The manufacture of stoneware vessels was known at a very early period of society. Frequent allusions to the potter’s wheel occur in the Old Testament, showing that the manufacture must have been familiar to the Jewish nation. The porcelain of the Chinese boasts of a very high antiquity indeed. We cannot doubt that the processes of the ancients were similar to those of the moderns, though I am not aware of any tolerably accurate account of them in any ancient author whatever.
Moulds of plaster of Paris were used by the ancients to take casts precisely as at present.[93]
The sand of Puzzoli was used by the Romans, as it is by the moderns, to form a mortar capable of hardening under water.
Pliny gives us some idea of the Roman bricks, which are known to have been of an excellent quality. There were three sizes of bricks used by the Romans.
1. Lydian, which were 1½ foot long and 1 foot broad.
2. Tetradoron, which was a square of 16 inches each side.
3. Pentadoron, which was a square, each side of which was 20 inches long.
Doron signifies the palm of the hand: of course it was equivalent to 4 inches.
Pliny has given a pretty detailed description of the precious stones of the ancients; but it is not very easy to determine the specific minerals to which he alludes.
1. The description of the diamond is tolerably precise. It was found in Ethiopia, India, Arabia, and Macedonia. But the Macedonian diamond, as well as the adamas cyprius and siderites, were obviously not diamonds, but soft stones.
2. The _emerald_ of the ancients (_smaragdus_) must have varied in its nature. It was a green, transparent, hard stone; and, as colour was the criterion by which the ancients distinguished minerals and divided them into species, it is obvious that very different minerals must have been confounded together, under the name of emerald. Sapphire, beryl, doubtless fluor spar when green, and probably even serpentine, nephrite, and some ores of copper, seem to have occasionally got the same name. There is no reason to believe that the _emerald_ of the moderns was known before the discovery of America. At least it has been only found in modern times in America. Some of the emeralds described by Pliny as losing their colour by exposure to the sun, must have been fluor spars. There is a remarkably deep and beautiful green fluor spar, met with some years ago in the county of Durham, in one of the Weredale mines that possesses this property. The emeralds of the ancients were of such a size (13½ feet, large enough to be cut into a pillar), that we can consider them in no other light than as a species of rock.