Zinc – Direct Process vs Indirect Process

​​​​​Hi -This is a bit of a long answer, but bear with me!
In terms of research in this area, I am not aware of anything outside the types of historical, commercially focused ones referenced in the article by Dawn Rogala that is cited, as well as another one by her from 2011, Industrial Literature as a Resource in Modern Materials Conservation: Zinc Oxide House Paint as a Case Study. ​There are several reasons that are likely for this. Most of the research on paint is driven by the commercial industry which has a very different set of concerns, centered around a much shorter time span, than conservation or artists in general. Add to that the fact that latex house paints were introduced in the 1940s and would eventually supplant oil-based ones as you get into the 1980s, you find that most of the fundamental research into traditional oil based paints basically starts to disappear in the 1950-60 period. Lastly, the indirect process has largely dominated the artist paint market because it is a purer product, with fewer contaminations. This is particularly true about lead, which appears in trace amounts when zinc is processed from ores. Unfortunately, it takes a very small amount of soluble lead (on the order of < 5 ppm) for it to trigger a health warning. I know when Golden Artist Colors (full disclosure I work as their Senior Technical Specialist) last looked into getting this pigment for testing, many years ago, we were unable to locate anything under that threshold but we have reached out again to see if anything has changed.
Before commenting on the post from Wet Canvas, let me just touch base on why I think it is doubtful that the direct process would have been used by the Pre-Raphaelites or other painters in Europe around that time. First, the French indirect process was discovered in the 1840s and was considered a major innovation, quickly spreading throughout Europe and adopted by both commercial and artist paint manufacturers. So any later import would have to over come this early lead and established market.  Plus the Pre-Raphaelites were formed right in the middle of this period, in 1848, and most of their major works date from the 50’s. This is important as the American direct process was only discovered in 1852, and it took to 1860 for processes to be worked out and production to be at a commercial scale. So definitely coming late to the Pre-Raphaelite party. Plus in 1865 you have the Civil War! Let's just say that supplying the European art market with pigment was not high on the list of priorities and most production was diverted to the war effort.
In terms of the article on the 1949 symposium Dawn Rogala wrote about, and the poster cites, there are a few things that I can point out and draw your attention to. The most important one is to realize is that Rogala is merely reporting on what the attendees to that symposium were presenting, as well as some of the contemporaneous research from back then – she is not supporting their findings as being true, simply that such and such was said. Which makes it easy to confuse things. For example, in a table she presents contrasting the favorable properties of direct vs indirect processing, it is easy to miss the description that states  "Comparative zinc oxide pigment characteristics as reported c. 1949 in the paint research literature”.  Much of her interest in commercial symposiums are not necessarily the findings as being factual, per se, but that the conversations around the research and the topics being thought about and discussed foreshadow the ones that now dominate current conservation and that things can be gleaned from them worth pursuing. 
In the paper from 2011 that I mentioned earlier, Industrial Literature as a Resource in Modern Materials Conservation: Zinc Oxide House Paint as a Case Study, Rogala​ actually makes clear that this type of commercial literature is fraught with bias as they are presented by the manufacturers themselves within a competitive context, each vying for advantages.
Just to share a couple quotes from that, which actually reference that 1949 symposium:
(pp.79-80) 
ZINC OXIDE IN INDUSTRIAL LITERATURE BETWEEN 1926-1950 
The most useful articles from this period appear mostly in industry journals and symposium post prints. Articles from this period focus mostly on market demand and product adaptation, which may not initially seem applicable to conservation, but in fact provide uniquely informative material that is available on​ly in the industrial literature. 
Competitive Bias 
As zinc oxide paint was adopted by the consumer market, the commercial debate shifted to determining the best raw material for successful paint formulations. The audience for these articles was the paint manufacturer, and accordingly, much of the information regarding raw materials was conveyed through papers presented at industrial symposia. Of the nearly fifty articles gathered from this period, the authors of approximately a quarter of the studies note their affiliation with a university or scholarly research center, while a far larger number of authors acknowledge their role as employees of paint and raw pigment manufacturers. Sponsored symposia articles are suspect, especially when the authors present the superior qualities of their product with little explanation of analytical methods and limited bibliographies (some examples are Kekwick 1938, Calbeck 1941, Davidson 1949). The 1949 Zinc Oxide Symposium, sponsored by the Victorian Section (Australian Branch) of the Oil & Colour Chemists’ Association and reproduced in a special issue of Paint Notes: A Journal of Paint Technology (1949), contains several examples of so-called “comprehensive” literature surveys whose bibliographies are limited to authors with similar agenda. For example, K. R. Bussell’s survey of literature promoting the use of acicular zinc, which begins with the statement: “the literature on zinc oxide is, of course, very extensive” (1949, 217) contains a bibliography of articles exclusively by industry representatives. Such publications should not be ignored, however. Symposia post prints also include papers by impartial authors who offer comprehensive citations and unbiased discussions of paint film behavior. The writings of F. L. Browne (1936 and 1941), D. W. Robertson (1935 and 1936), J. R. Rischbieth (1949) and F. C. Schmutz (1935) stand out because of their inclusive references and accessible language. Despite an irregular citation style, bibliographies from these articles are invaluable in building comprehensive period literature lists. Period post prints also contain pertinent information about period additives (such as surplus postWWII rubber plasticizers) or industrial formulations based on engineered failure properties, a topic of particular relevance to the conservator.
(pp 80)
For example, Rischbieth’s article from the Australian symposium (1949) focuses on zinc oxide paint performance in Australia, but also notes a global industrial preference for acicular zinc pigment, purposefully used because the brittle acicular zinc oxide paint films will preferentially micro-fissure during failure. Such widespread chalking caused the upper layers of zinc oxide house paint to slough off in the rain, creating the appearance of a perpetually clean paint surface​

Lastly, in case it is not clear, I simply bring all of this up as a way to say things are usually more complex than they seem. That said, acicular zinc DEFINITELY should be examined and tested – no question. And we are sympathetic to the desire to find an overlooked process or material that can bolster the argument for zinc, or render it safer. 

Hope some of this helps!

Sarah Sands
Senior Technical Specialist
Golden Artist Colors

I read your comment with tremendous gratitude for your appreciation and generosity. Thank you so much. ​Truly. I can’t think of a sweeter gift to serve as a coda to my career. This is my last week before retiring, and so your words will likely be the last response to a last posting, and I simply cannot imagine a better note to go out on.

My greatest privilege​ over the 26 years I have worked in this field – first for Williamsburg back in the 90s, and then Golden from 2002 on – is engaging with so many, like you, who have made me stretch and learn and stay curious because you yourselves are curious. Its been a beautiful way to live and have a career at the same time. But other things beckon – time in the studio chief among them, but also simply a world to explore with my curiosity intact.

Keep asking questions. Keep painting. Enjoy deeply your life.
Sarah​

​To just add another wrinkle to the Pre-Raph part of this discussion. You can see that cracking is more pronounced where zinc white predominated (see Our English Coasts by W. H. Hunt) AND as J. Thompson indicates, what the Pre-Raphs were purchasing may have been a mixture of zinc and lead white. Finally, the painting into a couch of white diluted with copal technique appears to have been only used a few times at most. It is now believed that his biographer greatly exaggerated this “technical innovation” in her biography to make Hunt appear more revolutionary.