RrnSTERALS IN ART AND ARCHEOLOGY — GETTENS 565 



mineral, either malachite or cuprite. The product, bright blue crys- 

 talline copper calcium silicate, is called "Egyptian blue." It was 

 the most widely used blue pigment of Classical times, and it is found 

 on ancient paintings from Mesopotamia in the East to Roman sites in 

 Scandinavia in the West. It is strange that this product seems to have 

 no counterpart in nature, although several copper silicate minerals are 

 well known. 



The lead minerals, cerussite (lead carbonate) and hydrocerussite 

 (basic lead carbonate), are not uncommon and were known to the 

 ancients, but they apparently never served as pigments. White lead, 

 the artificial counterpart of liydrocerussite, has been made artificially 

 probably since it was known how to win lead metal from galena. Like- 

 wise, zinc oxide and titanium oxide, both widely used by artists, are 

 made artificially, the former mostly from sphalerite and the latter 

 from ilmenite. 



Like the sculptor, the painter has had to go to the earth for the ma- 

 terials of his craft. The painter can bring together on a single panel 

 or canvas the minerals and mineral-derived products of several coun- 

 tries and climes. In order to express fully his ideas it may be neces- 

 sary for him to assemble on his palette minerals and chemical com- 

 pounds that represent as many as three dozen of the chemical elements. 

 In addition to paint pigments, the painter has other mineral needs. 



NONMETALLIC MINERALS IN ART 



There is still another class of minerals which has always played an 

 important if less glamorous role in the creation of art. Among paint 

 manufacturers they are called "inert pigments," but others call them 

 nonmetallic minerals. They are mostly white, or nearly white, bulk 

 minerals with low refractive index and include such familiar minerals 

 as gypsum and anhydrite, chalk, China clay, and others. 



In Italy, gypsum and anhydrite were abundant, and in addition to 

 their use in sculpture they were employed to make gesso (It. = gypsum) 

 wliich was used with glue binder to undercoat wood panels, picture 

 frames, and furniture to ready them for painting and gilding. Eaw 

 gypsum, which is calcium sulphate dihydrate, requires heating to a 

 temperature only slightly above the boiling point of water to dehy- 

 drate it and permit it to be ground easily to a fine powder. The prod- 

 uct, calcium sulphate hemihydrate (plaster of paris), can be used 

 directly for casting purposes, and for m.ortars and plaster finishes. 

 Some of it is especially refined to make gesso a oro^ or gilding base 

 for use on the wood of furniture or picture frames. Microscopic 

 examination of the gesso undercoating or ground of most Tuscan 

 paintings shows that it rontains mostly fine particles of anhydrite. 

 Apparently an impure gypsum-anhydrite mixture was the raw ma- 



