No, 2 (1921) REMARKS ON CANNING 4/ 



of very thin sheets of steel coated with tin, and is usually packed 

 in boxes containing 1 12 sheets ; these boxes weigh very differently 

 according to the area and thickness of the plates, e.g., from 70 to 

 160 lb. The selection of plate is a most important matter in 

 every way ; if cans solid drawn from the flat are made, the plate 

 must be of specially ductile character or the bodies cannot be 

 struck without great loss from fractured or torn metal. If the areas 

 of the plates are not carefully adapted to the intended size of the 

 cans, so as to get the maximum number of bodies or tops and 

 bottoms with the minimum of waste strips, the cans will be costly 

 and the metal wasted. If the gauge is too thick there will be 

 unnecessary cost since the plates are charged for by weight, while 

 the cans will be uselessly heavy and hard to cut open ; if the 

 gauge is too thin the cans will be weak and, especially if processed 

 under pressure, may ' spring ' and bulge with a permanent ' set ' 

 owing to expansion, so that the tops or bottoms will have the 

 appearance of ' swells ' and be rejected by dealers and consumers 

 though in reality perfectly good; such cans, in which the top or 

 bottom can be pressed in and out like a loose drum head, are 

 technically called ' springs ' to distinguish them from ' blown tins ' 

 or 'swells' which are caused by decomposition, and of which the 

 tops and bottoms are both convex and cannot at all be pressed in. 

 See paragraphs 136 — 139 below. 



Rusty plate must of course be avoided both by reason of bad 

 appearance and of effect upon the contents and sealing of cans. 

 Solder does not adhere to rusty edges ; this is a fruitful source of 

 leaks especially in hot and humid climates and when cans are 

 processed under pressure (see below s.v. ' Processing ')• 



26. The quantity of tin per square inch of plate is very 

 important; cheap plate is apt to be defective both in the quality 

 of the metal and the quantity of tin used. Since the tin protects 

 the iron from rust and from corrosion by fruit and other acids, no 

 plate should be used which is not up to high standard. For it is 

 principally the action of acids and other elements on the iron of the 

 tin plate that causes improper flavours and not the action on the 

 tin, which is usually unimportant in fact except as regards appear- 

 ance (e.g., the blackening of prawns), though very occasionally 

 found in the goods. Fortunately iron salts are innocuous. 



27. Solder. — This essential item is often insufficiently attended 

 to, though perhaps, in India, the most costly single item in a can. 

 In a full-sized quarter tin there are 29 inches of soldered seam 



