(a) Iron which is to be tinned must be previously steeped in acid materials, such as sour whey or distillers' wash; then scoured and dipped in melted tin, having been first rubbed over with a solution of sal ammoniac The surface of the tin is prevented from calcining by covering it with a coat of fat.
(6) This process is of little importance as a protection for iron, as the layer of tin is a mere film, but it may be useful when thicker coats of tin are to be applied by other processes. For the bath, dissolve with the aid of heat, in an enamelled cast-iron kettle, ammonia-alum, 11 oz., and fused tin protochloride, 1/3 oz., in 4 1/2 gal. soft water. The pieces of iron, previously cleansed and rinsed in cold water, are steeped in the solution as soon as it boils. They are immediately covered with a film of tin of a fine white dead lustre, which may be rendered bright by friction. The bath is maintained at the proper strength by small additions of fused tin protochloride. This bath is convenient for a preliminary tinning of zinc; when the ammonia-alum may be replaced by any other kind of alum, or by alumina sulphate; but for wrought and cast iron and steel this substitution cannot be made.
If the saucepan is an old one it most be put on the fire and allowed to get nearly red hot, which will get rid of all the grease; then make a pickle of the following proportions; - Oil of vitriol, 1/2 lb.; muriatic acid, 1/4 lb.; water, 1 gal. If the saucepan can be filled so much the better, if not keep the pickle flowing over it for s,ay 5 minutes, pour out, rinse with water, and scour well with sand or coke dust with a wisp of tow, rinse well with water; if the pan is clean it will be of an uniform grey colour, but if there are any red or black spots it must be pickled and scoured again till thoroughly clean. Have ready zinc chloride, that is, muriatic acid in which some sheet zinc has been dissolved, some powdered sal ammoniac, some tow, about 18 in. of iron rod 1/4 - 3/8 in. thick, one end flattened out and bent up a little and filed clean, and some bar tin; dip a wisp of tow in the zinc chloride, then into the powdered sal ammoniac, taking up a good quantity, and rubwell all over the inside.
This must be done directly after the scouring, for if allowed to stand it will oxidise; put on the fire till hot enough to melt the tin, the end of the bar of tin being brushed over the heated part till melted; run down about half the bar, and with the flattened end of the iron rod rub the tin well over the surface, taking care not to heat too large a surface at once, nor to let it get too hot, which may be known by the tin getting discoloured, when some dry sal ammoniac must be thrown in.. Having gone all over it, wipe lightly with a wisp of tow, just made warm enough that the tin does not stick to it; when cold scour well with sand and tow, rinsing with plenty of water.
File bright the piece of iron required to be tinned, and mix up the following solution; - In a pennyworth of spirits of salts, put a piece of zinc the size of a shilling, the spirits of salts will eat it away; wet the places required to be tinned with the solution, then while wet use a copper-bit with fine solder, and it will immediately tin.
The following is Heeren's process for giving iron wire the appearance of silver. This is done by a thin film of tin. The iron wire is first placed in hydrochloric acid, in which is suspended a piece of zinc. It is afterwards placed in contact with a strip of zinc in a bath of 2 parts tartaric acid dissolved in 100 of water, to which is added 3 parts tin salt and 3 of soda. The wire should remain about 2 hours in this bath and then be removed, and made bright for polishing, or drawing through a polishing iron. By this galvanic method of tinning, wire which has been wound in a spiral, or iron of other shape, can be made quite white, which is an advantage over most other methods, where the wire is tinned in the fire and then drawn through a drawing plate.