Tinning is the process of thinly coating sheets of wrought iron or steel with tin, and the resulting product is known as tinplate. The term is also widely used for the different process of coating a metal with solder before soldering.
It is most often used to prevent rust, but is also commonly applied to the ends of stranded wire used as electrical conductors to prevent oxidation (which increases electrical resistance), and to keep them from fraying or unraveling when used in various wire connectors like twist-ons, binding posts, or terminal blocks, where stray strands can cause a short circuit.
While once more widely used, the primary use of tinplate now is the manufacture of tin cans. Formerly, tinplate was[clarification needed] used for cheap pots, pans, and other holloware. This kind of holloware was also known as tinware and the people who made it were tinplate workers.
The untinned sheets employed in the manufacture are known as black plates. They are now made of steel, either Bessemer steel or open-hearth. Formerly iron was used, and was of two grades, coke iron and charcoal iron; the latter, being the better, received a heavier coating of tin, and this circumstance is the origin of the terms coke plates and charcoal plates by which the quality of tinplate is still designated, although iron is no longer used. Tinplate was consumed in enormous quantities for the manufacture of the tin cans in which preserved meat, fish, fruit, biscuits, cigarettes, and numerous other products are packed, and also for the household utensils of various kinds made by the tinsmith.
- 1 History
- 2 Plate production methods
- 3 Tinning processes
- 4 Alternatives
- 5 See also
- 6 Kalai - the art of coating vessels with tin in India
- 7 References
- 8 Further reading
The practice of tinning ironware to protect it against rust is an ancient one. This may have been the work of the tinner. This was done after the article was fabricated, whereas tinplate was tinned before fabrication.
The manufacture of tinplate was long a monopoly of Bohemia, but in about the year 1620 the industry spread to Saxony. 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.
Andrew Yarranton, an English engineer and agriculturist, and Ambrose Crowley (a Stourbridge blacksmith and father of the more famous Sir Ambrose Crowley III) were commissioned to go to Saxony and if possible discover the methods employed. They visited Dresden in 1667 and found out how it was made. In doing so, they were sponsored by various local ironmasters and people connected with the project to make the River 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 two of the sponsors, the ironmasters Philip Foley and Joshua Newborough, erecting a new mill, Wolverley Lower Mill (or forge), in 1670. This contained three shops: one being a slitting mill, which would serve as a rolling mill, the others being forges. In 1678 one of these was making frying pans and the other drawing out blooms made in finery 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 one William Chamberlaine renewing a patent granted to him and Dud Dudley in 1662. Yarranton described the patent as "trumped up".
The slitter at Wolverley was Thomas Cooke. Another Thomas Cooke, perhaps his son, moved to Pontypool and worked there for John Hanbury (1664–1734). According to Edward Lhuyd, by 1697, John Hanbury had a rolling mill at Pontypool for making "Pontypoole Plates" machine. This has been claimed as a tinplate works, but it was almost certainly only producing (untinned) blackplate. However, this method of rolling iron plates by means of cylinders, enabled more uniform black plates to be produced than was possible with the old plan of hammering, and in consequence the English tinplate became recognized as superior to the German.
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. The tinplate was shipped from Newport, Monmouthshire. This immediately follows the first appearance (in French of Réaumur's Principes 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 ironmaking regions in England and Wales, but later mainly in south Wales. In 1805, 80,000 boxes were made and 50,000 exported. The industry continued to spread steadily in England and especially Wales, and after 1834 its expansion was rapid, Great Britain becoming the chief source of the world's supply. In that year her total production was 180,000 boxes of 108 lb each (around 50 kg, in America a box is 100 lb), in 1848 it was 420,000 boxes, in 1860 it reached 1,700,000 boxes. But subsequently the advance was rapid, and the production reached about 2,236,000 lb in 1891. One of the greatest markets was the United States of America, but that market was cut off in 1891, when the McKinley tariff was enacted there. 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.
In 1891, the United States made 11,000 tons of tinplate and imported 325,100 tons, but in 1899, it made 360,900 tons, importing only 63,500 tons (mostly for re-export). British exports were further hindered by the Dingley tariff, which removed the advantage of Welsh plate on America's Pacific coast, had by 1900 increased to more than 849,000,000 lb, of which over 141,000,000 lb were terne-plates. The total imports in that year were only 135,264,881 lb. In later years, again, there was a decline in the American production, and in 1907 only 20% of the American tinplate mills were at work, while the British production reached 14 million boxes.
Despite this blow, the industry continued, but on a smaller scale. Nevertheless, there were still 518 mills in operation in 1937, including 224 belonging to Richard Thomas & Co. However the traditional 'pack mill' had been overtaken by the improved 'strip mill', of which the first in Great 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 in about the 1960s.
Plate production methods
The pack mill process
The pack mill process begins with a tin bar, which is a drawn flat bar that was usually purchased from an ironworks or steel works. The tin bar could be wrought iron or mild steel. The cross-section of the bar needed to be accurate in size as this dictates the length and thickness of the final plates. The bar was cut to the correct length to make the desired size plate. For instance, if a 14 in × 20 in (360 mm × 510 mm) plate is desired the tin bar is cut to a length and width that is divisible by 14 and 20. The bar is then rolled and doubled over, with the number of times being doubled over dependent on how large the tin bar is and what the final thickness is. If the starting tin bar is 20 in × 56 in (510 mm × 1,420 mm) then it must be at least finished on the fours, or doubled over twice, and if a thin gauge is required then it may be finished on the eights, or doubled over three times. The tin bar is then heated to a dull red heat and passed five or six times through the roughing rolls. 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. The plate is then reheated and run through the finishing rolls.
If the plate is not finished on singles, or without doubling the plate over, it is doubled over in a squeezer. The squeezer was like a table where one half of the surface folds over on top of the other and a press flattens the doubled over plate so the rolled end will fit in the rollers. It is then reheated for another set of rolling. This is repeated until the desired geometry is reached. Note that if the plate needs to be doubled over more than once the rolled end is sheared off. The pack is then allowed to cool. When cool, the pack is sheared slightly undersized from the final dimensions and the plates separated by openers.
At this point, the plates are covered in scale and must be pickled. This involves dipping the plates in sulfuric acid for five minutes. The pickling turns the scales into a greenish-black slime which is removed via annealing. The plates are annealed for approximately 10 hours and then allowed to slowly cool. At this point the plates are known as pickled and annealed black plates. These plates were commonly sold for stamping and enameling purposes.
After this, the plates are rough and not straight, so they are cold rolled several times. The rolling lengthens the plates to their final dimension. They are then annealed again to remove any strain hardening. These plates are called black plate pickled, cold rolled, and close annealed (black plate p. cr. and ca.). To attain perfect cleanliness the plates are pickled again in a weak sulfuric acid. Finally they are rinsed and stored in water until ready to be tinned.
The tinning set consists of at least one pot of molten tin, with a zinc chloride flux on top, and a grease pot. The flux dries the plate and prepares it for the tin to adhere. If a second tin pot is used, called the wash pot, it contains tin at a lower temperature. This is followed by the grease pot, which contains oil and a tinning machine. The tinning machine has two small rollers that are spring-loaded together so that when the tinned plate is inserted the rolls squeeze off any excess tin. The springs on the tinning machine can be set to different forces to give different thicknesses of tin. Finally, the oil is cleaned off with fine bran and dusted clean.
What is described here is the process as employed during the 20th century. The process grew somewhat in complexity over time, as it was found that the inclusion of additional procedures improved quality. 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
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Early hot rolling strip mills did not produce strip suitable for tinning, but in 1929 cold rolling began to be used to reduce the gauge further, which made tinning achievable. The plate was then tinned using the process outlined above.
There are two processes for the tinning of the black plates: hot-dipping and electroplating.
Tinplate made via hot-dipped tin plating is made by cold rolling steel or iron, pickling to remove any scale, annealing to remove any strain hardening, and then coating it with a thin layer of tin. Originally this was done by producing individual or small packs of plates, which became known as the pack mill process. In the late 1920s strip mills began to replace pack mills, because they could produce the raw plates in larger quantities and more economically.
In electroplating, the item to be coated is placed into a container containing a solution of one or more tin salts. The item is connected to an electrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electric current is passed through the circuit, metal ions in the solution are attracted to the item. To produce a smooth, shiny surface, the electroplated sheet is then briefly heated above the melting point of tin. Most of the tin-plated steel made today is then further electroplated with a very thin layer of chromium to prevent dulling of the surface from oxidation of the tin.
- Terne-plate is a similar product to tinplate, but the bath is not of tin, but of tin and lead mixed, the latter metal constituting from 7.59% of the whole. The name derives from 'terne' - meaning dull or tarnish. Terne-plates began to be produced in England about the middle of the 19th century, and are widely employed in the United States for roofing purposes.
- For many purposes, tinplate has been replaced by galvanised (zinc-coated) vessels, though not for cooking as zinc is toxic, where stainless steel is often used. Zinc protects iron electrolytically, that is, the zinc will oxidise and turn to a white powder to preserve the iron, whereas tin will only protect the iron if the tin-surface remains unbroken, as it electrolytically cannibalises unprotected iron to preserve itself.
Kalai - the art of coating vessels with tin in India
The art of Kalai (Kalhai or Qalai) is the process of coating the alloy surface i.e. Copper or Brass by deposition of metal tin on it. The word "Kalai" is derived from the Arabic language which means "white wash or tin". A cultural Sanksirt work by Keladi Basava called "Sivatattva Ratnakara" (1699) mentions "Kalaya-lepa" in the chapter of cookery or "Supashashtra" which means applying Kalai on utensils. People practicing the art of Kalai are called Kalaiwala or Kalaigar. Basically, Kalaigars or Kalaiwalas are community craftsmen.
Vessels with Kalai, both on its interior and exterior have been found in the excavations of Bramhapuri at Kolhapur, Maharashtra which adds to the archeological evidence of Kalai Art. From this evidence, P K. Gode, who studied tin coating on metallic vessels in India, stated that the history of tin coating dates back to 1300 C.E. The history of Kalai is also recorded in "Parsibhashanushasana" of Vikaramasimha (before Samvat 1600 i.e. C.E. 1544) and also in the famous Ain- I -Akbari (C.E. 1590) by Abul Fazal. The art of Kalai was first practiced by the Muslims and is current even today.
Spiritual approach to Kalai
The copper vessels with Kalai were used to store water and cook food earlier because of a spiritual belief that copper attracts and transmits a divine consciousness also called "Chaitanya". The spiritual approach to the use of copper vessels to store water is that Copper and Tin have Sattva-Raja (the basic component of creation/universe) component that is transferred to water.
Scientific approach to Kalai
Kalai is required to be done on the vessels after approximately every two months of the doing the same. Earlier, copper and brass vessels were used because of their high conductivity. High conductivity of copper vessels reduces the fuel cost. Tin also conducts heat almost as quickly as copper, so there's no question of copper losing its conductivity because of the Kalai. The vessels had to be tin-coated frequently to maintain the taste of the food as after sometime the food tastes bitter and it led to even food poisoning. The copper, in traces gets dissolved in water when the water is stored in a copper vessel for a long period of time. This scientific process is called "Oligodynamic effect". Copper reacts with certain foods and can lead to gastrointestinal symptoms. By doing Kalai, the deposition of tin layer works as a safe substance between the copper and the food. Tin will melt if the temperature is above 425 degrees Fahrenheit (218.333 degrees Celsius). Tin from the copper vessel wears away with time so usage of metal utensils, so wooden or silicon spatulas should be used. Cooking of acidic foods should be avoided. A chemical reaction between copper and oxygen called Oxidization turn the copper vessels black. If Kalai is not done on the copper vessels, copper will react with the air's moisture and create copper carbonate, which is green in color. This can make a person severely ill.
The process of Kalai
The steps of Kalai are as follows:
- First step is to clean the utensil with water. There are two ways of cleaning the utensil further to remove any impurities such as dust. The first is to clean it with caustic soda. The other is to wash it with dilute acid solution which contains gold purifying compound known as 'Sufa'. If the latter is used, the utensil should be cleaned immediately after applying the dilute acidic solution as it may bear a mark if not done immediately.
- After the cleaning, the vessel is heated on burning coal for about 2 to 3 minutes. The Kalaiwala, Kalaigar, or Kalaikar then digs a small pit in the ground to burn the coal. He/she prepares a temporary blast furnace to do Kalai and blows air through bellows.
- After the vessel turns pinkish hot, virgin grade tin (called 'Ranga' in Hindi) in the form of strips is applied on the hot vessel. This step is called 'casting' by the Kalaigars.
- Ammonium chloride powder (also called 'sal ammoniac' in mineral form, or 'Nausadar' in Hindi) is sprinkled on the vessel as a flux. The tin melts rapidly which is then rubbed evenly on the utensil with the help of a cotton cloth or a swab of cotton. The rubbing process is known as 'Majaay' in Hindi. A whitish smoke with the peculiar smell of ammonia is released when the 'Nausadar' powder is rubbed on the utensil. A silvery lining appears on the vessel with a shine.
- The final step of Kalai is to dip the utensil in cold water.
Kalai was earlier done with silver instead of tin but now it would be expensive for the Kalaigars to use silver. As the stainless steel and aluminum ware came into being, the usage of copper and brass utensils reduced which led the Kalaigars to suffer losses. Now-a-days only some hotels and a very few people use vessels with Kalai done on it. As a result, there are a very few Kalaigars left. The art of Kalai is vanishing.
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