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26
DE NATURA FOSSILIUM
used, those which are soft to intermediate are given preference over the hard. All must be unctuous. Unctuous and dense clays are used to make the crucibles and scorifiers which are not affected by fire and are used in refining ores and metals. These clays are also used to make vessels which neither absorb nor exude liquids. Such vessels are used in mixing and storing the solutions for parting gold and silver. Dense clays are more difficult to work by hand than the intermediate and porous varieties. Harder clays, although more tractable, require a great deal more work to prepare than soft clays. Vessels that do not absorb liquids are made not only from dense clays but also from sand which is mixed and burnt. These vessels such as those from Waldenburg are in great demand by pharmacists for holding liquids and syrups since they last longer than others. These also withstand fire for long periods of time.
Some containers that absorb and even exude liquids are made from unctuous porous clays. These are the only pottery that are not completely burned. They cannot stand a high heat for when the moisture is com­pletely driven off and their unctuousness destroyed they develop flaws and cracks and may even be reduced to a powder. For this reason potters place amphorae, deep dishes, and small pots made from this class of clay in the sun to dry and after decorating them with various colors sell them in the unburned state. When sand and straw are mixed with this class of clay before burning it does not break so easily in the furnace. It is a common practice to mix sand with any clay that is available and use this mixture in making cooking vessels. Loose textured clays, even though mixed with sand, become porous during firing and will exude liquids. The jugs from Gislan, from which we drink new wine, are made by adding sieved sand to clay and thoroughly mixing the two by treading. Clays that are naturally sandy will be discussed later. A little dry sand is often added to Waldenburg pottery before firing to make it rough with the result that some of this pottery has the appearance of a sea urchin.
Since unctuous clays are not of a uniform color the pottery made from them also varies in color. Some clay is white and because of this whiteness it is called άργιλλο? by the Greeks, argilla by certain Latin writers. The Eisleben clay that sparkles with silver-mica belongs to this class. Some clays such as the Waldenburg are grayish-white. The triangular crucibles used by people who coin gold and silver are made from gray clay from Ipsa. Yellow clay is found at Midbeida. Red clay occurs in the silver veins at Annaberg and this is used in making crucibles and scorifiers. Black clays are found at Glogovian in Ligyes. Some clays have variegated colors, for example, the white and yellow clay of Midbeida.
A potter will choose first one clay then another for making different kinds of pottery but the sculptor always chooses an easily worked clay. Potters have made countries, islands, and towns famous, for example, the pottery from Murviedro, Spain; Polenza, Asta, Modena, and Arezzo, Italy; the goblets from Sorrento, Italy; the dishes of Reggio and Cumae,