This is a question that has come to us on several occasions, so wanted to take a moment to lay this concern to rest.
Like so many of these often-heard, historical adages, this one has survived because there is some element of truth or logic that it was originally built on. In this case, the concern has its roots in the fact that earlier production of cadmium pigments (Cadmium Sulfides and Cadmium Sulfoselenides) contained some percentage of free sulphur, which could potentially react with basic lead carbonate to form the brownish/blackish compound known as lead sulphide. Along with Cadmiums other pigments that were thought to be of concern included Vermilion (Mercuric Sulfide) and Ultramarine Blue (Sodium Sulfosilicate). In addition, of course, sulphur compounds were a major problem in air pollution during the 19th and well into the 20th centuries, so environmental exposure was yet an additional factor. Ultimately, however, discoloration was almost solely a concern with water-based media and only possibly – and to this day, still mostly anecdotally – with some oil paintings in the past. Research has simply never born out any clear evidence of lead whites in oil turning black, either from exposure to air or admixture with sulphur-bearing pigments such as the above mentioned, first generation of Cadmiums or the broadly used Ultramarine Blue. Indeed, it appears that the oil binder itself is a sufficient insulator to prevent these types of interactions. One can find a summary of this position in Painters’ Methods to Prevent Colour Changes Described in Sixteenth to Early Eighteenth Century Sources on Oil Painting Technique by Margriet van Eikema Hommes, Looking Through Paintings: The Study of Painting Techniques and Materials in Support of Art Historical Research , London, 1998. In that essay the author states:
“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 aqueous binding medium would also apply to oil painting.”
This is also echoed in the fairly authoritative entry on Lead White in Artists’ Pigments, A Handbook of Their History and Characteristics, Vol. 2, National Gallery of Art, Ashok Roy, editor, 1993. There the authors Rutherford J.Gettens, Hermann Kuhn, and W. T. Chase state:
“Lead white locked in drying oil film and protected with varnish endures for centuries without blackening; witness the white collars and cuffs in Dutch portraits. Although lead white is theoretically incompatible with sulfide pigments, and should form black lead sulfide in contact with them, no glaring examples can be cited. Flesh tones, lead white tinted with red mercuric sulfide (vermilion) especially in oil-medium, have stood for centuries without change; as similarly mixtures with ultramarine in skies and draperies. Lead white mixed with cadmium-sulfide also seems unaffected.” (p.72)
Nearly all the known cases of lead turning black appear restricted to watercolor or fresco where exposure to sulphur is much easier and more common.
If nothing else, we hope the above will calm any concerns about the use of Lead Whites with the full range of colors on one’s palette, including Cadmiums, but if you still have any further questions about any of this, simply let us know.