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Update on Zinc – Search for the Dividing Line

21 thoughts on “Update on Zinc – Search for the Dividing Line”

  1. I appreciate your testing this. However have you looked into the variety of Zinc White that you’re testing. Acicular (or American/Direct process) Zinc White has a history of greater durability than the finely divided and more commonly available French process Zinc White that was introduced after WWII.

    “The Victorian Branch of the Oil and Colour Chemists’ Association held a meeting in Melbourne, Australia in the summer of 1949 to discuss a sudden and marked increase in problems associated with house paint formulations containing zinc oxide pigment, and to investigate any possible relationship between this increased failure rate and recent changes in manufacturing methods for the pigment.”

    “C.H.Z. Woinarski, then Senior Chemist at Hardie Trading Ltd., outlined the shift in paint production and performance observed during World War II, when new methods of pigment manufacture emerged—at newly built or converted facilities—to meet an increased demand for zinc oxide pigment. Direct Process (also called American Process) methods of pigment production were replaced by Indirect Process (sometimes called French Process) methods, and this manufacturing shift was accompanied by a precipitous rise in failure rates of oil‐based house paints containing zinc oxide pigment produced using the new method.”

    “Nelson confirms that Direct Process oxides contained various percentages of acicular variations on the crystalline form, some joined to form “twins” and “threelings” (referred to as “brush‐heap” formations by Bussell), while Indirect Process zinc oxides (often marketed under the term “Seal” oxides) were typically irregularly shaped particles of uniform size distribution.”

    See this report:

    • Hi CF –

      I replied more fully on MITRA, where I saw that you posted a longer version of this question, asking about any research comparing direct (French) and indirect (American) processed zinc, and wondering if perhaps the Pre-Raphaelites might have used the acicular zinc that the direct method produced. So let me go ahead and simply paste a lightly edited version of that reply below, even though it goes somewhat far afield from the more narrow and streamlined question above.

      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 you link to at the bottom of your question, 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 with lead, which usually appears in trace amounts when zinc is processed from ores. Unfortunately, it takes only a very small amount of soluble lead (on the order of 5 ppm) for it to trigger a health warning. I was told that many years ago, when we first looked into getting this pigment for testing, we were unable to locate anything under that threshold but we are reaching out again to see if anything has changed.

      On whether the Pre-Raphaelite’s might have used acicular zinc, let me share why I think it is doubtful. First, the French indirect process was discovered in the 1840s and considered a major innovation that quickly spread throughout Europe and was adopted by both commercial and artist paint manufacturers. So any later import from the US would have to overcome 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 that article by Dawn Rogala which you linked to, concerning a 1949 Symposium, there are a few things that I can point out and draw your attention to. The most important one is to realize that Rogala is merely reporting on what the attendees to that symposium were presenting – she is not supporting their findings as being true, simply that such and such was said. Which makes it easy to confuse things and take the passages at face value. Much of her interest in commercial symposiums are not necessarily that the findings are 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 of quotes from that, which actually reference the same 1949 symposium:



      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, just to make it absolutely clear, I bring all of this up not as a way to beat up on acicular zinc, but only to show that things are usually more complex than they seem. But hand’s down, 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. And like you and others we scan the literature looking for clues and so truly appreciate it when folks share what they have found as well. We’ll keep digging on our end of the tunnel and see if we can all meet up somewhere!

      Hope some of this helps.

      Sarah Sands
      Senior Technical Specialist
      Golden Artist Colors

    • Hi Ron –

      You are so very welcome. And I know you have been one of the faithful followers of our testing over many years and I truly do appreciate that as well.

  2. Could it be possible that mixing this 2% zinc white paint mixed with another pigmented color that dries to the harder end of the scale, might also be pushed beyond the ideal limits, though the zinc percentage is lowered still further?

    • Hi Mark – My immediate response, informed by 26 years of head-scratching while studying oils, and accompanied by a slight chuckle, is who knows, at this point, it seems anything is possible! But more seriously, I think combinations with a range of other colors are something we still need to look into. Certainly, we know that zinc mixed with lead will form a particularly brittle film – which is counterintuitive when recalling that lead forms the most flexible film of any pigment. So, just given that fact, I am not sure if the hardness of a color will be the key more than its chemical interactions. As you can imagine, part of the difficulty in this research is just how complex it can become. But we will keep chipping away at it.

  3. Dear Sarah,
    Thank you for your report on investigations into the embrittlement of Titanium/Zinc White oil paint films. However, a number of issues struck me as being unexamined and needing further elaboration. For example, you state:

    “The paints in the following tests used a basic formula of pigment, alkaline refined linseed oil, beeswax, and barium sulfate with a minimum amount of drier, with only the percentage of zinc vs titanium changing from sample to sample”.

    My question relates to the experimental design. Why were beeswax, barium sulfate, and drier (an undescribed type) added and used in the various formulations? I thought the number of variables being tested should be predicated on varying only the percentage of zinc oxide pigment used. Even at a fixed percentage in all the formulations employed, could barium sulfate, or the beeswax, or the unnamed drier have an effect upon embrittlement? I felt it might be important to examine the role that these other ingredients have in addressing the proverbial “elephant in the room” question about how to configure the investigation itself. In other words, why wasn’t a control set of samples included, which had the same gradated amounts of zinc oxide pigment present, but made without barium sulfate, beeswax, and the drier, also run? Thanks for the feedback and best wishes on your continued experimentation.

    • Hi Michael – First a thousand and one thank you’s for pointing this out as in fact what you caught was a mistake on my part – an overlooked mismatch between the description of the paints vs the actual table and data I wrote about!! I have adjusted the text accordingly while the table that was shown was already the one based precisely on a series of blends using only pigment and oil for all the reasons you mentioned. While we did do parallel testing on the types of basic paints that were described, those are not included here and, in any case, roughly followed a similar pattern. You can sense the inadvertent mismatch later on when I describe the tests on yellowing and state that those “mixtures were also composed solely of pigment and alkaline refined linseed oil”, clearly a reference back to a prior group. So yes, you were absolutely right to call out the elephant in the room – which thankfully sent me running back to the article where I was relieved to discover, after careful double-checking, that it was merely the ghost of a prior elephant that had somehow strayed onto the scene.

      Again, cannot thank you enough for pointing this out. One benefit of writing for JustPaint is that I have a thousand editors willing to hold me to account.

    • Hi Randy – In terms of embrittling acrylic paints, no. The interaction is solely limited to oils and alkyds due to the nature of how zinc interacts with drying oils.

      As for painting with oils over acrylics having zinc in them, the simple answer is we don’t really know and will need to test this combination. Luckily Zinc is not commonly used in acrylics since a transparent white can be created easily with a medium or gel, but until we truly have data on it, we would advise caution about painting oils on top of anything that contains zinc, including acrylics, just to err on the side of caution.

  4. Hi Sarah,

    And how about zinc in egg tempera – any concern with brittleness there?

    Thanks, as always, for your commitment to artists and their materials, and the research that you and Golden do.

    Koo Schadler

    • Hi Koo – I would be the first to admit that we do not really have any testing on egg tempera’s interactions with various pigments, and my knowledge of the literature in that area is also very spotty. I would think Brian Baade or Kristen deGhetaldi, over at MITRA, might be better equipped to answer this.

      And thank you for the warm words about our testing and contribution. We certainly love offering what we can.


  5. You’re the best, Ms. Sands. You always go the extra mile.
    But, I wasn’t the one posting on MITRA. Someone was quoting what I’d originally posted on WetCanvas (Antonin). I didn’t read your answer there until now ;).

    “…in 1834, just two years after the paint company was founded, Winsor & Newton introduced a calcine zinc oxide which they called ‘Chinese White’ (named after the type of porcelain that was popular in Europe at the time). It was heated at high temperatures and was denser and more opaque than other whites available.” Many years ago, I remember reading articles that specifically stated that the original WN Chinese White was the Acicular form of Zinc White. I had never heard that term before.

    The pigment crystal morphology is said to play a role in the brittle fracture of zinc oxide:
    “The use of the acicular form of zinc oxide (American process) leads to a very considerable improvement and gives much more durable paints than the amorphous (French process) zinc oxide which consists of small round particles. “Instead of the end of their life being signalised by the usual checking of the film, such paints showed no signs of it and finally began to disintegrate much later by chalking.”
    “And further;
    …in the case of acicular zinc oxide paint, failure “begins with small uniform cracking which relieves strain and prevents more intensive failure”, and that very high acicularity is undesirable because it involves too high an oil absorption.”


    “There are two processes used in producing pigment grade zinc oxide, the American process and the French process. With the American process, a more acicular particle is derived as opposed to the French process which is more nodular in structure. Typically, the acicular pigments are less reactive in coatings systems and are therefore considered more stable.
    Grade 417W is an acicular, American process pigment which provides the maximum in stability and a narrow particle size distribution. It is our most commonly used coating grade and is maintained in inventory in our Clearwater warehouse.”


    The particles are nodular in shape and the individual primary ZnO crystallites are 30–2000nm in size. Scanning electron microscope images of typical French process ZnO are shown in Fig. 5. The surface area of French process ZnO is generally 3–5 m2 g−1 but can reach 12 m2 g−1 by carefully controlling combustion conditions such as air flow and flame turbulence or the distance between the suction hood and nozzle (which affects the air velocity). If the flame temperature increases, the specific surface area will drop. By increasing the excess of reactant air (oxygen) by making a better circulation of air or forced flow of compressed air in the combustion zone, ZnO quenching becomes faster and finer particles can be achieved, resulting in higher specific surface area.”

    “There are various implementations of the French process. Older technology principally uses a batch process that takes place in a crucible with a long cooling duct, most of which is horizontal. Newer technologies use a semi-continuous process with a vertically-designed cooling duct to save space. A batch is recharged with zinc ingots at approximately four hour intervals whereas in the semi-continuous process a zinc ingot (often 25 kg) is added to the furnace every 6 min. The productivity of the semi-continuous process is often higher than that of the batch process. The semi- continuous system is rarely shut down unless for an overhaul and it is generally very compact.”

    “3.3. Morphology of zinc oxide particles
    The morphology of ZnO particles can be controlled by varying the synthesis technique, process conditions, precursors, pH of the system or concentration of the reactants. A wide variety of shapes are possible. The French and American process zinc oxides have nodular-type (0.1–5􏰀m) or acicular-type (needle-shape, 0.5–10 􏰀m) particle shapes. Wet-process ZnO may have a sponge-like form with porous aggregates being up to 50􏰀m diameter. There are, however, a large number of other morphologies, each produced under some specific set of conditions. Many of these have been given whimsical names. The possibilities include nanorods, nanoplates , nanosheets , nanoboxes , irregularly-shaped particles, polyhedral drums , hexagonal prisms, nanomallets, nanotripods, tetrapods, nanowires, nanobelts, nanocombs and nanosaws, nanosprings and nanospirals and nanohelixes, nanorings, nanocages, nanoneedles, nanotubes, nanodonuts, nanopropellers, and nanoflowers.”

    There’s an illustration in the article that shows some of these variations. Could one or more of these morphologies provide a more stable Zinc White?

    These are excerpts from the Zinc Oxide entry:
    “Variations in particle morphology in zinc oxides have been identified and are divided into two main types. The acicular-type crystals, which occur as individual, twinned or tetrahedral combinations, are found in zinc oxides derived from the slow burning of zinc vapour, as mentioned by Kühn, usually from the French process. The nodular-type particles are the most common and have a more rounded appearance, being produced from faster burning syntheses. The manufacturing methods yield different grades of pigment based on particle size which are sold under different brands of Seal. Varieties which require little oil (usually those with larger particle size) are often known as ‘zinc white double’ or ‘zinc oxide double’.
    However, Kühn also states that drying processes, particle morphology, impurities and lattice defects are also now known to play an important role on the photochemical properties of zinc white.
    Although zinc white is stable in light, studies have shown that it tends to saponify the fatty acid component of certain oils, with the degree of soap formation being dependent upon the particle size of the pigment, with the finest particles resulting in more rapid soap formation.”

    Perhaps something was lost between the original French process and the current French process that involves “space saving” and “semi-continuous” techniques, resulting in a more finely divided and reactive pigment.

    • Hi CF – Many of these sources are ones we know about and we are on board with the thought that this might offer a more stable film – or at least something that fails less dramatically since micro-fissuring and chalking are not exactly a great selling point. But then perhaps those issues could be overcome. However the real block in pursuing testing of this line of zinc is not needing to be convinced, but the fact that the pigment from all the suppliers we have contacted reveals too high a level of basic lead carbonate. Now, it might be that some people would be willing to use a Zinc White containing a Lead Warning, but we feel it presents issues. And of course one would need to assure people that there is no embrittlement not simply over the short term but further out, and given the current concerns with Zinc, that would present a high hurdle to overcome. But we will continue to look and examine this area.

  6. Perhaps it would be useful to do a series of tests on trying to make a titanium white based paint without zinc and lead but with different additions to see if one could be made without so much yellowing or embrittlement?

    I am thinking of various percentage additions of: Zinc Sulfide, Strontium Titanate, Calcium Carbonate, Kaolin, Barium Sulfate, Hydrated Calcium Sulfate, Hydrated Magnesium Silicate.


    • Hi Richard – In terms of yellowing, we have done testing with a good number of formulations but not necessarily all the ones you mention, and we definitely know that other additions besides zinc can improve yellowing. For example look at Image 5 in our article On the Yellowing of Oils from 2019 and you can clearly see the improvements. But always room to test other combinations.

      In terms of embrittlement, Titanium White doesn’t have an issue except when zinc oxide is added, so really no need to adjust a zinc-free Titanium White to improve flexibility, although without some form of addition you can get a very soft, somewhat sticky film.

  7. Hi Sarah,

    I meant to include beeswax in my comment as I remembered you had found good results with it! I was wondering if another material would strengthen the film and/or reduce the yellowing. That’s why I was thinking of a comprehensive set of tests 🙂

    I know it’s a bit of a cheat but Permawhite included a very small amount of ultramarine blue to counteract the yellowing.

  8. I have a commercially prepared stretched linen canvas primed for oil paintings.
    The label states that the canvas was first double-sized, then primed with zinc bound in linseed oil,
    then primed again with titanium. Should I be worried about using this canvas, because
    it was primed with zinc first?

    • Hello Martin,
      While there are some who claim that pre-primed canvases with zinc or titanium/zinc are fine, we have seen enough embrittlement from zinc pigment to err on the side of caution. Keep in mind, that zinc can cause issues even when it is buried under other layers of oil color. The zinc can cause other layers to become brittle and in some cases can push other layers off the surface!
      We hope this helps!


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