Coloring Agents for Greek Glazes

Abstract

number of papers have been written on classical black glaze (see references at end of article: 1,2,3) and the intentional red glaze of the fifth century has been successfully reproduced (4). Very little technical study has been undertaken, however, for prehistoric pottery or even for Corinthian. During a year in Greece refiring experiments on sherds from different sites to determine the technique of manufacture were made by one of us (M.F.), and a representative collection was selected for further analytical study in America. technical history of decorations on pottery is mainly a history of raw materials and firing techniques for, to a large extent, the ancient Greeks used iron-bearing clays to achieve their multi-color effects. Iron-bearing clays used for glaze are, if reduced, black; if partially reduced or, for certain clays, when fully oxidized, brown; if oxidized, purplish, red, yellow or creamy white, according to the amount of iron present and, generally to a lesser extent, according to other impurities present in the clay. It should be noted, however, that if any appreciable amount of manganese is present in the clay, the color is always black, brownish-black or brown, whether the firing atmosphere is reducing or oxidizing. This variation in manganese color is influenced by the percentage of iron present, as well as by the percentage of manganese and possibly by minor impurities. Thus, parallel to multi-color effects obtained by controlled or even by primitive firing, there are also multi-color effects obtained by using manganese clays or ores for the black and iron-bearing clays for the fabric and the red. This particular and easier method of achieving red and black together was used over a large geographic area and for a considerable length of time. Manganese was, for example, used very extensively in Thessaly, particularly during the Neolithic II Period, but its use can be followed east during later times and examples as late as the sixth century B.c. have been found at Gordion (pl. 87: 12). In writing about surface decorations and finishes on pottery one is always faced with the problem of what to call them. It is inconsistent and sometimes confusing to call the surface finishes of certain periods paint and ot er periods It is possible to raise many objections to either term, but Attic black as well as some others may correctly be called glaze or more properly Greek The black iron pigment is bound by a glassy p ase which protects it fr m further oxidation and the glaze has a vitreous luster. It is true, however, that it does not melt completely during firing, as, for instance, a lead glaze does, but only partially melts or sinters and thus seals in the black iron pigment and pr tects it from reoxidation to the red iron pigment. For the sake of consistency all black or red surface coating, whether of settled clay or other minerals, which re applied to the pot for decorative purp ses, are called glaze by us. While this paper is concerned primarily with the decorations o pottery rather than with the fabric, the fabric cannot be ignored entirely for the two are closely related. A good clay makes a good potter, a d no ne in ancient times had a better clay than the classical Athenian potter. This clay appears to be similar to the clay now being used at Amarousi. U ed as it comes, or partially settled to remove some of the coarser sand and other minerals and mixed with a white cl y to reduce its plasticity and make it more workable, it fires a good red. The very finest particles make an excellent glaze. As can be shown by x-ray diffraction, this fraction is high in illite, a mica-like clay mineral whose plate-like structure makes it a good reflector of light and gives the black glaze its characteristic luster. Since both glaze and fabric are made from the same clay, the two have similar coefficients of expansion and the glaze adheres firmly. Among the ancient potters, the Corinthians had as unsatisfactory clay as any and yet they were able to circumvent almo t insoluble difficulties and to produce a fairly satisfactory ware over a long period of time. All the clay in the neighborhood of Corinth appears to be calcareous and unsuitable for glaze. That the ancient Corinthians used clays high in