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Mirach The Pack Mill Process

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. This was drawn into a flat bar (known as a tin bar) at the ironworks or steel 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 form 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 30inches 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 the folded in half 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 is 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 (suing powered shears) and the plates separated by openers (usually women). 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 remove 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 sense polished surface. They were then annealed again(but 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 and oil) and removes 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 14 inches by 20 inches, doubles twice that. A box weighed approximately a hundredweight.

 

What is described here is the process as employed during the 20th century. The process grew somewhat in complexity with the passage of time, as gradually it was found that the inclusion of additional procedures improved quality. 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) and is like a big water tank.