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Moderator Answer
I would like to respond to some of the comments about lithopone and especially to the "SIMPLIFIED SUMMARY" where the sense that it should not be recommended is mentioned. As a pigment lithopone has been in use since 1880 and while never widely adopted as an artists' paint, it was and continues to be used to prime canvases, including many from that earlier period such as those used by Van Gough (see "Investigation of the Grounds of Tasset et L'Hote Commercially Primed Canvas Used by Van Gogh in the period 1888 to 1890", 2013). It later became a commercially significant as a white in architectural coatings, capturing some 60% of the market prior to the introduction of titanium in the 1920's - although broad use of titanium did not really take off until the better chalk resistance and opacity of the rutile form was introduced in the1940's. Today Lithopone still finds significant use commercially in a variety of industries, including plastics, rubber, paints, and as an inert extender.
So, taken as a whole, lithopone has been in continuous use for almost 140 years and during that time it had enough prominence in industrial use that it seems inconceivable, to us at least, that a fundamental weakness such as soap formation would not have been noted or any cases of delamination or embrittlement due to soaps formed by lithopone noted. This is especially true given that the commercial paint world was highly aware of these issues in zinc oxide, and cases concerning zinc soap formation appear constantly in the literature of the time. In fact, given the long period of lithopone's adoption in commercial coatings, one would certainly expect any failures along these lines to be quickly noted. And yet the literature is absolutely devoid of any cases. While obviously the mere absence of a report does not and cannot act as proof of a positive, it does argue for considering lithopone innocent until proven guilty and that any weight of evidence suggesting there IS a problem needs to be higher than mere speculation or theory. Surely we should at least wait until one can actually prove it is an issue in an actual example. So far, none that we know of exists.And if anything, there is at least one recent conservation report, "Zinc oxide-centred deterioration in 20th century Vietnamese paintings by Nguyen Trong Kiem (1933–1991)", Gillian Osmond, 2014, that specifically point to the lack of failure of lithopone found in the ground. For example, on page 9:
"In Children playing at the beach, size or unpigmented layers are present between zinc oxide based paint and lithopone-based retouching and ground layers. In these instances pigment dissolution is visible in zinc oxide-based layers while lithopone remains intact"
As to our own testing, we did outdoor exposures in South Florida for a full three months, as well as indoor accelerated lightfast testing in a Xenon chamber. In both cases we followed ASTM’s D4303 Lightfastness Testing Standard for Artists Materials, which is meant to be a very strenuous test of exposing a sample to high levels of UV within a very compressed period. In all the tests the color performed exceptionally well and would have the equivalence of an ASTM rating of I, or excellent. There was no discoloration or darkening, an early concern around lithopone that is mentioned in the literature but which was subsequently resolved through a change in the manufacturing process. Finally, we have done flexibility tests on naturally aged samples that are currently 4+ years old. That is within the same time frame that samples of zinc have repeatedly shown embrittlement but currently we are not seeing any negative impact with the lithopone samples – all would be classified as acceptable and certainly as flexible as other whites, such as Titanium.
We do find that the issues Kristin shares from Dr. Chris Petersen to be interesting and certainly worth study. In particular the issue of sensitivity to high levels of moisture makes sense. After all, keep in mind that zinc sulfide is a component of nearly all modern cadmium yellows, which are listed as cadmium zinc-sulfides, and that cadmium yellows in particular are known to degrade when in high humidity outdoors. For that reason, in fact, cadmiums are not recommended for exterior use. So, given that concern, it would make sense to not use lithopone outdoors until more testing was available, but of course that is an almost negligible use for the modern oil painter, where works are largely kept inside under reasonable conditions.
In summary, with any new introduction of a pigment – even when an old one – there can obviously be concerns with longevity. We continue to feel that it has never, outside of the early darkening issues, been cited in the commercial technical literature as being prone to embrittlement or a cause of concern structurally, and commercial testing – while not along the same line always as that conducted for art materials – is often quite strenuous since they expect pigments, especially white ones, to perform in a reliable way. Certainly if there had been an obvious structural weakness in the more than 140 years of use, we think that would have been spoken to at some point. But, that said, we will continue to test the durability of the pigment going forward and provide updates through our technical journal and website, http: www.justpaint.org
Sarah Sands
Senior Technical Specialist
Golden Artist Colors
