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Additional recommended knowledge
The term type metal (sometimes called "hot metal") represents a range of metal alloys that are used in traditional typefounding and mechanical typesetting.
Although the knowledge of casting soft metals in moulds was well established before 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 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.
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, be it individual character mechanical casting for hand setting, mechanical line casting or individual character mechanical typesetting and stereo plate casting.
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:
type of metal 10b, or 613
Exceptionally soft, malleable and ductile but with little tenacity. Easily fusible with other metals. Does not produce sharply defined castings.
type of metal 24sno
hard, stiff, tougher than lead. Improves casting properties by making the molten alloy more fluid.
type of metal 623
Crystalline appearance, brittle, fusible. When alloyed with lead, strengthens the alloy and improves casting detail.
Typical type metal proportions
Alloys for mechanical composition
Most mechanical typesetting is divided basically into two different competing technologies: line casting (Linotype and Intertype) and single character casting (Monotype).
The manuals for the Monotype composition caster (1952 and later editions) mention at least five different alloys to be used for casting, depending the purpose of the type and the work to be done with it.
Although in general Monotype cast type characters can be visually identified as having a square nick (as opposed to the round nicks used on foundry type), there is no easy way to identify the alloy aside from an expensive chemical assay in a laboratory.
Apart from this the two Monotype companies in the USA and the UK also made moulds with 'round' nicks. Typefounders and printers could and did order specially designed moulds to their own specifications: height, size, kind of nick, even the number of nicks could be changed.
Type produced with these special moulds can only be identified if the foundry or printer is known.
Type metal alloys mentioned in the UK-Monotype-caster manuals
In Switzerland the compagny "Metallum Pratteln AG, in Basel had yet another list of type-metal alloys. If needed, any alloy according to customer specifications could be produced.
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 some grey powder -the dross- will be left. This dross still contains type metal.
Dross must be processed at specialized companies, in order to extract the pure metals in conditions that would prevent environmental pollution.
General requirements for type-metal
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.
The basis for type metal is lead, a relatively cheap metal, that melts at 621 °F. It is easy to handle and makes alloys with many other metals. It is however very soft, and castings with pure lead are not sharp enough for printing.
Melts at 1166 °F, this highly crystalline metalloid, gives type metal hardness and a much better and sharp cast from the matrix.
This soft metal has a low melting point at 450 °F. Tin promotes the fluidity of the molten alloy and makes the type tough, giving the alloy resistance to wear.
Behaviour of bipolar alloys
Pure metal melts and solidifies in a simple manner at a specific temperature. This is not the cases with alloys. There we find a range of temperatures with all kinds of different events. The melting temperature of all mixtures is considerable lower than the pure components.
Example: addition of a small amount of antimony some 5 or 6% :
Although the melting point of antimony is 1166 °F, this mixture will be completely molten and a homogeneous fluid at 700 °F.
Cooling down, at 671 °F, the melting point of pure lead, nothing will happen. After cooling down until 555 °F, lead crystals will start to grow. This will make the fluid more and more pasty. The temperature will drop until 486 °F, before solidification starts. Only when the fluid has become completely solid, and all melting-energy is lost to the environment, the temperature will lower again.
A mixture of 10% antimony and 90% lead: the formation of the crystals will only start later at a lower temperature of some 500 °F. After this the temperature will drop to 486 °F, and remain there until solidification is complete again.
An alloy with 12% antimony and 88% lead has a sharp melting point at 486 °F. No crystallisation will occur above this temperature.
This mixture is called: eutectic.
Higher contents of antimony will raise the temperature where crystallisation starts. This crystals contain a high content of antimony depleting the fluid of this metal, until the fluid becomes eutectic at last.
The resulting solid contains smaller and larger crystals of antimony surrounded by a small crystalline -almost fibric- eutectic.
Adding tin to this bipolar-system complicates the behaviour even further. Some tin enters into the eutectic. Which has the composition of 4% per cent Tin, 12 per cent Antimony and 84 per cent lead, this mixture solifies at 464 °F.
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.
Metals used on typecasting machines
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.
Present usage of type metal
The fierce competition between the different mechanical typecasting systems like Linotype and Monotype has given rise to some lasting fairy tales around typemetal. Linotype users looked down on Monotype and vice versa.
Monotype machines however can utilize a wide variety of different alloys, maintaining a constant and a high production meant a strict standardisation of the typemetal in the company. To reduce by all means any rupture of the production. Repeated assays were done at regular intervals to monitor the alloy used. Because 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 for a good deal, 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 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.
Contamination of type metals
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 is of inferior quality, the solution being to discard and replace it 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 this way the metal in the pot.
Iron is hardly dissolved into type metal, although the molten metal is always in contact with the cast iron surface of the melting pot. It's symbol Fe comes from the latin word Ferrum.
Historic references to type metals
Joseph Moxons', in his Mechanick Exercises, mentions a mix of equal amounts of antimony and iron nails. This mixture is heated red hot in an open-air furnace, and when all is molten and finished, red hot heated lead from another melting pot is added and stirred thoroughly.
The "antimony" here was in fact stibnite, antimony-sulfide Sb2S3, the surplus of iron was burned away in this process, reducing the antimony while removing the unwanted sulphur.
Only while casting small characters and narrow spaces some tin was added, otherwise the mould would not be filled in a properly. The good properties of tin were well known, at different times the use of tin was minimized to save expenses.
Much of this toxic work was done by a labour force that included children. As an antidote to the inhaled toxic metal fumes, the workers were given a mixture of red wine and salad oil.
N.N., The Monotype Casting Machine, The National Committee for Monotype Users' Associations, London, UK, 1952.
Moxon, Joseph, Mechanick Exercises, reprint of the 1896-edition, Thoemmes Press, Bristol, UK.
Fry's Metal Foundries, Printing Metals, Great Britain, self published, revised edition 1966.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Type_metal". A list of authors is available in Wikipedia.|