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  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  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.