Type metal

In printing, type metal (sometimes called hot metal) refers to the metal alloys used in traditional typefounding and hot metal typesetting. Lead is the main constituent of these alloys. Antimony and tin are added to make the character produced durable and tough while reducing the difference between the coefficients of expansion of the matrix and the alloy.

Although the knowledge of casting soft metals in moulds was well established before Johannes Gutenberg's time, his discovery of an alloy that was hard, durable, and would take a clear impression from the mould (because it did not shrink as much as lead alone when cooled) represents a fundamental aspect of his solution to the problem of printing with movable type. (His other contributions were creation of inks that would adhere to metal type and a method of softening handmade printing paper so that it would take the impression well.)

The enormous effort to create an alloy with the characteristics needed in an ideal type metal is often underestimated.

Cheap, plentifully available as galena and easily workable, lead has many of the ideal characteristics, but on its own it lacks the necessary hardness and does not make castings with sharp details because molten lead shrinks and sags when it cools to a solid.

After much experimentation it was found that adding pewterer's tin, obtained from cassiterite, improved the ability of the cast type to withstand the wear and tear of the printing process, making it tougher but not more brittle.

Despite patiently trying different proportions of both metals, solving the second part of the type metal problem proved very difficult without the addition of yet a third metal, antimony.

Alchemists had shown that when stibnite, an antimony sulfide ore, was heated with scrap iron, metallic antimony was produced. The typefounder would typically introduce powdered stibnite and horseshoe nails into his crucible to melt lead, tin and antimony into type metal. Both the iron and the sulfides would be rejected in the process.

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