The surface of the iron is cleaned from scale by vitriol or sulphuric acid, and then scoured with sand. It is now coated with a strong solution of chloride of zinc, and dipped into melted tin. The tin will instantly adhere to every spot that is clean.
The chief point which requires attention in this matter is that the tinning of iron in the cold cannot succeed at all, unless the bath contains, in solution or suspension, an organic substance like starch or glucose, although no precise scientific explanation of this indispensi-ble condition has been hitherto given. To 100 litres of water are added 3 kilos, of rye meal; this mixture is boiled for half an hour, and next filtered through cloth; to the clear but thickish liquid are added 106 kilos, of pyrophosphate of soda, 17 kilos, of protochloride of tin in crystals (so-called tin-salt), 67 kilos, of neutral protochloride of tin, 100 to 120 grms. of sulphuric acid; this liquid is placed in well-made wooden troughs, and serves more especially for the tinning of iron and steel wire (previously polished) for the use of carding machines. When instead of the two sorts of tin just named, cyanide of silver and cyanide of potassium are taken, the iron is perfectly silvered.
A Bavarian serial contains a method of brightening iron recommended by Boden. The articles to be brightened are, when taken from the forge or the rolls, in the case of such articles as plates, wire, etc., placed in dilute sulphuric acid (1 to 20), cleansing the articles, which are then washed clean with water and dried with sawdust. They are then dipped for a second or so in nitrous acid, washed carefully, dried in sawdust and rubbed clean. It is said that iron goods thus treated acquire a bright surface, having a white glance, without undergoing any of the usual polishing operations. This is a process that those interested can easily test for themselves. Boden states that the action of the sulphuric acid is increased by the addition of a little carbolic acid, but it is difficult to see what effect this can have, and it may very well be dispensed with.
Rub lightly with a sponge or rag dipped in diluted sulphuric, nitric, or hydrochloric acid. When the discoloi*ation is removed, carefully wash the article, dry it by rubbing, warm it and give a coat of oil or it will rapidly rust.