The group of metals which concern the printing trade are mainly lead, antimony and tin, which, when alloyed, form the type metal series.
Lead is a soft, silver-grey metal with a melting point of 327o centigrade (620oF.), found principally in nature as galena, a lead sulphide. The lead supplied from Broken Hill (NSW, Australia), which constitutes one of the largest commercial deposits in the world, is also of the highest purity.
Lead alone is too soft to be used as a type metal and lacks many other valuable properties needed for type metal purposes.
Antimony is a white, brittle crystalline metal, with a melting point of 630oC. (1166oF.), and is obtained from Stibnite, which is a compound of antimony and sulphur. It is used in type metals from 3% to 23% to increase hardness and fluidity.
Tin is a silver-white metal melting at 232oC (457oF.). It occurs in nature as an oxide termed cassiterite. The ore is widely distributed throughout the world, but main world supplies are drawn from S.E. Asia and Bolivia. Used with antimony and lead, tin produces a series of low melting point alloys which constitite the printing metals.
Chief of the many qualities which make these alloys specially suitable for type production is the ability to produce sharp-edged smooth-faced impressions. This results from a low surface tension of the molten metal and a reduced amount of casting shrinkage. The relatively low melting points of the printing metals avoid damage to mould materials, thus simplifying and cheapening reproduction costs. The basic material upon which the alloy series is built is a metal consisting of 4 parts Tin, 12 parts Antimony and 84 parts of lead, commonly described as 4-12-84 Linotype formula.
This metal has the lowest melting point of all type metal alloys, runs freely when melted, and hardens abruptly on cooling to produce a dense faced casting.
By varying the composition to include increasing amounts of tin and antimony, a useful hardness increase can be obtained to meet applications requiring heavy pressure, and to provide superior resistance to severe wear arising from various paper finishes.
Herewith is the range of metal composition generally employed in printing applications, together with the important initial and final freezing points and hardness.