The treatment of unfermented wine or must with partly dehydrated gypsum is a practice now commonly called "plastering." In this treatment a reaction occurs between the added calcium sulfate and the potassium bitartrate present in solution whereby calcium tartrate is precipitated and soluble tartaric acid and potassium sulfate are formed. The precipitated calcium tartrate then carries down various suspended impurities, thus greatly clarifying the must. The removal of the potassium bitartrate also makes the coloring matter more soluble, so that the color of the wine is improved. Moreover, the fermentation is rendered more rapid and complete, and the wine is said to keep better. In spite of these obvious advantages, however, the practice is somewhat objectionable, because potassium sulfate is left in solution in the wine. In some countries the addition of plaster of Paris is regarded as an adulteration of wine: either it is forbidden by law or a restriction is placed on the amount of potassium sulfate that may be present in the finished wine.
If the emendation in this passage is sound, as seems very likely, this remark of Theophrastus is the earliest known allusion to the practice of treating wine with gypsum.
67. And painters employ it for some parts of their art. Though finely ground calcium sulfate is a satisfactory white paint pigment, and chemical analyses show that it was sometimes used in antiquity for this purpose, at least in Egypt,516 no ancient author seems to include it among the colors used for painting. Hence it was probably not used to any great extent as a true paint pigment but only in the preparation of a white ground for painting. That it was actually so used in ancient Greek times is shown by some analyses of Rhousopoulos,517 who found that the white ground on painted Athenian lecythoi of the fifth century b.c. consisted of calcium sulfate.
67. and so do fullers, who sprinkle it on clothes. In section 64 Theophrastus has mentioned the use of Samian earth and of Tymphaic earth or gypsos for treating clothes. The primary
516 Lucas, Ancient Egyptian Materials and Industries, p. 399.
517 Diergart, Beitrage aus der Geschichte der Chemie dem Gedachtnis von Georg W. A. Kahlbaum, p. 181.
