Zinc White (PW4) issues

In general, I caution against using zinc white with any medium that contains free fatty acids if there are reasonable alternatives. This includes drying oils, egg yolk, mixtures of those two, and possibly even encaustic. This is probably overreaching and there are times when titanium white is not appropriate. It also suffers from deficiencies, especially in drying oils where it makes a very poor film. This does create a situation where there are compromises either way.  I will contact individuals in the industry to give their perspective.

​We have not seen any studies that indicate problems with zinc oxide in egg tempera, but caution is appropriate based on the formation of zinc soaps in lipids and fatty acids. In addition, the use of zinc oxide in egg tempera has not been as widespread as in oils, so we may not be aware of any issues at this time. The precaution would be more important with tempera grassa since this medium employs drying oils.

​At Golden we havn’t done any research into egg tempera-based zinc paint. But one study that pertains to your question was published last year, where it was found that metal soaps are more crystalline in water based egg tempera paint layers. The water that evaporates from a drying egg tempera paint film creates microscopic pin holes and the porosity of the paint layer can help the growth of crystalline soaps. For this study they used a 15th century panel painting that consisted of both egg tempera and oil layers, but no zinc, only lead, calcium and copper soaps. ( https://www.researchgate.net/publication/330290821_Identification_and_distribution_of_metal_soaps_and_oxalates_in_oil_and_tempera_paint_layers_in_15th_Century_altarpieces_using_synchrotron_radiation_techniques)