Tin plate can be classified, according to the iron used, as follows: Charcoal plate, puddled iron, coke plate, and steel plate. In a few works sheet iron is still made of iron refined with charcoal. Of course an excellent quality of pig iron must be used to make puddled iron of good and best quality. Steel plate is made of very tough steel made by different processes. The so-called charcoal tin is made by refining pig iron and scrap with charcoal, and is very dense and strong. For this reason tin plate made from it is rather harder to work, but will stand longer and is better than that made from softer iron. Only puddled iron is generally used for coke plate, since a better quality is rarely required for such tin.
The iron used in making tin plate is prepared as follows: The blooms, weighing 40-50 cwt., as they come from the puddling or refining furnaces, are first placed under steam hammers, then rolled into thin bars, which are cut up and tied in bundles. These bundles are strongly heated in the reheating furnace, thoroughly wrought, heated again, rolled into bars in calibrated rolls, then cut in lengths corresponding to the different sizes of plate, and called platins or plate iron. These bars are then rolled out with hard rollers into sheets, which are trimmed with huge scissors to the exact sizes met with in commerce. The sheets must be pickled to remove the coating of oxide (rust), either hydrochloric or sulphuric acid being used according to circumstances.
The material is rendered so hard and brittle by this treatment that it has to be annealed before proceeding to the next step, namely, smoothing and polishing. This is accomplishel by heating in tightly closed boxes or muffles, the plates being packed tightly together. These muffles are placed on wagons and run in a warming furnace, where they are left 10-15 hours. The polishing is performed by drawing the sheets of iron, after they have been pickled and tempered, between polished rolls of hard cast steel heavily weighted.
To get a clean metallic surface, such as is requisite to receive the tin, the iron must be dipped repeatedly into quite dilute sulphuric or hydrochloric acid, then polished and scoured, each one separately, with sharp sand over the entire surface. It is now ready to receive the tin, and passes to the tinning room.
In this room are a number of kettles, all of the same height, placed in a row and heated with fires beneath them. They are called the grease kettle, the tinning kettle, the brush kettle, the fine tin or roller kettle, and the grease kettle. The different operations performed in these kettles take place in this order: The pickled and scoured plates are put in the first kettle and thoroughly coated with grease, usually pure tallow, but sometimes palm oil is used. Then it goes to the tin kettle, in which it is moved about until evenly tinned all over. From this it goes to the third kettle, also containing tin. Here each individual plate is taken out and brushed with an oakum brush or pad of hemp to remove the coarser particles. It is next put in the fine tin; then in the last kettle, that also contains hot grease, on a grating, or moved up and down in it by rollers. When the plates come from this kettle they are placed on racks to cool, The tinning is now completed, but they do not look very nice, owing to the adherent grease.
To remove this they are drawn through three or four large boxes filled with slaked lime, sawdust, bran, or flour; flour is the best of all, for it cleans them better, and after it gets saturated with grease the flour can be used for cattle feed.
After the tin plates leave these boxes they go to the polishing bench to remove the dust. This bench consists of a table covered with woollen cloth, or a sheep pelt, and the sheets are rubbed singly with a rubber made of wool or sheepskin, which brings out the pure, fine lustre of the tin.
The tin is next assorted by a careful inspection of both sides, and classified as first, second, or third quality. Sheets that are imperfectly tinned are sent back to the tinning room, while the rest are packed in wooden boxes and the brand is burned on.
Attempts have been made to replace the fat with tin chloride, but tin plate made in this way was found to be inferior to that made by the old process, because it is far more prone to rust. At present scarcely any tin plate is male with tin chloride, hut some manufacturers use this process for tinning cooking utensils.
Another improvement consists in passing the tin, as it comes dripping from the last bath of melted tin, between rollers that squeeze off the excess of tin and leave a uniform coating of any desired thickness according as they are set close or far apart.
Elm is the wood generally used for boxing tin.