The iron doors of such an oven open into the room where the varnishing operations are carried on, so that the atmosphere of this room in which the men work is always more or less charged with irritating vapour. Such an oven as this, if ventilated at all, is ventilated by means of a pipe proceeding from it through the roof of the building.

The process of enamelling leather or pasteboard differs very little from that of making table - covers. The enamel is laid on by means of the scraper before mentioned, and by rubbing it gently on by the hands of the workman. The material is then heated in an oven such as has been described, but so arranged that the material is slid in horizontally upon the sliding shelf on which the varnishing is performed. The varnishing and heating are repeated as often as may be necessary to ensure the proper thickness of enamel.

Brattice cloth, for use in mines, is made at Marsden's factory at Manchester by covering coarse hempen cloth on both sides with a coarse black paint or varnish made of boiled oil and lampblack. It is then dried in a chamber heated to about 120° F. (49° C.) by means of steam pipes. The evolution of irritating vapour is such that it is said that the windows have to be left open for ventilation for 2 hours before the workmen can enter to remove the cloth.

Such trades as these, unless carried on with due precaution, are apt to give rise to almost intolerable nuisance. The two ordinary sources of nuisance are the manufacture of the varnish and the escape of the pungent acrolein vapours from the drying - rooms and ovens into the external atmosphere. In those works where the only ventilation of the oven is into the workshop itself, when the oven door is opened, although the workmen suffer from the vapour more than they ought to do, the neighbourhood suffers less, probably because much of the vapour is condensed by the cool air of the workshop. The mode of preventing that part of the nuisance which proceeds from the manufacture of the varnish, has already been described. At the Leather Cloth Co/s Works at Stratford, the nuisance proceeding from the escape of vapour from the drying - rooms has been thoroughly obviated by carrying a 10-in. iron pipe from within each room, at its outer wall, down the outside of the building, to a main underground flue, which enters the main flue of the boiler furnace at a point where the flame can reach the vapour, and consume it before it is discharged from the chimney - shaft.

A fan may be used to assist the draught.

Metals

In enamelling metals, the enamel is fused by heat upon the surface of the object, and is incorporated by fusion with its surface. Enamel for metals must therefore be indestructible by heat. There are two kinds of enamel, the transparent and opaque; the first is the base of all the coloured enamels, which are produced by adding some metallic oxide to this transparent flux. The transparent enamel is produced by fusing the following materials, which are first ground, then dried, fused, and again ground for use:-3 parts silicious sand, 1 chalk, 3 calcined borax; or, 3 parts broken crystal glass, 1 calcined borax, 1/4 nitre, 1 diaphoretic antimony (well washed). Dead white enamel, or calcine, is produced by calcining 2 parts zinc and 1 of lead together. The calcine, or combined oxides, is mixed up with crystal and manganese in the proportion of 1 part combined oxides or calcine, 2 fine crystal (glass) powder, 40 manganese. These are ground together and fused; when fusion is completed, the vitreous mass is poured into water, ground, and fused anew, and this operation is repeated several times, and much care must be exercised, for the smallest portion of oxide or copper will spoil the enamel completely.

Other colours are obtained by adding to the transparent ground enamel the following materials, according to the colour desired: - Blue enamel, by adding oxide of cobalt or some of its combinations, with the addition of a little nitre. Black enamel, by peroxide of manganese or iron and a little cobalt. Clay produces, with about J protoxide of iron, a fine black enamel (clovet). Yellow enamel, by phosphate or sulphate, or some preparation of silver, or oxide of lead and antimony. Thus, 1 part white oxide of antimony, 2 to 3 white lead, 1 alum, 1 sal - ammoniac. Green enamel is obtained directly from oxide of copper or oxide of chromium. Red enamel is more difficult to secure than many others. The protoxide of copper is used in a hydrogen or carbonaceous flame, so as to keep the copper from peroxidizing, or the preparations of gold, e.g. the purple of Cassius. Violet enamel: by adding peroxide of manganese and a little nitre, any shade of violet to amethystine colour, or even black, can be obtained.

Enamelling is performed in an oven or by lamp. (Building News).

Copper

In Germany and France the following process has lately come into use, especially for enamelling copper culinary vessels:-12 oz. white fluorspar, 12 oz. gypsum, 1 oz. borax, are finely powdered, intimately mixed, and fused in a crucible; the mass produced is poured out, allowed to cool, and rubbed up to a paste with water. The paste is then brushed over the inside of the vessel to be enamelled; after thorough drying, the vessel is gradually heated till the enamel fuses. The coating thus produced is firmly adherent to the copper, is white and opaque, does not easily chip off, and is proof against vegetable acids. It also gives a beautiful alabaster surface for ornamental purposes. (Ding. Polyt. Jl.)