566 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



terial used for making gesso in that region. Gypsum has always been 

 a key material in the craft of the Italian painters. 



In the northern countries — England, France, and the Low Coun- 

 tries — calcite in the form of chalk served the same purpose as gypsmn 

 did in Italy. Lump chalk needed only grinding and bolting to produce 

 the white inert for the multiple-layer ground coat of artists' panels 

 and canvases. When examined at high magnification, a sample of the 

 wliite underlayer of a Flemish or Dutch painting often shows presence 

 of coccoliths, the calcareous skeletons of ancient minute marine organ- 

 isms which formed the layers of chalk in some bygone geological age. 

 In this way natural chalk can easily be distinguished from precipi- 

 tated chalk. 



The Chinese mural painters employed kaolin for the same purpose. 

 The Japanese, however, used mostly a lime wliite from burnt shells for 

 both undercoat and white pigment purposes. 



All these white inert pigments have been employed as the substrates 

 or carriers of vegetable and animal dyes in the production of organic 

 pigments, especially those with high tinctorial power, to give the dye 

 pigment body and some opacity. In modern paints, ground barite 

 is frequently used as an extender and bulking agent. Diatomaceous 

 earth, chalk, mica, talc, and even ground silica have special uses in 

 both artists' and commercial paints. 



A wide variety of minerals are used in making and coloring ceram- 

 ics, which are among the finest expressions of art ever conceived. All 

 the minerals of ceramics, however, are much modified in the ceramic 

 furnace, and in the finished product they appear as entirely new inor- 

 ganic species. Kaolin, feldspar, and quartz, when strongly heated, 

 are changed into compomids which correspond to sillimanite, cristo- 

 balite, mullite, and others. These are minerals which in nature are 

 produced in contact zones by chemical reactions and physical changes 

 like those which take place in the potter's kiln. 



Mosaic is a form of art which employs minerals in a quite different 

 way. The design is executed in small cubes or tessarae of mineral, 

 stone, or glass and fixed in a setting bed of plaster. Various colored 

 marbles and semiprecious stones are used to produce shading and 

 brilliance. Small tessarae of malachite and azurite have been seen on 

 Kussian and Byzantine icons done in fine mosaic (pi. 7, fig. 2). 



MINERAL ALTERATION PRODUCTS ON ANCIENT METALS 



Collectors of ancient bronzes admire those pieces which have ac- 

 quired a fine green or blue "patina." Patina is made of mineral 

 alteration products formed by reaction of copper in the aUoy with 

 corrosive agencies of air or earth (pi. 8, fig. 2). The most admired 

 bronze patinas are the adherent crusts of malachite or azurite inter- 



