Alum, Ro′man

Syn . Red alum* , Roach A. , Roche A. , Rock A.* ; Alu′men Roma′num , A. ru′brum , A. ru′peum , ., L.; Alun Romain , A. de roc he , Fr.; Alume di rocca , It. In small fragments, covered with a reddish powder ( ALUMEN RUBRUM VE′′RUM ); originally imported from Civita Vecchia, where it occurs native. It is much esteemed by dyers from being nearly free from iron-alum. That now sold for it in England is ordinary alum coloured with Venetian red, Armenian bole, or rose-pink ( ALUMEN RUBRUM SPU′′RIUM ). This is done by shaking the fragments in a sieve over a ve el of hot water, and then stirring them up with the colour, until the surface is uniformly tinged with it. In genuine roach-alum the colour not only covers the surface, but also partially pervades the substance of the crystals. The name was formerly also applied to a pure white variety of alum, prepared at Tolfa; but it is now, in English commerce, exclusively given to common alum artificially coloured.