A thin steel sheet coated with tin by dipping or electrolytic deposition. It is used for light robust containers and protective constructions. First tried out in the late 17th-c, it was not used to any extent until the invention of canning in the early 19th-c, since when it has attained worldwide industrial importance.
Tinplate consists of sheet steel covered with a thin layer of tin.
Tinplate is made by rolling iron or steel in a rolling mill, cleaning it of scale (rust) by pickling it in acid and then coating it with a thin layer of tin. This was formerly carried out by producing the plates (or small groups of them) individually in what was subsequently known as a 'pack mill'.
Formerly, tinplate was used for cheap pots, pans and other holloware, but galvanised vessels (coated with zinc) are now used. This is because zinc protects iron electrolytically, the zinc being dissolved in preference for iron, whereas tin will only protect the iron if the tin-surface remains unbroken.
This kind of holloware was also known as tinware and the people who made it were tinplate workers.
History
The practice of tinning ironware to protect it against rust is an ancient one. This was done after the article was fabricated, whereas tinplate was tinned before fabrication. Tinplate was apparently produced in the 1620s at a mill of (or under the patronage of) the Earl of Southampton, but it is not clear how long this continued.
The first production of tinplate was probably in Bohemia, from where the trade spread to Saxony, and was well-established there by the 1660s. In Saxony, the plates were forged, but when they conducted experiments on their return to England, they tried rolling the iron. This led to the ironmasters Philip Foley and Joshua Newborough (two of the sponsors) in 1670 erecting a new mill, Wolverley Lower Mill (or forge). This contained three shops, one being a slitting mill (which would serve as a rolling mill), and the others were forges. It is likely that the intention was to roll the plates and then finish them under a hammer, but the plan was frustrated by one William Chamberlaine renewing a patent granted to him and Dud Dudley in 1662. He had a slitting mill there and was also producing iron plates called 'Pontpoole plates'. Edward Lhuyd reported the existence of this mill in 1697.
Tinplate first begins to appear in the Gloucester Port Books (which record trade passing through Gloucester, mostly from ports in the Bristol Channel in 1725.
Further mills followed a few years later, initially in many ironmaking regions in England and Wales, but later mainly in south Wales. This caused a great retrenchment in the British industry and the emigration to America of many of those who could no longer be employed in the surviving tinplate works. Nevertheless there were still 518 mills in operation in 1937, including 224 belonging to Richard Thomas & Strip mills rendered the old pack mills obsolete and the last of them closed in about the 1960s.
The pack mill process
The raw material was bar iron, or (from the introduction of mild steel in the late 19th century), a bar of steel. The cross-section of the bar needed to be accurate in size as this would be the cross-section of the pack of plates made from it. It was then passed four or five times through the rolls of the rolling mill, to produce a thick plate about 30 inches long. Between each pass the plate is passed over (or round) the rolls, and the gap between the rolls is narrowed by means of a screw.
This was then rolled until it had doubled in length. The plate was then folded in half ('doubled') using a doubling shear, which was like a table where one half of the surface folds over on top of the other. When cool, the pack was sheared (using powered shears) and the plates separated by 'openers' (usually women).
In the pickling department, the plates were immersed in baths of acid (to remove scale - i.e. The plates were then rolled cold through highly polished rolls to remove any unevenness and give them a sense polished surface.
The tinning set consisted of two pots with molten tin (with flux on top) and a grease pot. The second tin pot (called the wash pot) had tin at a lower temperature. However the practice of hot rolling and then cold rolling evidently goes back to the early days, as the Knight family's tinplate works had (from its foundation in about 1740) two rolling mills, one at Bringewood (west of Ludlow) which made blackplate, and the other the tin mill at Mitton (now part of Stourport, evidently for the later stages.
The strip mill
The strip mill was a major innovation, with the first being erected at Ashland, Kentucky in 1923. This provided a continuous process, cutting out the need to pass the plates over the rolls and to double them. At the end the strip was cut with a guillotine shear or rolled into a coil. Early (hot rolling) strip mills did not produce strip suitable for tinplate, but in 1929 cold rolling began to be used to reduce the gauge further. The first strip mill in Great Britain was opened at Ebbw Vale in 1938 with an annual output of 200,000 tons.
The strip mill had several advantages over pack mills:
It was cheaper due to having all parts of the process, starting with blast furnaces on the same site. Larger sheets could be produced at lower cost and this reduced cost and enabled tinplate and steel sheet to be used for more purposes.
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