TINNING; TIN-PLATE. The art of tinning, or of coating other metals with a thin layer of tin, so as to protect them from oxidation, was known to the ancients, although it does not appear to have been very extensively practised. During many centuries, England procured tin-plate from Bohemia and Saxony, where the manufactory was esta blished near the tin-mines of the Erzgebirge mountains, which were the most extensive in Europe after those of Cornwall. From the time of the invention of tin-plate down to the close of the 17th century, if not later, both England and the whole continent of Europe depended upon the above-named countries for their supply of tin-plate ; but about the year 1665 an attempt was made to introduce the manufacture into England. The manufacture was permanently established at Pontypool in Monmouthshire about the year 1730, and soon afterwards in France. About 1740 the manufacture was brought to such perfection in England that very little was imported from foreign countries ; the British manufacture was superior to the foreign in glossiness of surface, owing to the plates being drawn under a rolling-mill, instead of being hammered, as was common in those made beyond sea. The difficulty of extending iron, in what may be deemed the infancy of the manufacture, into thin uniform sheets, with a perfectly smooth and clean surface, which is essential to the adhesion of the tin in an equal film, was one of the principal obstacles to the progress of this department of the art of tinning.
The process of tinning depends upon the strong affinity which exists between tin and the metals to which it is applied. The tinning of sheet-iron, as the most important application of the process, will be first noticed. The finest English or Welsh bar-iron, prepared with charcoal instead of mineral coke, and known to the trade as tin-iron, is used for making tin-plates. This material is first made into fiat bars, or slabs, about 30 inches long, 6 inches wide, and weighing eighty pounds. These bars are made red-hot, and extended by passing them repeatedly between rollers, until they are reduced to about 3-8ths of an iuch in thickness. When cooled, the pieces are cut by shears, worked by machinery, into plates about 10 inches by 6, which are repeatedly re-heated and rolled, until they are reduced to as thin a state as the process will conveniently allow. The sheet is then doubled and again rolled until reduced in thickness one-half, after which it is doubled again, and rolled until still further diminished in thickness. When
thus brought to the required tenuity, the thin sheet is cut into plates of the sizes required to suit the market (most commonly about 13 inches by II)), and then the several thicknesses or are separated. After shearing, the plates aro piled in heaps, one being laid cross-wiso at intervals, to separate the number required to form a box. This name is technically applied to 225 plates in all the subsequent pro cesses, although it is not until they are completed that the plates aro actually placed in boxes.
The next operation to be performed is the removal of every particle of oxide or other impurity from the surface of the plates. For this purposo each is steeped for a few minutes in a leaden trough con taining a weak solution of muriatic acid. The plates are taken out, arranged on the floor in rows, and then removed, by means of an iron rod, to a reverberatory furnace or oven, in which they are submitted to a red heat. The heat to which the plates are exposed, combined with their previous washing in the acid, causes them to throw off a scale of rust or oxide. The plates are then flattened by beating them upon a cast-iron block, and are submitted to a second or cold rolling, which removes any warping acquired in the previous processes, gives a high degree of smoothness to their surfaces, and imparts elasticity to the iron. After the cold rolling the plates are immersed singly, in a vertical position, in an acidulous preparation consisting of water in which bran has been steeped for nine or ten days, until it has fermented and become slightly acid. In this the plates are kept for ten or twelve hours, and occasionally turned, to insure an equal exposure of every part of their surface; and from the lye-trough they are transferred to a leaden vessel containing diluted sulphuric acid. This trough and the lye-trough are slightly heated by flues, to assist the action of the acid mcnstrua. The plates are usually agitated in the weak sulphuric acid for about an hour, until they become bright and free from black spots. They are then removed into pure water, in which they are scoured with hemp and sand, to remove any remaining oxide; and iu this bath of pure water the plates remain until wanted for tinning; because, even if left for mouths, they will remain perfectly free from rust.