Enamelling of Cast-Iron

Wagner in his ‘Chemical Technology’ gives the following account of this proce :—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 po ible, 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 gla , 20 1 ⁄ 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 ve els is frequently of a le harmle kind than that described by Wagner. Mr Tatlock states that in some instances the milk-white porcelainous enamel, with which cast-iron cooking ve els 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 di olved out in large quantity during cooking proce es. Mr Tatlock gives the analysis of three samples of enamel from the interior of three cast-iron pots obtained from different manufacturers. These iron ve els 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 di olved out. He demonstrated that the ratio of the bases to the silica in No. 1 was 1 to 1·58; in the No. 2, as 1 to 0·79; and in the No. 3, as 1 to 0·76. A one per cent. solution of citric acid boiled in the No. 1 did not affect it in the slightest, while in the case of the No. 3, the gla y surface of the enamel was at once roughened and destroyed, and lead di olved out to such an extent as to give immediately a dense black precipitate with sulphuretted hydrogen. He thought that no enamel was fit to be used unle it were totally unaffected by boiling with a one per cent. solution of citric acid, which was a very moderate test, and gave it as his opinion that either the use of such poisonous ingredients as lead and arsenic in large quantity should be entirely abandoned, or that the composition otherwise of the enamel should be of such a character as to ensure that none of the poisonous substances could be di olved out, in the circumstances under which the enamelled ve els are used.