Tinplate consists ofsheets ofsteel coated with a thin layer oftin to impederusting. Before the advent of cheapmild steel, the backing metal (known as "backplate") waswrought iron. While once more widely used, the primary use of tinplate now is the manufacture oftin cans.
In thetinning process, tinplate is made by rolling the steel (or formerly iron) in arolling mill, removing any mill scale by pickling it in acid and then coating it with a thin layer oftin. Plates were once produced individually (or in small groups) in what became known as apack mill. In the late 1920s pack mills began to be replaced bystrip mills which produced larger quantities more economically.
Formerly, tinplate was used fortin ceiling, andholloware (cheap pots and pans), also known as tinware. The people who made tinware (metal spinning) were tinplate workers.
For many purposes, tinplate has been replaced bygalvanised metal, the base being treated with azinc coating. It is suitable in many applications where tinplate was formerly used, although not for cooking vessels, or in other high temperature situations—when heated, fumes from zinc oxide are given off; exposure to such gases can produce toxicity syndromes such asmetal fume fever.[1] The zinc layer prevents the iron fromrusting throughsacrificial protection with the zinc oxidizing instead of the iron, whereas tin will only protect the iron if the tin surface remains unbroken.
The practice oftin mining likely began circa 3000 B.C. in Western Asia, British Isles and Europe. Tin was an essential ingredient of bronze production during the Bronze Age.[2][3][4]The practice of tinning ironware to protect it against rust is an ancient one. This may have been the work of thewhitesmith. 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 inBohemia, from where the trade spread toSaxony, and was well-established there by the 1660s.Andrew Yarranton and Ambrose Crowley (a Stourbridge blacksmith and father of the more famousSir Ambrose) visited Dresden in 1667 and learned how it was made. In doing so, they were sponsored by various localironmasters and people connected with the project to make theriver Stour navigable. 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 ironmastersPhilip Foley and Joshua Newborough (two of the sponsors) in 1670 erecting a new mill,Wolverley Lower Mill (or forge) inWorcestershire. This contained three shops, one being aslitting mill (which would serve as arolling mill), and the others were forges. In 1678 one of these was makingfrying pans and the other drawing outblooms made infinery forges elsewhere. It is likely that the intention was to roll the plates and then finish them under a hammer, but the plan was frustrated by William Chamberlaine renewing apatent granted to him andDud Dudley in 1662.[5][6]
The slitter atWolverley was Thomas Cooke. Another Thomas Cooke, perhaps his son, moved toPontypool and worked there forJohn Hanbury.[7] He had a slitting mill there and was also producing iron plates called 'Pontpoole plates'.[8]Edward Lhuyd reported the existence of this mill in 1697.[9] This has been claimed as a tinplate works, but it was almost certainly only producing (untinned) backplate.
Tinplate first begins to appear in the Gloucester Port Books (which record trade passing throughGloucester), mostly from ports in theBristol Channel in 1725. The tinplate was shipped fromNewport, Monmouthshire.[a] This immediately follows the first appearance (inFrench) ofReamur'sPrincipes de l'art de fer-blanc, and prior to a report of it being published in England.
Further mills followed a few years later, initially in many iron-making regions in England and Wales, but later mainly in south Wales, most notably theMelingriffith Tin Plate Works,Whitchurch, Cardiff, which was founded some time before 1750. In 1805, 80,000 boxes were made and 50,000 exported. The industry continued to grow until 1891. One of the greatest markets was the United States, but that market was cut off in 1891 when theMcKinley tariff was enacted. This caused a great retrenchment in the British industry and the emigration to America of many of those were no longer employed in the surviving tinplate works.
Despite this blow, the industry continued, but on a smaller scale. There were 518 mills in operation in 1937, including 224 belonging to Richard Thomas & Co. The traditional 'pack mill' had been overtaken by the improved 'strip mill', of which the first inGreat Britain was built by Richard Thomas & Co. in the late 1930s. Strip mills rendered the old pack mills obsolete and the last of them closed circa the 1960s.
The raw material wasbar iron, or (from the introduction ofmild steel in the late 19th century), a bar of steel. This was drawn into a flat bar (known as a tin bar) at theironworks orsteel works where it was made. 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. The bar was cut to the correct length (being the width of the plates) and heated. 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. It is then put into a furnace to be heated until it is well 'soaked'. This is repeated until there is a pack of 8 or 16 plates. The pack is then allowed to cool. When cool, the pack was sheared (using powered shears) and the plates separated by 'openers' (usually women).[11] Defective plates were discarded, and the rest passed to the pickling department.
In the pickling department, the plates were immersed in baths of acid (to remove scale, i.e., oxide), then in water (washing them). After inspection they were placed in an annealing furnace, where they were heated for 10–14 hours. This was known as 'black pickling' and 'black annealing'. After being removed they were allowed to cool for up to 48 hours. The plates were then rolled cold through highly polished rolls to remove any unevenness and give them a polished surface. They were then annealed again at a lower temperature and pickled again, this being known as 'white annealing' and 'white pickling'. They were then washed and stored in slightly acid water (where they would not rust) awaiting tinning.
The tinning set consisted of two pots with molten tin (with flux on top) and a grease pot. The flux dries the plate and prepares it for the tin to adhere. The second tin pot (called the wash pot) had tin at a lower temperature. This is followed by the grease pot (containing an oil), removing the excess tin. Then follow cleaning and polishing processes. Finally, the tinplates were packed in boxes of 112 sheets ready for sale. Single plates were 20 by 14 inches (51 cm × 36 cm); doubles twice that. A box weighed approximately ahundredweight (cwt; 112 pounds or 51 kilograms).[12][b]
The strip mill was a major innovation, with the first being erected atAshland, Kentucky in 1923. This provided a continuous process, eliminating 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 1929cold rolling began to be used to reduce the gauge further. The first strip mill inGreat Britain was opened atEbbw Vale in 1938 with an annual output of 200,000 imperial tons (203,210 tonnes or 224,000 short tons).
The strip mill had several advantages over pack mills:
