Malleable Bronze

The Encyclopedia of Founding (1892)

Important: Safety Warning

This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.

This information has been left in for historical purposes but should not be acted upon.

Hard bronze may be made malleable by the addition of from 1/2 to 2 per cent mercury, which may be combined with either of the metals composing the mixture before the bronze is finally made. It can be put into the melted copper at the same time the tin is added, or can be used as an amalgam with the tin. See BRONZE; ALLOYS; BRASS. Malleable Cast Iron is made by a process of de-carburizing the articles made from cast or pig iron in the annealing furnace, where they are subjected to an oxidizing atmosphere somewhat below the fusing-point. The furnaces employed for this purpose consist of iron plates to enclose the necessary space, which serve also as guides for the doors, which are raised perpendicularly, being balanced by a weight at the back of the furnace. The inside of the furnace consists of fire-space and the oven proper, an arch extending over both, the fire-space being separated from the oven by a bridge wall extending nearly to the arch, leaving only a narrow space through which the flame is forced by the blast, completely filling the oven. The gases escape through small outlets in the corners to the flues below, which are fitted with a damper operating from the front. The castings to be rendered malleable are placed in cast iron covered boxes or saggers, along with oxide of iron, and subjected to a red heat in these furnaces for from two to six days, according to the magnitude of the castings. They are then allowed to cool slowly. See DECARBONIZE.

What Has Changed in the Last Century

The 1892 recipe of adding half a percent to two percent mercury to soften bronze has no place in modern foundry practice: mercury boils at 357 °C, far below the roughly 1,100 °C of molten copper, so little of it could survive the melt, and its use is now barred on health grounds under the 2013 Minamata Convention. Today malleability in copper alloys is obtained by composition and heat treatment instead — low-tin and phosphor bronzes, aluminium bronzes, and controlled annealing. The account of malleable cast iron is closer to the mark: castings are still packed with iron oxide and annealed for days to decarburise them, though the practice has been largely displaced since Keith Millis's 1943 discovery that magnesium treatment produces ductile (nodular) iron straight from the mould, without the long anneal.

Written for this edition, 2026.