Prep. 1. From ‘frit’ or ‘flux,’ fused with oxide of lead, and a little red oxide of iron.
2. Lead, tin, ashes, litharge, antimony, and sand, of each 1 oz.; nitre, 4 oz.; mix, fuse, and powder; and add the product to ‘flux’ or ‘frit,’ q. s.
3. White oxide of antimony, alum, and sal-ammoniac, of each 1 part; pure carbonate of lead, 1 to 3 parts, or q. s. (all in powder); mix, and expose them to a heat sufficiently high to decompose the sal-ammoniac. Used as the last. Very bright coloured.
4. (Wynn.) Red lead, 8 oz.; oxide of antimony, and tin, calcined together, of each 1 oz; mix, and add of ‘flux’ (No. 5), 15 oz.; mix well and fuse.
5. Pure oxide of silver added to the metallic ‘fluxes.’ The salts of silver are also used, but are more difficult to manage. If a thin film of oxide of silver be spread over the surface of the enamel to be coloured, exposed to a moderate heat, then withdrawn, and the film of reduced silver on the surface removed, the part under will be found tinged of a fine yellow. (Clouet.)
Enamelling of Cast-Iron. Wagner in his ‘Chemical Technology’ gives the following account of this process:—The surface of the cast-iron to be enamelled is first carefully cleaned by scouring with sand and dilute sulphuric acid, next a somewhat thickish magma, made of pulverised quartz, borax, feldspar, kaolin and water is brushed over the clean metallic surface as evenly as possible, and immediately after a finely powdered mixture of feldspar, soda, borax, and oxide of tin, is dusted over, after which the enamel is burnt in by the heat of a muffle. In France an enamel is applied which consists of 130 parts of flint glass, 201⁄2 parts of carbonate of soda, and 12 parts of boric acid fused together, and afterwards ground to a fine powder.
It would appear, however, from the statements contained in a paper read by Mr Tatlock, F.R.S.E., F.C.S., that the enamel used for iron vessels is frequently of a less harmless kind than that described by Wagner. Mr Tatlock states that in some instances the milk-white porcelainous enamel, with which cast-iron cooking vessels are now so commonly prepared, has a composition such as to render it highly objectionable, on account of the facility with which it is acted upon by acid, fruits, common salt, and other ordinary dietetic substances, by which means lead, and even arsenic, are dissolved out in large quantity during cooking processes.
Mr Tatlock gives the analysis of three samples of enamel from the interior of three cast-iron pots obtained from different manufacturers. These iron vessels were all employed for cooking:—
| No. 1. per cent. | No. 2. per cent. | No 3. per cent. | |
| Silica | 61·00 | 42·40 | 42·00 |
| Alumina | 8·00 | 2·88 | 6·06 |
| Oxide of iron | 1·10 | 2·04 | 4·04 |
| Lime | 3·02 | 0·16 | 0·78 |
| Magnesia | 0·28 | 0·10 | 0·21 |
| Oxide of lead | absent | 25·89 | 18·48 |
| Potash | 5·61 | 7·99 | 6·46 |
| Soda | 20·67 | 14·67 | 19·25 |
| Phosphoric acid | trace | trace | trace |
| Arsenious acid | 0·02 | 0·42 | 1·02 |
| Carbonic acid | 0·30 | absent | absent |
| Borax | absent | 3·45 | 1·70 |
| ——— | ——— | ——— | |
| 100·00 | 100·00 | 100·00 | |
| ——— | ——— | ——— | |
| Total bases | 38·58 | 53·73 | 55·28 |
The author showed that it was not so much on account of the presence of large proportions of lead and arsenic that the enamels are so dangerous, but because they are so highly basic in character, that they are acted upon with facility by feebly acid solutions, the lead and arsenic being thereby easily dissolved out.