate BaCO3, Orthorhombic
Refractive index 1.53 &#; 1.677
In plane polarised light, ground witherite twinkles in Meltmount. Witherite tends to form flat tablets due to perfect cleavage in one direction, but in other directions the cleavage is less perfect and the particles tend to be more irregular in outline than calcite or dolomite. It also lacks both the lamellar twinning and the rhombohedral form which is likely to be found in both dolomite and calcite. Large particles are likely to show an orange edge if viewed in Meltmount with a central stop dispersion staining objective. Pseudo-rhombs are likely to be rare and are most unlikely to appear to have symmetrical extinction: this lack of particles that extinguish symmetrically probably forms the best method of separating it from calcite and dolomite.
Witherite occurs in large quantities in Britain. It is softer than barytes and consequently can be prepared more cheaply, but it has a higher oil absorption and, perhaps for that reason, has chiefly been used in the preparation of water paints [Heaton]. Artificial barium carbonate was sometimes sold under the name of blanc fixe and it is unfortunate that this is visually indistinguishable from barium sulphate, because unlike the sulphate, which is insoluble in water and completely harmless, the carbonate is relatively soluble and as a result is poisonous. It seems likely that the use of barium carbonate was mainly restricted to the nineteenth century, though it may have been used in Great Britain between the wars. However, Gettens and Stout note that barium carbonate was sometimes mixed with titanium white.
Zinc white
Zinc oxide, ZnO Uniaxial: refractive index 2.00 &#; 2.02
The European version of the pigment is usually composed of particles so small that their shape can seldom be seen clearly with the optical microscope. In transmitted light small groups of particles appear green with most achromatic objectives, but larger groups appear yellow even when an apochromatic objective is used. Between crossed polars, clumps appear pure first-order white with soft cloud-like edges. An &#;acicular&#; form of the pigment was described as &#;recently produced&#; in the first edition of an Introduction to Paint Technology published in . In this type the crystals are elongated and much larger than the normal type of zinc oxide, and the rod shaped crystals are often combined into radiating groups of three crystals. The extinction is parallel and the crystals are length slow. Mattiello says that zinc white produced by the American process contains a wide variety of crystal sizes and shapes including acicular ones. This is reputed to provide a better weathering resistance than the finer particles produced by the French process where the particles are often confined to the 0.2 &#; 0.3 micrometre range. This probably explains the claim by McCrone that when viewing zinc white at high magnification one can see particles that take the shape of &#;C&#;s, &#;Y&#;s or &#;T&#;s. The authors have not observed this feature in European zinc white, but neither have the authors had the opportunity of examining samples of American made pigment.
In the French process, the pigment is made by burning either metallic zinc in an oxidising atmosphere, or zinc ores in a reducing one. The acicular form is produced by controlling the conditions of oxidation. The pigment fluoresces yellow under long wave ultra-violet light.
In or 82, Courtois of Dijon was the first person to manufacture zinc white on a commercial scale. It may have been produced on a small scale in England before the end of the century as patents were taken out in the name of John Atkinson, a colour maker, in and . By Sorel, who had experimented with galvanizing metals, had erected a furnace at Grenelle in France for the manufacture of zinc oxide. By the middle of the century, the manufacture of the pigment was inextricably mixed up with the production of spelter. Until the end of the century practically the whole of the zinc oxide used in Britain was imported, but the -18 war provided a considerable incentive to British manufacturers to increase their production of the pigment. Zinc white had the advantage over lead white that it is not poisonous, and is not visibly affected by hydrogen sulphide (it is merely converted to zinc sulphide which is also white). Against this, it has less body than lead white, and does not dry well in oil. During the nineteenth century, the use of an oil which had been made siccative by being boiled with manganese dryers was recommended. Zinc white was more expensive than the traditional lead white. At the end of the eighteenth century Watin says that it is too expensive to be used by decorators, and the end of the nineteenth Hurst makes the point that &#;being a rather costly pigment [it] is very liable to adulteration with other white pigments, such as china clay, barytes, whiting, terra alba &c&#;. It seems to have been first accepted by artists as a watercolour. In Winsor & Newton introduced it under the name of &#;Chinese white&#;, and it gradually gained ground against the other white pigments used in this medium.
In spite of the fact that zinc white has a reputation for drying badly in oil when used as an artist&#;s colour, by the early twentieth century this difficulty had been overcome in the field of commercial paint manufacture. Improved mixing and grinding techniques allowed the amount of oil required to form a paste paint to be reduced from about 22% of the weight of the dry pigment at the end of the nineteenth century, to as little as 12% twenty years later. In addition it was found that zinc white combined readily with resins, and this, with its ability to form fine dispersions, made it particularly suitable for use in enamel paints.
Although zinc white in oil dries slowly, it eventually forms hard brittle films which are liable to crack and flake. In consequence it is better to use white lead in under painting, grounds and undercoats in decorative painting. The acicular form shows much better resistance to weathering than the finely divided form, however it does not appear to be in widespread use.
Mactaggart, P. & Mactaggart, A. (June ) &#;White Pigments&#; In: Pigment ID using Polarised Light Microscopy from: https://academicprojects.co.uk/white-pigments/
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