Type metal
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The term type metal represents a range of metal alloys that are used in traditional typefounding and mechanical typesetting.
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[edit] History
Although the knowledge of casting soft metals in moulds was established before Gutenberg's time, it represents an aspect of his solution to the problem of printing with movable type.
The enormous effort to bring about an alloy with the characteristics needed in an ideal type metal should not be underestimated.
[edit] Characteristics
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.
To solve the first problem, after much experimentation, it was found that adding the pewterer's tin, obtained from cassiterite, improved the ability of type to better withstand the wear and tear of the printing process, making the the type metal tougher but at the same time 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.
Since alchemists had shown that when stibnite, an antimony sulphide 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 sulphides would be rejected in the process.
The addition of antimony conferred the much needed improvements in the properties of hardness, wear resistance and especially, the sharpness of reproduction of the type design, given that it has the curious property of diminishing the shrinkage of the alloy upon solidification.
[edit] Composition of type metal
Although type metal alloy are basically conformed by the same three metals, the proportions vary according to the intended final use.
Generally speaking, the proportions are somewhere in the order of: lead 50‒86%, antimony 11‒30% and tin 3‒20%. The basic characteristics of these metals are as follows:
[edit] Lead
Melting point 327 °C
Exceptionally soft, malleable and ductile but with little tenacity. Easily fusible with other metals. Does not produce sharply defined castings.
[edit] Tin
Melting point 232 °C
Soft, ductile, tougher than lead. Improves casting properties by making the molten alloy more fluid.
[edit] Antimony
Melting point 630 °C
Crystalline appearance, brittle, fusible. When alloyed with lead, strengthens the alloy and improves casting detail.
[edit] Typical type metal proportions
| Eutectic alloy | tin 4% | antimony 12% | lead 84% |
| Slugcasting alloy | tin 3% | antimony 11% | lead 86% |
| Stereotype alloy | tin 7% | antimony 15% | lead 78% |
| Monotype alloy | tin 10% | antimony 16% | lead 74% |
| Stereotype alloy | tin 18% | antimony 28% | lead 54% |
