Tin-plate. The manufacture of this article forms a branch of the iron trade. The art of tinning plate-iron is said to have been invented in Bohemia, about the beginning of the 16th century, although the tinning of copper was known earlier. Tin-plate was first made in England about 1670. Sheet-iron for tin-plates is made either of charcoal-bar or coke-bar, which has been rolled with particular care, in order to avoid scales on the surface. Before tinning the plates are called 'black plates.' When the iron has been cut to the required size the plates are 'pickled'—i.e. they are immersed in hot sulphuric or hydrochloric acid which has been diluted by 16 parts of water to 1 of acid, the use of the acid being to remove all oxide. After this the plates require to be washed several times in water, and then annealed. The plates are next passed two or three times through chilled iron rollers highly polished with emery and oil, to give them a well-polished surface. Once more they are sent to the annealing furnace, passed again through dilute sulphuric acid, which is followed by another washing, but this time in running water, and then scoured with sand. This should leave them quite clean and bright.
Each plate is now put singly into a pot of melted grease (which has become sticky by use), and left till it is completely coated, after which the plates are taken in parcels and plunged into a bath of melted tin covered with grease, called the 'tin-pot.' The plates are afterwards put in parcels into the first of the two compartments of a vessel, where they receive a coating of purer tin than that of the 'tin-pot,' and are then withdrawn one by one, and wiped on both sides with a hemp brush, the marks of which are obliterated by another dipping in the second compartment of the 'wash-pot.' This last dipping also gives the plates a polish. The superfluous tin is removed by immersing the plates in a pot containing tallow and palm-oil, maintained at a temperature just high enough to allow the tin to run off. The final treatment consists in working the plates separately in troughs of bran with a little meal, and then rubbing them with flannel. Of late years tin-plates of mild steel have been manufactured, and for some purposes they are preferred to iron tin-plates.
There is a variety of tin-plates called 'terne-plates,' coated with an alloy of tin and lead, in which the proportions vary from 1 of lead and 2 of tin to 2 of lead and 1 of tin. Now 'tin-plates' are frequently made of steel.
The manufacture of tin-plates is chiefly carried on in South Wales and in the United States. In 1890 the total exports from Great Britain amounted to 421,797 tons, valued at £6,361,477; of this quantity 321,109 tons were sent to the United States. By the McKinley tariff of 1890 a high protective duty ( cents per lb.; lowered to ; and again increased in 1897) was imposed on tin-plates imported into the United States, in order to protect the manufacture there. In 1895 English exports were 366,120 tons, value £4,239,193.