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  • Zinc Sulfide in Drying OilApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2016-11-12 17:11:58 ... Most recent comment 2016-11-12 17:16:00
    Drying Oils Oil Paint Pigments
    Question
    Is zinc sulfide embrittlement comparable to that of zinc oxide? As a pigment does it pose the same risks in a paint film?
Answers and Comments
  • ApproveRejectUn-ApproveUser CommentThere is surprisingly little published information on the formation of zinc sulfide and its potential to form zinc soaps which is the primary concern with zinc oxide (zinc white) but our scientists on the moderating board may have additional insight on this topic that I am unaware of. I will include some excerpts by an article that was only recently published by Rebecca Capua in 2014 (in the Journal of the American Institute for Conservation) who writes both about the history of zinc sulfide (specifically lithopone) as a pigment but also includes scientific and anecdotal history as well: Lithopone had high marks in all categories, with one huge disadvantage. As lyrically described by Orr's Zinc White 25th anniversary brochure in 1923: “[T]he good gifts of the fairy godmother are often accompanied in the world of fact as in that of fiction by a malevolent touch of a malicious witch. The new pigment, with all its claims to favour, had disadvantages, the greatest of which was the property of darkening in the direct rays of the sun. The cause was a mystery, never satisfactorily explained, the apparently paradoxical effect being that the trouble was reversible, for, while the paint went dark in the sun's rays, it reverted to white in the darkness (Robbins, A. 1923. Orr's Zinc White: A Romance of Modern Industry.http://www.catalyst.org.uk/collection/collection.htm.” The source of the problem, described by Rigg (Rigg, J. G. 1956. The history, manufacture and uses of lithopone. Journal of the Oil and Colour Chemists’ Association 39:809–831.), was that alkali metal chloride impurities found in some zinc sources would tend to catalyze the transition from the blende or sphalerite crystalline form of zinc sulfide to wurtzite during calcination, and wurtzite is more readily reduced in the presence of ultraviolet (UV) radiation to produce metallic zinc. In later formulations small quantities of cobalt salts were added prior to calcination, which had the effect of stabilizing the wurtzite crystal lattice and thus increasing the light-fastness of the lithopone (Rigg, J. G. 1956. The history, manufacture and uses of lithopone. Journal of the Oil and Colour Chemists’ Association39:809–831.). According to Rigg, Orr's Zinc white was prepared from materials free of alkali metal chlorides. The difficulties that the pigment industry had in producing a light-stable version of lithopone led to a far-wider adoption within manufacturing industries compared to its use in artists’ oil and watercolor paints. Lithopone was extremely important to the rubber, linoleum, and interior house-paint industries (Robbins, A. 1923. Orr's Zinc White: A Romance of Modern Industry.http://www.catalyst.org.uk/collection/collection.htm. Perhaps because of the stigma of its early difficulties, the usage of lithopone in paints for fine arts is difficult to uncover. As to the suggestion that lithopone used as an artists’ watercolor pigment did not actually discolor: it seems likely that protection from sun exposure during painting would have averted the reduction reaction. Many scientific accounts, such as those of O'Brien, point to experimental results that reveal that “moisture is necessary for the darkening” (O'Brien, W.J. A study of lithopone. The Journal of Physical Chemistry 29:113–144., 126), and that the same results would occur whether the lithopone was mixed with water, linseed oil, or varnish. It is reasonable to conclude that the La Farge and Van Gogh watercolors were painted indoors or otherwise protected from sun exposure, and once dried were not vulnerable to reduction to the same extent. It is hard to believe that artists’ paintings with lithopone did not experience any darkening given the contemporary scientific evidence, but they were certainly less vulnerable than a white-washed fence or carriage exposed to the elements, circumstances that seem particularly worried-over by industrial chemists grappling with this problem. However, the question then arises: if lithopone used as an artists’ pigment was not particularly prone to darkening, why did it have a poor reputation as an artists’ pigment? Indeed, the problem is circular; the best-case scenario is that this study leads to increased attention to the identification of lithopone on works of art. Only with more data points to provide a statistically meaningful set for comparison can research go beyond hypothesizing the answers to these questions
    2016-11-12 17:56:09
  • EditDeleteModerator AnswerThere is surprisingly little published information on the formation of zinc sulfide and its potential to form zinc soaps which is the primary concern with zinc oxide (zinc white) but our scientists on the moderating board may have additional insight on this topic that I am unaware of.

    I will include some excerpts by an article that was only recently published by Rebecca Capua in 2014 (in the Journal of the American Institute for Conservation) who writes both about the history of zinc sulfide (specifically lithopone) as a pigment but also includes scientific and anecdotal history as well:

    Lithopone had high marks in all categories, with one huge disadvantage. As lyrically described by Orr's Zinc White 25th anniversary brochure in 1923:

    “[T]he good gifts of the fairy godmother are often accompanied in the world of fact as in that of fiction by a malevolent touch of a malicious witch. The new pigment, with all its claims to favour, had disadvantages, the greatest of which was the property of darkening in the direct rays of the sun. The cause was a mystery, never satisfactorily explained, the apparently paradoxical effect being that the trouble was reversible, for, while the paint went dark in the sun's rays, it reverted to white in the darkness (Robbins, A. 1923. Orr's Zinc White: A Romance of Modern Industry.http://www.catalyst.org.uk/collection/collection.htm.”

    The source of the problem, described by Rigg (Rigg, J. G. 1956. The history, manufacture and uses of lithopone. Journal of the Oil and Colour Chemists’ Association 39:809–831.), was that alkali metal chloride impurities found in some zinc sources would tend to catalyze the transition from the blende or sphalerite crystalline form of zinc sulfide to wurtzite during calcination, and wurtzite is more readily reduced in the presence of ultraviolet (UV) radiation to produce metallic zinc. In later formulations small quantities of cobalt salts were added prior to calcination, which had the effect of stabilizing the wurtzite crystal lattice and thus increasing the light-fastness of the lithopone (Rigg, J. G. 1956. The history, manufacture and uses of lithopone. Journal of the Oil and Colour Chemists’ Association39:809–831.). According to Rigg, Orr's Zinc white was prepared from materials free of alkali metal chlorides.

    The difficulties that the pigment industry had in producing a light-stable version of lithopone led to a far-wider adoption within manufacturing industries compared to its use in artists’ oil and watercolor paints. Lithopone was extremely important to the rubber, linoleum, and interior house-paint industries (Robbins, A. 1923. Orr's Zinc White: A Romance of Modern Industry.http://www.catalyst.org.uk/collection/collection.htm. Perhaps because of the stigma of its early difficulties, the usage of lithopone in paints for fine arts is difficult to uncover.

    As to the suggestion that lithopone used as an artists’ watercolor pigment did not actually discolor: it seems likely that protection from sun exposure during painting would have averted the reduction reaction. Many scientific accounts, such as those of O'Brien, point to experimental results that reveal that “moisture is necessary for the darkening” (O'Brien, W.J. A study of lithopone. The Journal of Physical Chemistry 29:113–144., 126), and that the same results would occur whether the lithopone was mixed with water, linseed oil, or varnish. It is reasonable to conclude that the La Farge and Van Gogh watercolors were painted indoors or otherwise protected from sun exposure, and once dried were not vulnerable to reduction to the same extent. It is hard to believe that artists’ paintings with lithopone did not experience any darkening given the contemporary scientific evidence, but they were certainly less vulnerable than a white-washed fence or carriage exposed to the elements, circumstances that seem particularly worried-over by industrial chemists grappling with this problem. However, the question then arises: if lithopone used as an artists’ pigment was not particularly prone to darkening, why did it have a poor reputation as an artists’ pigment? Indeed, the problem is circular; the best-case scenario is that this study leads to increased attention to the identification of lithopone on works of art. Only with more data points to provide a statistically meaningful set for comparison can research go beyond hypothesizing the answers to these questions.

    Kristin deGhetaldi (CAS)
    2016-11-12 18:02:04
  • EditDeleteModerator AnswerAfter doing a bit of research I would like to share the following information regarding zinc sulfide but with the full confession that my scientific colleagues may very well correct or even dispute this observation. I was able to converse with Dr. Chris Petersen who is an organic chemist and works often with cultural heritage objects. He suggested looking a bit more at the Ksp values of these two compounds (zinc sulfide and zinc oxide) which essentially tells us how likely they are to disassociate (separate out into separate ions) in water....much like salt does. I realize this may seem "backwards" but zinc oxide has a higher Ksp (approx. 1.7 x10^-17) than zinc sulfide (approx. 2 x 10^-25), meaning zinc oxide will dissolve more readily than zinc sulfide. While I realize we are not talking about suspending these pigments in water (but rather in oil) these materials can still react when exposed to higher levels of humidity leading to similar reactions that play out over an extended period of time. In summary, I would suspect that while zinc sulfide (lithopone) may still form zinc soaps when bound in oils, alkyds, waxes, and possibly tempera, it seems that it is less likely to do so when compared to zinc white based on Ksp values. Plus zinc white already has a fairly reactive oxygen on it vs. sulphur. So YES this really all boils down to chemistry....
    Kristin deGhetaldi (CAS)
    2016-11-15 20:41:33
  • ApproveRejectUn-ApproveUser CommentCan I offer a SIMPLIFIED SUMMARY (for artists in a hurry)? ie: Lithopone (zinc sufide) is not recommended: still uncertain, needs more research. Might darken in sunlight and form metal soaps (problematic) but Lithopone (zinc sufide) may be preferable to the more commonly used white zinc oxide oil paint. Stay tuned :J
    2016-11-16 00:11:56
  • ApproveRejectUn-ApproveUser CommentOn Painting Best Practices Facebook forum (moderated by George O'hanlon of Natural Pigments) I posed the question about using the new/old white oil paint lithopone W5 in safflower oil named "Porcelain White" now offered by Williamsburg Oils. It is appealing to some artists since it is less opaque than titanium and has a translucent quality .Also, it may be preferable to problematic zinc oxide paints. Here is a quote from Williamsburg Oils "Porcelain White is based on a new pigment for Williamsburg, PW 5, a complex co-precipitate of barium sulfate and zinc sulfide more commonly known as Lithopone. Created in the 1870s and for a while a rival to both lead and zinc, it captured some 60% of the market for white pigments in 1928 before declining to 15% by 1945 in the face of a growing dominance by titanium dioxide (Alphen 1998). It holds a renewed interest for us as an alternative to Zinc White, which forms a brittle film and has been linked to issues of cracking and potential delamination when used extensively. Historically known as Enamel, Orr’s, or Charleston White, we chose the one we felt was the most descriptive: Porcelain White." Is there any way to get a definitive answer on lithopone W5? What sort of testing would be needed? Wouldn't Williamsburg do some testing of their product before bringing it to the fine art supply market? Several artists who primarily paint in oils are interested, intrigued and enticed by the properties of "porcelain white", but with a healthy dose of skepticism and caution as well.
    2016-11-17 02:03:34
  • EditDeleteModerator AnswerI 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
    2016-11-17 17:47:29
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