MITRA Forum Question Details

The primary mechanism behind increased transparency involves the formation of lead soaps which is due to the complexes formed between the free fatty acids in the medium and the lead carbonate. There are secondary mechanisms as well but they play a lesser role. So fatty acids are in abundance in both oils and alkyds. Transparency can therefore occur more readily in oil films (and probably alkyds) that contain lead white. Increased transparency can also occur in egg tempera but to a much lesser degree. Because shadows in dark areas would inherently contain much less lead white they are unlikely to become more transparent beyond the slight change in refractive index of the binder. Even this change would be somewhat mitigated by the fact that dark transparent layers tend to be "fattier" and therefore tend to darken over time....this counteracts to some degree any increased transparency that may occur. Finally, one might then ask: why do we now see underdrawing under areas of red lake when it was likely not initially visible? This is usually because painters that exploited this technique (early Netherlandish/Flemish painters) often covered their underdrawings with a lead-white containing imprimatura layer, a layer which over time eventually becomes more transparent. This is yet another thing to consider when loading your oil paints with lead driers...to my knowledge there has been no research into whether an abundance of lead ions from driers within an oil or alkyd matrix would lead to an increase in transparency...but the chemistry behind this mechanism would be the same and therefore leads me to believe that it very well could.

Lead is the primary source of chemical, as opposed to physical, increased transparency in old master paintings. However, zinc oxide also forms metal soaps that can increase transparency, as can probably other reactive metals in pigments, but to a much lesser degree. So, in the absence of these issues, chemically induced increased transparency is the result of a slight elevation in the refractive index of the binder over time. This is certainly a secondary and a lesser cause.

Probably, one should not worry too much about this. Frankly, you probably see more of a change when you varnish your egg tempera paintings. I would still caution against working on very dark grounds where a slight increase in transparency would create a profound change, unless that process is necessary to your aesthetic aims.

I should also mention that there are also physical causes for this visual defect. Lower layers are sometimes more visible than originally painted due to the abrasion of the surface layers from poor restoration procedures (eg some of the overly green faces seen on early Italian tempera paintings). This can be easily avoided by proper care and storage and only using qualified conservators to conserve one’s artwork.