For the manufacture of sheet copper, the metal is cast either direct from the refining furnace, or from remelted pigs, into flat cakes 3 or 4 inches thick. As soon as these cakes have "set" they are dumped into cold water to make them soft for rolling. They must be not less than 99.75% pure for rolling, though all of the impurity allowed must not be wholly arsenic, bismuth or antimony, because the cakes may crack slightly on the surface in rolling.

As soon as the cakes have cooled, their surfaces are examined for spots of cinder, oxide, scale, etc. Large spots are chipped out, or the whole cake may be pickled in dilute sulphuric acid and then scrubbed and rinsed to remove all surface scale. The cakes are taken over to the rolling mill and several are heated at a time to redness in a reheating furnace in order to keep the rolls supplied.

One cake at a time is taken from the furnace and passed repeatedly through rolls such as are shown in Fig. 55. This view shows three 26-inch pull-over sheet mills. This type of mill is used for rolling thin sheets of commercial metals, and is called a pull-over mill because a man on the far side of the rolls receives the sheet as it comes through and, by aid of tongs and a guide bar, lifts it so that the man on the front side can pull it over the top roll to pass it again between the rolls.

Fig. 55. - Sheet Mills, Pull-Over Type.

If the sheet is to be finished as soft copper, it is rolled hot down to the thickness required, is then annealed in a furnace to make it of uniform softness throughout, and is pickled to remove roll and furnace scale. The sheet is then straightened by sending it through straightening rolls, or it may be straightened in a stretching machine which grips the ends and pulls the sheet.

If the sheet is to be finished as hard copper, suitable for making pipes, or tanks which must stand pressure, it is rolled hot nearly to the finished thickness, pickled, and then rolled cold in the finishing rolls clown to gage. It is then straightened and trimmed to size.

To avoid the formation of furnace scale when heating copper and brass to be rolled, it is a common practice to do the heating and annealing in muffle furnaces.

Cold rolling increases tensile strength, but makes copper and brass more brittle and springy, which is a decrease in ductility.

Sheets of copper, known as planished copper, which are springy and show highly polished surfaces, are cold rolled after having been hot rolled and pickled.