Black pottery

Touch needle A small bar of gold, either pure, or alloyed with silver in a known proportion, for trying the fineness of a gold or sliver article by comparing the streaks made by the article and the bar on a touchstone. Touch paper 2059 Paper impregnated with potassium nitrate. It bums steadily without flame and is used for Igrrltlng fireworks. Saltpeter paper. Touchstone 3190 A stone or piece of black pottery used for assaying. 

Primary clay, for example kaolin, is colorless, and when such clay is heated to a high temperature it produces white ceramic materials. Most pottery, however, is colored its color is due to the fact that most of it was, and still is, made not from primary but from secondary clay. Secondary clay contains minerals other than clay, and colored metal ions in them endow the pottery with their color. Iron ions (in iron oxides), for example, tend to make pottery yellow, brown, or red, and manganese ions (in pyrolusite, a mineral composed of manganese oxide) make it either dark or black. 

Clark, R. J. H., Curri, L., Henshaw, G. S., and Laganara, C. (1997). Characterization of brown-black and blue pigments in glazed pottery fragments from Castel Fiorentino (Foggia, Italy) by Raman microscopy, X-ray powder diffractometry and X-ray photoelectron spectroscopy. Journal of Raman Spectroscopy 28 105-109. 

Sabbatini, L., Tarantino, M. G., Zambonin, P. G., and De Benedetto, G. E. (2000). Analytical characterization of paintings on pre-Roman pottery by means of spectroscopic techniques. Part II Red, brown and black colored shards. Fresenius Journal of Analytical Chemistry 366 116-124. 

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