A few notes on the chemical nature of pigments, their reactions, and compatibility.

When painting in the indirect method one needs to remember the fat-over-lean rule: always paint a more flexible layer over a less flexible one. With that in mind, the brittleness of paint films created with Titanium or Zinc whites make them less suitable for this method of painting than Lead white.

However, in dry pigment or water based paints, certain pigments are reactive with lead, causing it to blacken. These pigments contain either free sulfur or sulfide salts which, in mixtures with lead, combine to form (the brown or black) lead sulfide. Hydrogen sulfide in the atmosphere can also cause this reaction if the painting is not properly sealed with a protective varnish.

Sulfide: (sulphide) a sulfur atom attached to two carbon atoms. A binary compound (as CuS) of sulfur usually with a more electropositive element or group: a salt of hydrogen sulfide. In mixtures with lead the resulting compound, lead sulfide, is black in color. Pigment examples: Cadmium Sulfide (all cadmium colors), Mercuric Sulfide (Genuine Vermilion), Sodium Sulfosilicate (Ultramarine Blue: sulfur is used in the preparation of Ultramarine Blue and its variants. One paint chemists told me that the sulfur content in ultramarine is so strong that it can be smelled in the dry pigment and that it should not be mixed with lead.)

Sulfates and Sulfites: are salts or esters of sulfuric or sulfurous acids. In mixtures with lead these salts produce lead sulfate, the white scales visible on storage batteries. Since the product of this reaction is white, it does not harm the color of a painting. Safe pigments include: Cerulean Blue which is made with cobalt sulfate calcined with stannous (tin) chloride. Manganese Blue combines barium manganate with barium sulfate. A sulfate is also used in the production of Ferric ammonium ferrocyanide (Prussian Blue) and Titanium Anatase white, while sulfuric acid is used to produce Thioindigoid red (PR 88).

***It is believed that a pigment coated in oil is protected from direct intermixing and chemical reaction of the pigment particles, thus slowing or eliminating the problem. (see quote below) However, the Pre-Raphaelite painter William Holman Hunt wrote of problems he suffered on account of lead and vermilion mixtures:

It seems as tho' I were struggling against Fate, every day sometimes including Sundays I have been toiling every hour, and just as I have got my task nearly completed the whole thing has fallen into disorder again. At last I have found out what has been the cause of this -- Roberson's tube of Orange Vermilion which I used without suspicion because 25 years ago they sold this colour absolutely pure, is adulterated with 10 per cent of villainy -- the greater part lead, which has blackened so rapidly that when it had got dry enough for the final glazings the flesh had got to such a colour that I nearly went crazy with the idea that my eye sight had acommodated itself to what could only be thought right by a fool when he was trying to delude himself. I have had the colour analysed and at the same time have taken the opportunity to have others investigated and find that the fraudulent habit is exercised in many other cases.
letter to John Lucas Tupper, August 1875

And again:
...the fact that vermilion, which was the first colour suspected by me, was analysed and found to contain between 10 and 12 per cent of foreign matter, principally lead; that this vermilion is on two of my trial canvases, and where mixed with flake white (lead white) it has turned to 'café au lait colour'.
Editorial letter to The Times of London

One might suspect that the problem was in the adulteration of the dry vermilion pigment with lead, and that chemically pure mercuric sulphide would not react in an oil binder. However, this does not explain why Holman Hunt had problems only when he mixed the vermilion oil paint with Flake White on his canvas. One would imagine that, if the adulterating lead in the pigment were the cause, then the vermilion would have darkened before ever being ground into the binder.

Margriet van Eikema Hommes reports on Lead / Sulfur incompatibilities in her article Painters' Methods to Prevent Colour Changes Described in Sixteenth to Early Eighteenth Century Sources on Oil Painting Technique, from the book Looking Through Paintings: The Study of Painting Techniques and Materials in Support of Art Historical Research , © 1998
Pg. 114-115 - ISBN 90 6801 575 3 and 1 873132 56 5.

(The above quotes from William Holman Hunt are taken from Melissa R. Katz' article, Textual Evidence in Aid of Technical Analysis from the same book, and deal with Holman Hunt's investigation into technical problems with "modern" manufactured artists' materials.)

van Eikema Hommes writes:

When mixed together in a binding medium, certain pigments can cause a chemical reaction that could discolour the paint. Pigments containing sulphur such as vermilion, ultramarine and orpiment can react with pigments consisting of either lead compounds (including lead white and lead tin yellow) or copper compounds (such as verdigris and azurite). However, laboratory tests have shown that in practice these reactions almost always occur in an aqueous binding medium and that the discolorations rarely affect oil paint. In fact, there is also very little evidence of compatibility problems in old oil paintings where less compatible pigments had been mixed. For instance, lead white (which easily reacts with sulphur) was used in combination with vermilion or ultramarine in countless paintings without causing any discoloration. Hence, it is remarkable that the historical texts contain so many warnings about intolerant mixtures....Perhaps the reason for the sources' caution was the assumption made by the painters that the problems of mixing in an acqueous binding medium would also apply to oil painting. However, the difference between the seventeenth century and our contemporary view may also be due to the fact that we still do not completely understand the problems surrounding the use of historical pigments in mixing paint.

These sentiments were echoed by Mark David Gottsegen, Chairman of Artists Paints & Related Materials, artist consultant to numerous companies, author of The Artists Handbook, and Professor, Department of Art, University of North Carolina, Greensboro. In a personal phone conversation, he stated that the oil binder prevents lead / sulfer reactions common to dry pigments or water based paints. As a test, Mr. Gottsegen recommends mixing these "incompatible" pigments in tube oil colors and setting them in a south facing window (in the sun) for six months.

I have carried out this test, exposing lead/sulfide mixes and synthetic dye-colors that may be used to replace sulfur-containing pigments to the southern sun from April 15th, 2000 to October 14th, 2000. The same test strips were then exposed to exterior weather and south sun from October 30th, 2000 until April 14th, 2001. The surprising results may be examined HERE.

As noted above, cadmium sulfide is reactive with lead, producing the black lead sulfide. This is true of lead-chromate as well as lead carbonate. Lead white is reported in one document not to have affected cadmium sulfide when the cadmium was pure, stating that the darkening usually associated with this mix was due to impurities of free sulfide. Some samples of cadmium in the early days of its European production had as much as 20% free sulfur content that created the instability. It was stated that "when properly washed, made from pure starting materials, and reacted so as not to produce free sulfur, the pigments produced by this procedure were stable." No subsequent heating (calcining) was employed at that time; calcining (now used in production) tends to eliminate formation of hydrates and sulfates from presence of sulfur crystals. This brought about the report that "very little problem remained with commercial pigments by the mid 1920s." However, it is reported that as little as 0.1% of sulfur, iron, copper, lead, or arsenic "would cause noticeable change" in cadmiums and , speaking of cadmiums produced before 1914, "even the best of these would probably not stand the test of time."

Ralph Mayer, in the Artist's Handbook (1991), still reports that lead may darken cadmiums. In discussions with the Engelhardt Company, manufacturer of cadmium pigment, and paint chemists, I have been told that cadmiums should not be mixed with lead or even placed next to it in paintings.

As noted above, cadmiums have also been reported to be reactive with copper-based colors as well, such as copper acetoarsenite (emerald green), though this pigment is no longer in use due to its own permanency problems. The copper issue may, however, raise questions in mixing cadmiums with copper phthalocyanine blues and greens. I have found no specific studies addressing this issue.

If van Eikema Hommes statement above is to be believed, the oil binder may make these compatibility issues of little or no concern. But beyond these compatibility problems, cadmiums, especially the yellows that do not contain selenium, are said by the manufacturer to chalk in tints with white and to fade in the presence of moisture (in commercial paints), especially in exterior uses where the action of air and UV radiation act upon them. This effect is present even in mixtures with Titanium and/or Zinc whites making it a questionable color, even for alla prima painting.

View the results of weather testing HERE

AZO PIGMENTS: Yellows and Reds

The popular trend among paint companies is to replace the cadmiums with the azo family of pigments for yellows and reds. This includes monoazo or arylides, disazo/diazo or diarylides, and benzimidazolones. These pigments run the gamut on lightfastness, weatherfastness, brilliance, and resistance to bleeding and solvents, often trading good characteristics in one area for another. As a whole, azo pigments tend to fade in tints with white and to bleed or be soluble in some organic solvents. It is therefore recommended that they be used only for glazing in full color for best permanence.

Unfortunately, in the bright yellow range, there is little else to choose from today. Some are experimenting with the Isoindolinone yellows (PY 109 and 110) which offer a greenish yellow and medium reddish yellow, though they have been criticized as not being as pleasant in hue as the azos, especially in mixtures with other colors. Anthrapyrimidine yellow (PY 108), an anthraquinone color, shows good possibilities if it can be found in artists' paint. The more traditional Cobalt yellow, or Aureolin, shows worse permanence than the azos with an ASTM lightfastness rating of II compared to a rating of I for many azo yellows. Cobalt is reactive with organic pigments, having a tendency to turn brown. Though duller, for masstone the artist is best served by using the earth and metal yellows: iron oxides (ochre, sienna, and mars yellows), nickel titanate for a good lemon yellow, and naples yellows for the middle range. Then, if greater brilliance is needed, the azos may be glazed (well mixed with a good medium) on top.

In the reds it has become easier to find brilliant replacements for the cadmiums and azos. The Diketopyrrolo pyrrole (DPP) family of colors are said to be very stable, permanent, and as opaque as the cadmiums while the Quinacridones are generally produced with greater transparency. To a lesser extent, some Anthraquinone reds show good stability in the brilliant reds as well. Unfortunately, isoindolin red (PR 260) does not show the promise of its yellow counterparts, bleaching excessively in reductions with white. As with the yellows, the artist is best served by the earth reds of iron oxides (burnt sienna, mars, indian, english, venetian, and other synthetic iron oxide reds, and the umbers) while glazing DPP or Quinacridones over them, over other colors, or over a grisaille underpainting.

History, Definitions, and Techniques | Drying Oils and Mediums
Resins and Varnishes | Pigments Past |Pigment Chemistry
Supports for Painting | Grounds on Canvas | Techniques of Past Masters
Discussion with National Gallery Conservator