At the time the regulations came into force, in 1903, there were about 708 file-cutting shops in the United Kingdom, of which 517 were in Sheffield. Immediately after they came into force 126 certificates of exemption were granted for use of beds containing less than 5 per cent. of lead, and every year fresh applications are received[16]. There does appear, however, to be difficulty in securing beds which conform to the standards laid down. Thus, in the four years 1907-1910, of 23 samples submitted as containing less than 5 per cent., 16 were in excess.
The number of reported cases of poisoning in the five years 1900-1904 was 151, and 51 in the five later years 1905-1909. The attack rate is about 10 per 1,000, but, although this may be low compared with other trades, the much greater severity of the attacks has to be borne in mind.
File-Hardening.
—The process consists in keeping the files in a bath of molten lead at high temperature, covered with charcoal. The file is removed when red-hot, straightened if necessary, and plunged into a solution of brine.
Dangers and Prevention.
—Poisoning is attributable to fume given off by the molten lead (a temperature of 850° C. was recorded by S. R. Bennett—see [p. 201]), risk of which can only be met by efficient hooding and exhaust unless an alternative method of hardening is adopted, dispensing with the lead bath—e.g., by exposure to heat on a hearth, in a gas furnace, or by other means. In this small industry in and around Sheffield three cases were reported within a year, each involving partial or complete paralysis of the extensors of the forearm. Of ten men employed at the work, three showed presence of a blue line, three were cachectic, and one had weakness of the arms and wrists. In two of the three factories attempts had been made unsuccessfully to carry the fumes away by a hood and duct[17].
Hardening of forks and similar articles in the same manner has also given rise to poisoning.
We are informed that methods of hardening and tempering drills, tools, etc., by the use of fused metallic salts, are being adopted in some works. By mixing together two or more salts in definite proportions, suitable fusing-points can in all cases be produced. These baths can be raised to any desired melting-point to suit the requirement of different steels. For example, sodium nitrate and potassium nitrate in certain proportions give melting-points from 220° to 340° C., and can be used for tempering baths up to 600° C. For tempering above 600° C. mixtures of sodium chloride and potassium chloride can be used, whilst for hardening, sodium chloride or barium chloride give adequate ranges of temperature. Similarly, mixtures of sodium sulphate (melting-point, 890° C.) and lithium sulphate (860° C.) can be made to give any melting-point from 605° to 860° C.
Tinning of Metals.
[18]—Cheap hollow-ware vessels such as kettles and frying-pans are often coated with a mixture of (usually) half lead and half tin by dipping them into a bath of the molten metal, after cleaning in hydrochloric acid.