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It has already been written quite a lot about problematic nature of Zinc white in oil. In case one is not sure about partcular oil color composition (e.g. no pigment info on a tube, or no longer existing company, or just customer service not answering to questions), is there any way how to detect the presence of zinc white?
Kremer pigmente offers various chemicals https://www.kremer-pigmente.com/en/shop/solvents-chemicals-additives/chemicals/?c2V0TnVt=YWxs
Is it possible to use some sort of chemical (liquid, powdered) which, when mixed with paint, will react with zinc white in a certain way (for example the hue of paint will change) and therefore prove its presence even if it is present in small amount?
Move this whole section up, swapping places with the section above it.
Let me consult with some fellow conservators about this. I
did want to first sat that white the list of chemicals sold by Kremer is impressive,
the all appear to be related to creating art materials (dyes, inks, laking
pigments, grounds, etc), creating art (materials for printmaking, etc) and a
large number for use by restorers/conservators.
We cannot think of anything that would work with wet oil
paint. The following fluorochrome stain works wonderfully. The problem is that
is used to stain cross-sections to be viewed under the action of UV light and
the UV induced visible fluorescence shows the zinc contained paint layers. You
would really need a ton of it if applying to films of dried oil paint on a
macro scale and viewing with a hand-held UV light.
The stain is described here: https://en.wikipedia.org/wiki/TSQ
Sorry, beyond that you would turn to viewing dispersed
pigment samples using a polarize light microscope, from the up there is analysis with X-ray
fluorescence analysis, and finally Scanning Electron Microscopy with Energy
Dispersed Spectroscopy. These and other more rarefied instruments are likely
way above any budget you would expend on this.
Perhaps there is an additional, less complicated method, I just have not thought of it.
What about a spot test for zinc oxide?
Dilute sulphuric acid is added to a solution of potassium dichromate (1% by weight, 50ml) until the pH reaches approximately 1·6. Silver nitrate solution (0·125M, 10ml) is added slowly with vigorous stirring. After two hours, the supernatant liquid is decanted from the small amount of precipitate formed and the reagent is ready to use. No special storage precautions are necessary and the reagent has a ‘shelf life’ of at least a month; the loss of efficacy of the solution is indicated by the appearance of a dark red precipitate of silver chromate.
Antoni Palet Casas & Jaime de Andrés Llopis (2002) A Spot Test for Zinc White, Studies in Conservation, 47:4, 273-276.
To link to this article: https://doi.org/10.1179/sic.2002.47.4.273
George, you are right and thanks. This procedure does require some toxic materials and others which
would require a purchase from Sigma or other chemical supply house. Not
everyone is comfortable or capable of pulling this off. For others, it would be
a cake walk.