The lead white pigment used in Rublev Colours Lead White Artists Oil is a modern process lead white with fine particle size (under 45 microns), which is typical of all other lead whites manufactured today. The median particle size is about 5 microns, which is also typical of basic lead carbonate pigments used in all artistsí colors made today.
Natural Pigments manufactures a stack process lead white and will also release its Krems lead white later this year. Both of these lead white pigments have a much larger particle size range and heterogeneous particle shape than that of lead white pigments made according to modern processes.
Lead white can form soaps in drying oils, such as linseed oil, especially when free fatty acids are present in the paint vehicle and in the dried paint film. This causes a certain amount of dissolution of pigment particles, since the particles interact with fatty acids to form soaps (in a process known as saponification). This is the primary although not sole cause of the increasing translucency of lead white paint films. Usually the paint film achieves equilibrium and the pigment does not completely dissolve into soaps, unless the equilibrium is disturbed during its life through other chemical and mechanical changes.
This means small particles may or may not dissolve completely, depending on the amount of free fatty acids present in the paint.
The drying time of a pure lead white paint film is based on the type and amount of drying oil employed in the paint, and the purity of the basic lead carbonate. In addition, the chemical composition of basic lead carbonate is somewhat variable, so that lead white pigment containing a greater amount of lead hydroxide than the typical amount for basic lead carbonate (usually 25-35%) will cause the paint to dry faster. To a lesser degree, the particle size will also have an influence on the drying rate.
At present no company today makes a ready-made tube paint using stack process lead white, so to test the differences in drying times between stack process and modern process lead white, we prepared several samples of stack process lead white and modern lead white using the same oil and same ratio of oil to pigment. The modern lead white was obtained from a manufacturer in the U.S. and the stack process lead white was our own make. We found a small variance in the drying time between the samples of modern and stack process lead white. However, the drying time was more greatly affected by the type of oil and when we varied the ratio of oil to pigment, as once would expect.
[ 25. August 2011, 13:31: Message edited by: Admin ]