PAINT DRIERS DISCUSSED IN 19TH-CENTURY BRITISH OIL PAINTING MANUALS
2 DRIERS IN COMMERCIALLY PREPARED MATERIALS
Aside from the driers that artists may have added themselves just prior to painting, which will be discussed below, it is important to consider the various means by which driers would already have been present in oil painting materials when they were purchased.
2.1 RAW OIL CLARIFICATION
Driers were sometimes called for in the preparation of raw oil. Although the mucilage or water-soluble component in the oil could be separated simply by standing, several authors described methods that made use of driers to assist in this step. Lead oxide, or litharge, was reported to be most efficient for causing the mucilage to precipitate out of solution. For example, an author from the late 18th century recommended adding 1 part litharge to 2 parts oil and shaking them together over several days. This would cause “a tallow-like grease” to be thrown down with the litharge (Williams 1787, 48–49). One of the leading 19th-century authors, the colormaker George Field, recommended the same method but called for 1 part litharge to 8 parts oil (Field 1835, 205). Another recipe for preparing oil, entitled “Liebig's,” was found in one of Roberson's recipe books and involved combinations of driers such as lead acetate and litharge with water (Roberson Archive, ca. 1850s, HKI MS 788–1993).
The preparation method that used sulfuric acid to remove mucilage and clarify the oil was also said to make it more drying, though most sources referring to this means warned that the resulting oil was inferior. One author reported, “The use of colours ground with acidified oils is one of the principal reasons why pictures crack.” He cautioned, “We should always be careful, before grinding the colours, to assure ourselves by means of litmus paper that the oil is not sour; which often happens after submitting it to operations said to be for purifying it and rendering it siccative” (Vibert 1892, 74).
2.2 DRYING OIL
Painters' oil was treated with driers in order to speed its drying time (see recipes below). Such a “drying oil” was used in painting in a number of ways: by itself, it could be added on the palette to specific slow-drying colors or glazes just prior to painting, or it could be applied as a separate oiling-out layer in preparation for further painting. Drying oil was also present in the painters' ready-prepared materials, as it was added to certain bladder or tube colors. For example, the medium for a vermilion oil paint found in one of Roberson's recipe books included 1 part drying oil to 2 parts linseed oil and 1 part mastic varnish (Roberson Archive ca. 1866 HKI MS 788–1993). Drying oil was also an important ingredient in artists' media such as megilp, which was sold throughout the century (Carlyle 1991, 1:141–48; Townsend et al. 1998).
2.2.1 Drying Oil Recipes
A source from 1758 describes the preparation of drying oil (mainly intended for “coarser work”) as involving boiling or simmering the oil in conjunction with a variety of driers used at once, including litharge, white lead, red lead, lead acetate, and sulfate of zinc (Dossie 1758, 148–49). Reports on the color of these early drying oils indicate that they could be very dark: “a good drying oil is not to be had of them [the colormen], what they furnish us with being so highly colored as to be improper for the purposes of fine painting, and only can be admitted in the darkest shades or back grounds” (Williams 1787, 25). By contrast, recipes provided in the artists' oil painting instruction books were largely directed toward the preparation of a drying oil that would be as colorless as possible; these tended to include only one type of drier.
One of the preferred methods was to use metallic lead, either by agitating the oil with lead shot or by “grinding” the oil with a leaden pestle in a lead-lined mortar. Prolonged storage in a leaden vessel was also recommended. Litharge (and very occasionally white lead) was sometimes substituted for metallic lead and used in a similar manner. For example, Rembrandt Peale's method was to add 2 tablespoons of litharge to and 8-ounce phial which was then filled with linseed oil. It was to be kept exposed to the sun or near a fire for a few days and shanken frequently (Sully 1873, 33).
Litharge was available in two varieties: silver and gold. Silver, or yellow litharge, was heated higher and was reported to be a harder material than the gold, or red litharge. Field (1841, 108) recommended the silver over the gold because it was more highly oxidized and would therefore make a more effective drier. Sources warned that litharge was often contaminated with iron or copper oxides and noted that it contained significant amounts of silica as an impurity. Although it appeared frequently in drying oil recipes, litharge was not recommended to artists for use by itself (to be added directly to the colors on the palette), and it was not listed as a separate article in the colormen's catalogs.
The earliest recipes for drying oils in the sources consulted contained the highest proportion of driers to oil. In later publications, the ratio of 1 part drier to 8 parts oil or 1 part drier to 16 parts oil became more common. The preparation of drying oil was dangerous because of the risk of fire and required skilled workmen to achieve a consistently high-quality product. Judging from orders for both oil and driers, it appears that Roberson's prepared its own drying oil between 1830 to 1853. Sometime after, the firm consistently purchased drying oils ready-made.
2.2.2 Japanner's Gold Size
The strongest drying oils also tended to be the most highly colored since they were prepared by extended boiling with driers. The strongest and darkest of them all was Japanner's gold size, which was sold by the colormen throughout the century. Because of its color, Japanner's gold size was usually recommended for mixing with dark colors. It was also used in the preparation of some media. For example, it was a component in a brown megilp (a variation of the popular gelled medium). However, despite its dark color, at least one source mentioned using it to assist the drying of lake colors that were used in glazing (Burnet 1861, 27).
Japanner's gold size was made by boiling oil with a number of driers in combination such as litharge, red lead, umber, zinc sulfate, and manganese. Field (1835, 206) noted that its preparation often involved the use of needless “if not pernicious” ingredients, and by the end of the century Winsor & Newton stated in its catalog that it contained “large quantities of turpentine and dryers” and “is not recommended where permanence is required” (Winsor & Newton 1900, 104).
2.3 LEAD HYDROXIDE AS A DRIER
In the later part of the 19th century, white-lead pigment was described as consisting of lead carbonate (PbCO3) and lead hydroxide (Pb H2O2). Depending on its manufacture, various ratios were reported. For example, analysis of a Krems white made by precipitation revealed 91.21 parts PbCO3 to 8.21 parts Pb H2O2, while an English Dutch-process white lead was reported to consist of 63.35 parts PbCO3 to 36.14 parts Pb H2O2 (Hurst 1892, 39).
Along with its reported qualities of adding body and helping to keep the pigment in suspension in the oil, the lead hydroxide was believed to increase the drying power of white-lead paint, as it “facilitates the absorption of oxygen by the oil, so that the paint dries up with the utmost rapidity” (Scott Taylor 1890, 39–40). In support of his view that lead hydroxide (hydrated oxide of lead or hydrate of lead) contributed to drying, Scott Taylor (1890, 40) noted, “We are often surprised to discover that White Lead was not esteemed for its drying power by the older painters; and we also find that the Venetians had a custom of grinding it repeatedly in Vinegar. Both of these facts seem to lead to the conclusion that there was little free hydrate of lead in the older varieties.”
Painters were warned that white-lead formulas containing as much as 30% lead hydroxide would have detrimental effects on their paint as the lead hydroxide would leave the white pigment in a “chemically active condition” and that “a chemically active white is not only bad for the oil, but bad for every color with which it is mixed” (Scott Taylor 1890, 40).
It was also believed that lead hydroxide would form lead soap more readily than lead carbonate, and the resulting soaps would, in turn, be more reactive to hydrogen sulfide from the air. It was felt that ultimately this white would darken in “foul air” more easily since “a larger proportion of the lead is seized upon by the sulfur and converted into black sulfide of lead” (Standage 1892, 25–27).
2.3.1 Lead Soaps
One modern account of the formation of lead soaps in an oil paint (Dunn 1973, 66) notes that they enhance the characteristics of the dried film:
White leads are chemically active pigments. They react with both the free acidic portions of vehicles and with the breakdown acids that develop from paint vehicles as paint films age. These reaction products … which are called lead soaps, reinforce the paint film. Fortunately these white lead reaction products are formed at a favorable (slow) rate that imparts the right type of plasticity to the paint film for good stabilization.
Elsewhere the same author observes that “all basic lead pigments form lead soaps” and tests determined that “red lead appeared to form the most soap, white lead a smaller amount, and chromated red lead the least amount” (Dunn 1975, 403–4). Unfortunately, the amount of lead soap formed by lead acetate (the most popular drier, see section 3.1) in relation to other lead compounds was not discussed.
Whether some of the problems associated with the use of excess drier (see below) had resulted because of the formation of excessively high proportions of lead soaps and whether one lead compound was more culpable than another remains to be seen. However, perhaps Scott Taylor (1890), then scientific director at Winsor & Newton, was justified in his concern that lead soaps were present in excess and were responsible for problems.
2.4 ZINC SULFATE
Zinc sulfate was a popular ingredient in drying oil recipes and in combinations of driers such as “Patent dryer,” but it was not offered as a separate product in the artists' colormen's catalogs. There were only two references in the literature to the use of zinc sulfate alone. One author noted that coach painters and painters of ornaments added it directly to their paint but warned that in this application it would cause the paint to crack and affect its texture (Tingry 1804, 100). Much later in the century, zinc sulfate was mentioned in passing as a drier for zinc-white paint (Martel 1859, 77). A varnish maker (Neil 1833, 55–56) valued dried zinc sulfate for its ability to remove “aqueous particles” from his varnish ingredients—oil, resin, and turpentine.
In 1835, Field recommended zinc sulfate over lead acetate but noted it was a less effective drier. He cautioned against the then apparently common practice of mixing the two, as he explained that the compounds zinc acetate and lead sulfate would result: zinc acetate was a poor drier, and lead sulfate was “insoluble and opaque” (1835, 56).
Regarding the drying power of zinc sulfate, Laurie (1895, 68) reported in 1895 that on its own it was quite ineffective and suggested that it was used in the past because of the presence of manganese (as an impurity), which did promote drying. However Laurie's position that zinc alone is “ineffective” is not true, for it does contribute significantly as an auxiliary drier:
Zinc driers are auxiliaries and are true driers almost in name only. Their effects are nevertheless highly advantageous. Zinc, in combination with cobalt, is used to prevent film wrinkling. This is accomplished by the ability of sinc to hold the film surface open longer and thus bring about a more uniform drying rate throughout the entire film cross section (Martens 1974, 398).
2.5 MANGANESE DRIERS
After the 1860s in Britain, widespread concern about the effect of lead compounds in oil painting materials was voiced by leading authorities who repeatedly warned artists that all materials containing lead were liable to darken and discolor as the lead reacted with hydrogen sulfide in the air. It was not simply white-lead pigment that was considered susceptible to sulfide-induced darkening: drying oils, media, or oil varnishes where lead driers were used were also cited (Carlyle and Townsend 1990). As a result of this view, lead driers were no longer recommended. The most popular alternatives were manganese-based driers.
Manganese as a drier for oils had been available for some time; it was first mentioned (in the texts consulted) in 1830 (Merimée 1830, 58). In the 1850s it was recognized that earth pigments such as umbers and Cappagh brown owed their drying qualities to the presence of this material (Field 1850). The trade literature for color manufacturing indicates that there was a strong interest in manganese driers as early as 1874 (Riffault et al. 1874, 578–79), but it was not until the 1890s that the artists' colormen featured “manganesed oils” while warning artists in their catalogs against the use of drying oils made with lead-based driers. Apparently manganese driers were not generally recommended until there was a strong incentive to stop using lead compounds. Early references were to manganese oxide, but by the 1890s it appears that manganese borate was preferred (Church 1890).