PULVERIZING THE INGREDIENTS.
Grinding.
The three ingredients are now ground separately to a very fine powder. The mills (vide [plate 1]) which effect this, and incorporate, are so similar, that a description will be given under the head of “Incorporation.” Screening.After being ground in this way, the saltpetre is passed through a slope cylindrical reel, covered with copper sieving wire of 60 meshes to the inch, which, as it revolves, sifts it to the required fineness, being then received in a box or bin underneath. The charcoal and sulphur are likewise passed through similar reels of 32 and 60-mesh wire respectively, and that which remains without passing through, is ground again under the runners. A very excellent machine has been invented by Mr. Hall, the engineer, of Dartford, for grinding charcoal, which makes a most useful addition to the Gunpowder factory. It consists of a conical drum, working in a conical box, on the same principle as a coffee-grinding machine, the axis being vertical. The mill is fed with charcoal by a hopper, and, as it passes through in fine powder, falls into a revolving reel, which sifts it in the same manner as before described, the whole being covered in, to prevent the great annoyance of dust, which was felt until lately, from the old charcoal mill. The three ingredients having been pulverized, are now fit for the mixing process.