The substance used by the enamellers as a polishing material is known by the name of rotten-stone, which is prepared by pounding, washing, decanting off the turbid water, suffering the fine suspend ed particles to subside from this water, and lastly levigating it upon a glass plate.
The work is then cemented to a square piece of wood, with a mixture of resin and brickdust, and by this means fixed in a vice.
The first operation of polishing is made by rubbing the work with rotten stone upon a small straight bar of pew= ter. Some delicacy is here required, to avoid scratching or producing flaws in the enamel by pressing too hard. In this way the piece is rendered perfectly even. But the last brilliant polish i,s given by a piece of deal wood and the same rotten-stone.
This is the general method of applying enamel ; but some colours require more precaution in the management of the fire. Opaque colours require less man agement than the transparent. A variety of circumstances must be attended to in transparent colours; every colour re quires gold of a particular fineness.
When different colours are intended to be placed beside one another, they are kept separate by a small edge or prominency, which is left in the gold for that purpose, and is polished along with the enamel.
The enamelling upon silver is effected nearly in the same manner as that of gold ; but the changes by the colours upon silver by toe ..ctaon of the fire are much more considerable than when gold is used.
Copper is not much used by enamel lers, on account of the difficulty which attends the attempt to fix beautiful co lours upon it. When this metal is used the common practice is, to apply a coat ing of opaque white enamel, and upon this other colours which are more fusi ble than the white.
A good effect is produced in toys by leaving part of the gold bare. For this purpose its surface is cut into suitable compartments by the engraver. This, however, is an expensive method, and is for this reason occasionally imitated, by applying small and very thin pieces of gold upon the surface of the enamel, where they are fixed by the fire, and af terwards covered by a transparent vi treous coating.
A method of taking off the enamel from any toy, without injuring the metal lic part, is often a desirable object. For this purpose a mixture of common salt, nitre, and ahem in powder, is applied upon the enamel, and the piece is put into the furnace. As soon as the fusion has taken place, the piece is to be sud denly thrown into water, which causes the enamel to fly off either totally or in part. Any part which may still remain is to be removed by repeating the same operation a second time.
To coat vessels of iron or copper for culinary purposes with an enamel capa ble of defending the metal from the ac tion of any solvent, and for enduring any heat, or transition from heat to cold, ap pears a desirable object; and many ex periments have been made on the sub ject by Mr. Soen Rinman of the Royal Academy of Stockhlom.
The following compositions he found answer very well on copper. 1. The white semi-transparent fluor spar and sul phate of lime, in equal quantities, pow dered, mixed, and calcinated in a white heat ; then powdered, made into a thin paste with water, and applied a little warm to the vessel, also warmed. Then dried and heated gradually to a certain point, a very strong heat, greater than is generally obtained in an assaying fur nace, is to be applied as quickly as pos sible. 2. Sixty parts of lime, one hun dred of fluor spar, sixty of gypsum, twenty of quartz, and one of manga nese, are calcinated, ground, and applied an a similar manner. 3. Four parts of fluor spar, four of gypsum, and one of li tharge, melted into a straw-coloured glass, ground and applied in the same way, required a much stronger heat. 4. Five parts of fluor spar, five of gyp sum, two of minium, two of flint glass, half a part of borax, the same of oxide of tin, and one-twenty-fifth of a part of oxide of cobalt, melted together, made an enamel, which, when ground and applied as the others. fused with a less degree of heat. This, M. Hinman imagines, would have been acted upon in length of time by sulphuric acid. The oxide of cobalt was prepared by saturating a solution of cobalt in aqua-fortis with common salt, and evaporating to dryness.