Nine Books on the Nature of Things

Let us continue to explain how the mortification of metals is effected. First, iron is mortified and reduced to crocus in this way. Beat steel into extremely thin plates, heat them until glowing and quench them in the best wine vinegar. Repeat until the vinegar has acquired a striking redness. When you have enough red vinegar, combine it all, distil off its moisture and coagulate the residue into dry powder. This is a very excellent crocus of Mars. There is another way of making crocus of Mars, partly surpassing the first and requiring far less cost and labour: layer extremely thin steel plates with equal quantities of sulphur and tartar, then reverberate. This produces a very fine crocus, which must be removed from the plates.