Pliny wrote that different substances were distinguished by the name of "alumen", but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. Pliny says that there is another kind of alum that the Greeks call "schiston", and which "splits into filaments of a whitish colour", From the name "schiston" and the mode of formation, it appears that this kind was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists chiefly of sulfates of iron and aluminium. One kind of "alumen" was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors.