Tinning

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.[9]

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.[10]

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.[11]

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.[12]

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.[13][14]

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.[15]

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.

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.

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.[16] The word "Kalai" is derived from the Arabic language which means "white wash or tin".[17] 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.[17] People practicing the art of Kalai are called Kalaiwala or Kalaigar.[18] Basically, Kalaigars or Kalaiwalas are community craftsmen.[19]

The art of Kalai

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.[17] 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.[17] 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.[17] The art of Kalai was first practiced by the Muslims and is current even today.[17]

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".[20] 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.[20]

Kalai is required to be done on the vessels after approximately every two months of the doing the same.[21] Earlier, copper and brass vessels were used because of their high conductivity.[21] 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.[21] 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".[21] Copper reacts with certain foods and can lead to gastrointestinal symptoms.[22][23] By doing Kalai, the deposition of tin layer works as a safe substance between the copper and the food.[23] Tin will melt if the temperature is above 425 degrees Fahrenheit (218.333 degrees Celsius).[23] Tin from the copper vessel wears away with time so usage of metal utensils, so wooden or silicone 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.[23] 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.[23]

The steps of Kalai[21] 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.[18][24][25]

The Kalaidzhi are a distinct subgroup of the Romani community, especially in Eastern and Southern Europe. As was common throughout the European Middle Ages and later centuries, large families of particular ethnic groups were represented in an industry or craft. The tinkerers among the Roma have been known for centuries as Kalaidzhi and have made a living by tinplating domestic vessels. The craft itself has been in decline for decades with the advent of mechanization, industrialization, and the growth of urbanism. The subgroup nomenclature has remained, however. This Deutsche Welle documentary follows a Kalaidzhi family's participation in a now-controversial long-lived marital tradition. There is a demonstration of the kalaidzhi tin plating process 15 minutes into the documentary.

Kalai was earlier done with silver instead of tin but now it would be expensive for the Kalaigars to use silver.[26] 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.[18] Now-a-days only some hotels and a very few people use vessels with Kalai done on it.[27] As a result, there are a very few Kalaigars left. The art of Kalai is vanishing.

• Brown, P. J. (1988), "Andrew Yarranton and the British tinplate industry", Historical Metallurgy, 22 (1), pp. 42–8
• King, P. W. (1988), "Wolverley Lower Mill and the beginnings of the tinplate industry", Historical Metallurgy, 22 (2), pp. 104–113
• Minchinton, W. W. (1957), The British tinplate industry: a history, Clarendon Press, Oxford
• Tregoning, W. H. (1901), "Proceedings - Institution of Mechanical Engineers, Parts 3-5", in Institution of Mechanical Engineers (Great Britain) (ed.), The manufacture of tin-plate, Institution by Mechanical Engineering Publications Ltd., pp. 1273–1282
•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Tin-plate and Terne-plate". Encyclopædia Britannica. 26 (11th ed.). Cambridge University Press.