We have indicum also, a substance imported from India, the composition of which is at present unknown to me.[1908] Dyers, too, prepare an atramentum from the black inflorescence which adheres to the brazen dye-pans. It is made also from logs of torch-pine, burnt to charcoal and pounded in a mortar. The sæpia, too, has a wonderful property of secreting a black liquid;[1909] but from this liquid no colour is prepared. The preparation of every kind of atramentum is completed by exposure to the sun; the black, for writing, having an admixture of gum, and that for coating walls, an admixture of glue. Black pigment that has been dissolved in vinegar is not easily effaced by washing. CHAP. 26.—PURPURISSUM. Among the remaining colours which, as already stated,[1910] owing to their dearness are furnished by the employer, purpurissum holds the highest rank. For the purpose of preparing it, argentaria or silver chalk[1911] is dyed along with purple[1912] cloth, it imbibing the colour more speedily than the wool. The best of all is that which, being thrown the very first into the boiling cauldron, becomes saturated with the dye in its primitive state. The next best in quality is that which has been put into the same liquor, after the first has been removed. Each time that this is done, the quality becomes proportionally deteriorated, owing, of course, to the comparative thinness of the liquid. The reason that the purpurissum of Puteoli is more highly esteemed than that of Tyre, Gætulia, or Laconia, places which produce the most precious kinds of purple, is the fact that it combines more readily with hysginum,[1913] and that it is made to absorb the colouring liquid of madder. The worst purpurissum is that of Lanuvium.[1914] The price of purpurissum is from one to thirty denarii per pound. Persons who use it in painting, place a coat of sandyx beneath; a layer on which of purpurissum with glair of egg, produces all the brilliant tints of minium. If, on the other hand, it is their object to make a purple, they lay a coat of cæruleum[1915] beneath, and purpurissum, with egg,[1916] upon it. CHAP. 27.—INDICUM. Next in esteem to this is indicum,[1917] a production of India, being a slime[1918] which adheres to the scum upon the reeds there. When powdered, it is black in appearance, but when diluted in water it yields a marvellous combination of purple and cæruleum. There is another[1919] kind, also, which floats upon the surface of the pans in the purple dye-houses, being the scum which rises upon the purple dye. Persons who adulterate it, stain pigeons’ dung with genuine indicum, or else colour Selinusian[1920] earth, or anularian[1921] chalk with woad.