This process is very similar to the process of galvanizing, but is more elaborate, and requires more care, as the product is used where resistance to corrosion is more essential.

Sheets to be tinned are rolled from sheet-bars and are then trimmed to measure 14 x 20 inches, the standard size for tin plate.

After rolling and trimming, the sheets are selected with more care than for galvanizing. They are then pickled and rinsed to expose smooth, clean surfaces, and are annealed on a covered tray. Annealing makes them too pliable for use, hence each sheet is given two passes through finishing rolls to make the surfaces smooth and to make the sheet springy or elastic. This annealing and rolling makes another pickling necessary, after which they are kept under water to avoid oxidation while waiting to go into the tinning pot.

The coat of tin is applied by immersing one sheet at a time in an iron pot containing molten tin. The sheet enters the tin through a flux of ammonium chloride (sal ammoniac) which floats on the surface of the molten metal and prevents the formation of dross, which would cling to the surface of the sheet. Fig. 59 shows a diagram of the upper part of the tinning pot, on the back edge of which is mounted a grease pot G filled with molten palm oil, in which revolve several pairs of rollers as shown. The sheet is lifted from the pot until gripped by the lowest pair of rolls, and it is carried upward by these and the other rolls which deliver it to a table above the grease pot. The palm oil serves to keep the plate hot and to prevent oxidation while the rolls are pressing off superfluous tin and smoothing that retained on the sheet. The slower running of the rolls allows a heavier coating of tin to stick to the sheet. The tin in the pot is kept molten by gas burners not shown. After emerging from the grease pot, each sheet is forced up, edge on. through a bin of sawdust and lime to clean it, and is then passed through buffing and dusting rolls, after which it is carried to the sorting tables. The work of sorting is usually done by girls, who grade all sheets as "primes" (1st grade) or "wasters" (2d grade). Some "wasters" having no defects except a few poorly tinned spots may be "mended" by cleaning and retinning, thus making "primes," but "wasters" with bad defects must either be sold as seconds or thrown out entirely for remelting and re-covering the tin.

Fig. 59. - Tinning Pot.

Tin plates are usually packed 112 sheets to the box.