Rock crystal is compared by Pliny with the stone from which the vasa murrhina were made; the former, in his opinion, had been coagulated by cold, the latter by heat. Though the ancients, as we have seen, were acquainted with the method of colouring glass, yet they prized colourless glass highest on account of its resemblance to rock crystal; cups of it, in Pliny’s time, had supplanted those of silver and gold; Nero gave for a crystal cup 150,000 sistertii, or 625_l._
Very little has been handed down by the ancients respecting the processes of dyeing. It is evident, from Pliny, that they were acquainted with madder, and that preparations of iron were used in the black dyes. The most celebrated dye of all, the _purple_, was discovered by the Tyrians about fifteen centuries before the Christian era. This colour was given by various kinds of shellfish which inhabit the Mediterranean. Pliny divides them into two genera; the first, comprehending the smaller species, he called _buccinum_, from their resemblance to a hunting-horn; the second, included those called _purpura_: Fabius Columna thinks that these were distinguished also by the name of _murex_.
These shellfish yielded liquor of different shades of colour; they were often mixed in various proportions to produce particular shades of colour. One, or at most two drops of this liquor were obtained from each fish, by extracting and opening a little reservoir placed in the throat. To avoid this trouble, the smaller species were generally bruised whole, in a mortar; this was also frequently done with the large, though the other liquids of the fish must have in some degree injured the colour. The liquor, when extracted, was mixed with a considerable quantity of salt to keep it from putrifying; it was then diluted with five or six times as much water, and kept moderately hot in leaden or tin vessels, for eight or ten days, during which the liquor was often skimmed to separate all the impurities. After this, the wool to be dyed, being first well washed, was immersed and kept therein for five hours; then taken out, cooled, and again immersed, and continued in the liquor till all the colour was exhausted.[82]
To produce particular shades of colour, carbonate of soda, urine, and a marine plant called _fucus_, were occasionally added: one of these colours was a very dark reddish violet--“Nigrantis rosæ colore sublucens.”[83] But the most esteemed, and that in which the Tyrians particularly excelled, resembled coagulated blood--“laus ei summa in colore sanguinis concreti, nigricans aspectu, idemque suspectu refulgens.”[84]