Metals in Horology – a paper by Jim Nicholson
Mans early use of metals
Early man exploited gold, silver and copper because these can be found ‘native’ or in the metallic state: subsequently their ores, as well as those of tin, were relatively easily reduced to the metallic state at comparatively low temperature.
Iron is very occasionally found in the metallic state in the form of meteorites. It is possible that the relative lack of iron meteorites today, compared with the frequency with which they are believed to have fallen to earth, is because they were exploited by early man. It has been reported that the Inuits of North America used such resources at least to the end of the 19th century.
It must have appeared magical to early man that the smelting of rock resulted in producing a metal and that an alloy of two soft metals (copper and tin) could result in a hard alloy capable of bearing a cutting edge (bronze).
Brass
An alloy of copper and zinc. This seems simple, but it must be remembered that zinc, in a metallic state was only available quite late in history. Copper was produced in the Bristol and Swansea areas in large quantities in the 18th & 19th centuries. At one time 50% of the worlds copper production was done here. Extraction was a complex process involving six or seven separate smelting and the slag from each stage was added to the charge two or three stages later on. Most copper ores are sulphides but also they contained arsenic.
The brass making process involved filling crucibles with the trimmings or punchings from the making of the sheet copper, plus zinc carbonate or calamine, which was found in the lead mines of Derbyshire and the Yorkshire Dales, and powdered charcoal. At first a moderate temperature was employed so that the charcoal reduced the zinc metal in the form of vapour. This was absorbed by the copper and the temperature was then raised to melt the alloy. Process control was lacking; the crude brass was then ‘shoaled’ by pouring through wicker screens into a tub of water. Relatively small variations in the mix could produce large variations in the quality and colour of the alloy.
Old texts mention such materials as Gilding metal, Tomback, Princes metal, Tinsel and ‘Similor’. These were produced by successive re-meltings which were believed to purify the alloy. In actual practice what happened was that each re-melting reduced the amount of zinc until the desired properties were obtained.
It is reported that the Geneva watchmakers used old brass pans as the source of materials for verge crown wheels. The variation of alloy content was first properly investigated by Charles Holzapfel in the 1840’s. He melted copper and added weighted quantities of pure Hamburg zinc, had the alloy analysed, then repeated the meltings many times. He found large loss of zinc on the first melting and smaller losses at each subsequent melting. The brassfounders used a high percentage of scrap brass in their melts and adjusted the mix to produce the alloy required for various applications, usually by casting small pieces and examining the colour and how it behaved under a hammer. Clockmakers saved their scrap brass and filings and sold them back to their casting supplier. A problem was the inclusion of iron and steel filings, which left hard spots in subsequent castings; separating the use of ‘drawing’ magnets was one was one of the first applications of strong permanent magnets. This is one of the reasons why some wheel cutters are reluctant to use expensive Thornton’s cutter on cast brass and still rely on a hand made flycutter for such jobs. Kelvin, when engaged in the production of the first transatlantic telegraph cable found the resistance of samples of copper wire could vary by a factor of two. This was traced to arsenic impurities. Modern copper is finally refined by electrolysis, giving an extremely pure metal and any traces of gold or silver are recovered from the sludge in the bottom of the electrolysis tanks.
The formation of zinc oxide in the air during melting of brass can give chest problems to the workers. Nearly all brass castings for candlesticks, door handles etc. are now produced in India where health and safety practices are lax.
Current grades of Brass
Cast/Cartridge Brass 70%Cu/30%Zn CZ106 Clock Brass 58-60%Cu/38-40%Zn/1.5-2.5%Pb CZ120 Rod Brass 58Cu/39Zn/2-3%Pb CZ121
Effect of too much lead (Pb) – cheap electrical fittings. Annealing CZ121 for wheel collets Quarter, half or full hard – depends on amount of rolling after final anneal – half hard usually for clocks.
Gilding Silver
90%Cu/10%Zn CZ101 85%Cu/15%Zn CZ102 80%Cu/20%Zn CZ103
German Silver
10-30%Ni/27-13Zn, Cu balance Usually used as a basis for silverplate – white in colour but no Ag content.
Invar
This is 36% nickel alloy, balance being Iron. Discovery by Guillame when working with I.B.W.M. at Sevres. The range of Nickel/Iron alloys is wide, but only for a very small range is the thermal expansion so remarkably low. Nowadays usually described as Nilo 36. Somewhat subject to random small changes in length. Can be stabilised by alternative heating and cooling cycles. (domestic chest freezer and back of radiators).
Iron and Steel
Bloomery process (never molten)
Yields a soft bloom of iron, with bits of slag and charcoal embedded. Forged by hand to expel the dross. Could be heated in contact with charcoal to give a very variable steel.
Blast furnace
Furnaces got larger and water powered bellows and the charge actually melted. A new material was discovered; cast iron with about 4% carbon. First used fire backs, grave markers and cannon. Eventually used for machine framing and steam engine cylinders. Still depended on charcoal and England was short of timber, Abraham Darby at Coalbrookdale developed the use of coke which got rid of much of the sulphur in the coal.
Henry Cort
Invented the puddling process in the 1780’s. Cast iron was re-melted on a bed of iron oxide scale in a reverberatory furnace. This could be fired by coal since the fuel did not come into contact with the charge. The melted cast iron was stirred and gradually the carbon was oxidised away, the melting point was raised and a ball of wrought iron produced. Like Bloomery iron it then required repeated heating and hammering to consolidate it. Slag streaks were still present but this became the main method for producing iron until the 1860’s and was a major structural materials. Due to remaining impurities it was unsuitable for making into steel.
The Sheffield methods
Comentation
Sheffield had from the time of Chaucer been well known for its cutlery. (A Sheffield thwitle bare he in his hose) The steel required was largely imported from Spain and Germany in the form of ‘gads’ – tiny pieces about four inches long. A better material was called for. This was found by importing Swedish and Russian iron – this had been smelted and worked to the bar form using only charcoal as fuel. This coupled with the low phosphorous and sulphur in the Dannemora ores gave a pure iron but which still had slag streaks in it.
Bars of iron were placed in stone chests and heated in contact with charcoal by external coal fires. The cycle occupied about two to three weeks. The resulting bars were broken into short lengths, piled together and hammer welded by water power to produce shear steel. The piling and forging could be repeated to produce a more homogeneous material, double or triple shear steel.
Crucible process
Every Horological text I have ever read and most popular treatises say that Benjamin Huntsman was led to his discovery by problems with his clock springs. This is utter nonsense, the man never made a clock spring in his life only ordinary long case clocks. His problem was cutting tools, the slag streaks in cemented steel were longitudinal, this did not matter for springs. Try breaking small pieces of French clock springs along and across the length, this shows the effect well.
Consider a screwing tap, slag streaks running through the tips of the threads would cause fracture in use.
Huntsman initially produced melts of only 6 pounds and it took him many years before he made steel suitable for springs. This required a lower carbon content and consequently had a much higher melting point.
Until the Bessemer process of 1858, these methods were the mainstay of steel production. Sheffield produced 80% of U.K. production and 50% of the world’s production.
What should one use?
A few suggestions on modern materials:
En 1A – free machining mild steel En 8 0.4% carbon – will harden in small sections. Good for pinions Silver steel 0.95 – 1.2% carbon. Cutting tools En24T Nickel/Chrome steel already heat treated to 65/75 tons. Good tough steel for pinions but doesn’t spoil cutters. Gauge plate. Ideal for pallets. Similar to silver steel but with small alloy additions. Will harden in either oil or water.