Type metal

Type metal is an alloy of lead. Pure lead is a relatively cheap metal, is soft thus easy to work, and it is easy to cast since it melts at 327 °C (621 °F). However, it shrinks when it solidifies making letters that are not sharp enough for printing. In addition pure lead letters will quickly deform during use; a direct result of the easy workability of lead.

Lead is exceptionally soft, malleable, and ductile but with little tensile strength.

Lead is a poison, that primarily damages brain function.

Tin (Sn) It promotes the fluidity of the molten alloy and makes the type tough, giving the alloy resistance to wear. It is harder, stiffer and tougher than lead.

Antimony (Sb) is a metalloid element, which melts at 630 °C (1,166 °F). Antimony has a crystalline appearance while being both brittle and fusible.[1]

When alloyed with lead to produce type metal, antimony gives it the hardness it needs to resist deformation during printing, and gives it sharper castings from the mould to produce clear, easily read printed text on the page.

The actual compositions differed over time, different machines were adjusted to different alloys depending on the intended uses of the type. Printers had sometimes their own preferences about the quality of particular alloys. The Lanston Monotype Corporation in the United Kingdom had a whole range of alloys listed in their manuals.

Regeneration-metal was melted into the crucible to replace lost tin and antimony through the dross.

Every time type metal is remelted, tin and antimony oxidise. These oxides form on the surface of the crucible and must be removed. After stirring the molten metal, grey powder forms on the surface, the dross, needing to be skimmed. Dross contains recoverable amounts of tin and antimony.

Dross must be processed at specialized companies, in order to extract the pure metals in conditions that would prevent environmental pollution and remain economically feasible.

1. Type metal should produce a true and sharp cast, and reproduce the matrix in all detail. The dimensions and form should be correct after cooling down.
2. Type metal should be strong and lasting, to endure wear and pressure while printing.
3. Type metal should be easy to cast, this means: a reasonable low melting temperature, iron should not dissolve in the molten metal, mould and nozzles should stay clean and easy to maintain.
4. The molten metal should be clean, while molten it should give as little dross as possible, to prevent loss of tin and antimony.
5. The economics have to be taken in account too: keeping the costs down would mean: keeping the content of tin and antimony as low as possible, and maintaining a high quality of the type produced.
6. Type metal should not adhere to the copper of the matrix.

Pure metal melts and solidifies in a simple manner at a specific temperature. This is not the case with alloys. There we find a range of temperatures with all kinds of different events. The melting temperature of all mixtures is considerably lower than the pure components.

The addition of a small amount of antimony (5% to 6%) to lead will significantly alter the alloy's behavior compared to pure lead: although the melting point of pure antimony is 630 °C, this mixture will be completely molten and a homogeneous fluid even at temperatures as low as 371 °C. Letting this mixture cool the alloy will remain liquid even through 355 °C, the melting point of pure lead. Once the temperature reaches 291 °C, lead crystals will start to form, increasing the cohesion of the liquid alloy. At 252 °C, the mixture will start to fully solidify, during which the temperature will remain constant. Only when the mixture has fully solidified will the temperature start to decrease again.

Using a 10% antimony, 90% lead mixture delays lead crystal formation until approximately 260 °C.

Using a 12% antimony, 88% lead mixture prevents crystal formation entirely, becoming a eutectic. This alloy has a clear melting point, at 252 °C.

Increasing the antimony content beyond 12% will lead to predominantly antimony crystallization.

Adding tin to this bipolar-system complicates the behaviour even further. Some tin enters into the eutectic. A mixture of 4% tin, 12% antimony, and 84% lead solidifies at 240 °C.

Depending from the metals in excess, compared with the eutectic, crystals are formed, depleting the liquid, until the eutectic 4/12 mixture is formed once more.

The 12/20 alloy contains many mixed crystals of tin and antimony, these crystals constitute the hardness of the alloy and the resistance against wear.

Raising the content of antimony cannot be done without adding some tin too. Because the fluidity of the mixture will dramatically diminish when the temperature goes down somewhere in the channels of the machine. Nozzles can be blocked by antimony crystals.

Eutectic alloys are used on Linotype-machines and Ludlow-casters to prevent blockage of the mould and to ensure continuous trouble-free casting.

Alloys used on Monotype machines tend to contain higher contents of tin, to obtain tougher character. All characters should be able to resist the pressure during printing. This meant an extra investment, but Monotype was an expensive system all the way.

The fierce competition between the different mechanical typecasting systems like Linotype and Monotype has given rise to some lasting fairy tales about typemetal. Linotype users looked down on Monotype and vice versa.

Monotype machines however can utilize a wide range of different alloys; maintaining a constant and a high production meant a strict standardization of the typemetal in the company, so as to reduce by all means any interruption of the production. Repeated assays were done at regular intervals to monitor the alloy used, since every time the metal is recycled, roughly half a per cent of tin content is lost through oxidation. These oxides are removed with the dross while cleaning the surface of the molten metal.

Nowadays this "battle" has lost its importance, at least for Monotype. The quality of the produced type is far more important. Alloys with a high-content of antimony, and subsequently a high content of tin, can be cast at a higher temperature, and at a lower speed and with more cooling at a Monotype composition or supercaster.

Although care was taken to avoid mixing different types of type metal in shops with different type casting systems, in actual practice this often occurred. Since a Monotype composition caster can cope with a variety of different metal alloys, occasional mixing of Linotype alloy with discarded typefounders alloy has proven its usefulness.

Mechanical linecasting equipment use alloys that are close to eutectic.

Copper has been used for hardening type metal; this metal easily forms mixed crystals with tin when the alloy cools down. These crystals will grow just below the exit opening of the nozzle in Monotype machines, resulting in a total blockage after some time. These nozzles are very difficult to clean, because the hard crystals will resist drilling.

Brass spaces contain zinc, which is extremely counterproductive in type metal. Even a tiny amount — less than 1% — will form a dusty surface on the molten metal surface that is difficult to remove. Characters cast from contaminated type metal such as this are of inferior quality, the solution being to discard and replace with fresh alloy.

Brass and zinc should therefore be removed before remelting. The same applies to aluminium, although this metal will float on top of the melt, and will be easily discovered and removed, before it is dissolved into the lead.

Magnesium plates are very dangerous in molten lead, because this metal can easily burn and will ignite in the molten lead.

Iron is hardly dissolved into type metal, although the molten metal is always in contact with the cast iron surface of the melting pot.