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Book VII marbles, gems in rings and other applications

Book VII marbles, gems in rings and other applications Page of 251 Book VII marbles, gems in rings and other applications Text size:minus plus Restore normal size   Mail page  Print this page
164
DE NATURA FOSSILIUM
added to one, pitch to the other. A lump of lime is first quenched in wine then crushed in a mortar with lard and either the sap of the fig tree or pitch. That made with pitch is darker and readily distinguished from the other. According to Pliny when a surface is coated with maltha, oil is rubbed over it. Metal workers also have a genus of maltha with which they fill cracks in the bottom of large crucibles so that the molten metal will not be lost through them. If the crucible is made of ash the molten metal produces flaws that eventually break it. Fresh maltha will hold molten metal just as old maltha holds water. The maltha of the metal workers is made from unslacked lime, ox blood and meal.
Lithocolla differs in composition and use from the two older malthas. The former is made from powdered marble and beef glue and is used to cement fragments of marble or rock to the parent mass. Today two kinds are in use each containing powdered marble or rock similar to the frag­ments of marble or rock that are to be cemented. White of egg is added to one variety, pitch to another. Some people add other ingredients. Lapidaries have their own variety that is used to cement rough stones to the dorp. This is usually made from powdered brick and pitch.
Since I have discussed sand, lime, maltha and lithocolla I should say something about artificial stones. There are two genera of these. One is made from stones and lacks a name, the other is made from earth and is called later (brick). The former can be made from any stone by simply giving it a new shape although the original color is lost. Thus one large stone can be made from a number of small ones and a small stone from several smaller ones as aetites, from the small pebbles found in a geode and flint from small pieces of flint. Similarly a whetstone can be made from silicious fragments to which silver or other materials are added. First the stone is crushed to a powder in a sandstone or stone mortar and then egg white, linseed oil and juniper gum is mixed with the powder. The mass can be molded to any form. If a large amount of egg white is added to the mixture it dries quickly and if a large amount of gum the rock is harder. Some of these stones can be colored by adding a pigment although if they are made with pitch they are always black.
Bricks are made from earth. They are usually made from whitish marl or red ocher. Luneburg brick is made from an unctuous aluminous earth. Good bricks are made from a pumiceous earth at Sandarlik, Asia Minor, and Calentus and Cartagena, Spain. These will float on water. In olden times bricks were made from earth to which sand, male sand, had been added. No matter what variety of earth is used first it is moistened with water. The bricks are then molded from this mud. They are usually made in spring and fall so they will dry during one uninterrupted period. If they are made in summer, according to Vitruvius, they are usually defec­tive since the sun, when directly overhead, burns so strongly the bricks shrink and soon have the appearance of being dry and yet are moist in-
Book VII marbles, gems in rings and other applications Page of 251 Book VII marbles, gems in rings and other applications
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