These articles are manufactured at Liverpool, by persons who make it their sole occupation, in the following manner:
They take crystals of sulphate of iron (green vitriol or copperas), immediately from the crystallizing vessels, in the copperas works there, so as to have them as clean as possible; and instantly put them into crucibles or cast iron pots, and expose them to heat, without suffering the smallest particle of dust to get in, which would have a tendency to scratch the articles to be polished. Those portions which are least calcined and are of a scarlet color, are tit to make rouge for polishing gold or silver, while those which are calcined or have become red-purple or bluish-purple, form crocus fit for polishing brass or steel. Of these, the bluish-purple colored parts are the hardest, and are found nearest to the bottom of the vessels, and consequently have been exposed to the greatest degree of heat.
Dissolve crystals of sulphate of iron in water; filter the solution to separate some particles of silex which are generally present, and sometimes are abundant; then precipitate from this filtered solution the protoxide of iron, by the addition of a saturated solution of soda, which must also be filtered. This grey oxide is to be repeatedly washed and then dried; put it in this state into a crucible, and very gradually raise it to a dull red heat; then pour it into a clean metal or earthen dish, and while cooling it will absorb oxygen from the atmosphere, and acquire a beautiful dark red color. In this state it is fit for polishing the softer metals, as silver and gold, but will scarcely make any impression on hardened steel or glass. For these latter purposes I discovered that it is the black oxide that affected the polish (and this gives to the red oxide a purple hue, which is used as the criterion of its cutting quality in ordinary), therefore for polishing the harder materials the oxide must be heated to a bright red, and kept in that state until a sufficient quantity of it is converted into black oxide to give the mass a deep purple hue when exposed to the atmosphere. I have converted the whole into black oxide; but this is liable to scratch, and does not work so pleasantly as when mixed with the softer material. The powder must now be levigated with a soft wrought iron spatula, upon a soft iron slab, and afterwards washed in a very weak solution of gam arabic, as recommended by Dr. Green in his paper on specula. The oxide prepared in this manner is almost impalpable, and free from all extraneous matter, and has the requisite quality in an eminent degree for polishing steel, glass, the softer gems, etc.
"I prepare the peroxide of iron by precipitation with water of ammonia from a pure dilute solution of sulphate of iron; the precipitate is washed, pressed in a screw press till nearly dry, and exposed to a heat which in the dark appears a dull low red. The only points of importance are, that the sulphate of iron should be pure, that the water of ammonia should be decidedly in excess, and that the heat should not exceed that I have described. The color will be a bright crimson inclining to yellow. I have tried both soda and potash, pure, instead of water of ammonia, but after washing with some degree of care, a trace of the alkali still remained, and the peroxide was of an ochrey color till overheated, and did not polish properly."