[ [9] Zinn: Berl. Klin. Woch., Nr. 50, 1899.
[10] Serafini: Le Morgagni, No. 11, 1884.
CHAPTER XII
PREVENTIVE MEASURES AGAINST LEAD POISONING
Amount of Lead Fume and Dust in the Atmosphere Breathed.
—Lead fuses at 325° C. and boils at between 1450° and 1,600° C. It is volatile when heated to a cherry-red colour—about 550° C.
Experiments[A] carried out in the laboratory of a lead smelting works in London to determine the temperature at which leady fumes rise from the surface of open baths of molten lead, showed that unless pure lead is heated to about 500° C., and at the same time stirred, no appreciable fume comes off, and that from lead, at the same temperature, under ordinary working conditions, little or no lead in the form of oxide passes into the air. From lead that has been unrefined or which contains zinc—that is, lead in the earlier stages of its manufacture (in the reverberatory furnace)—leady fume was not given off at temperatures less than 760° C. even when stirred, because at a temperature of 600° C. the surface of the molten metal became covered with fluid slag, which will not allow any oxide to be given off. Impurities such as tin or antimony prevent the oxidation of molten lead at lower temperatures, and give it a bright, shiny colour. When heated to about 600° C., these impurities form a slag on the surface of the lead containing antimoniates and stannates of lead, which do not evolve lead fumes unless heated to temperatures never likely to be reached in open lead pots. The reason why molten refined lead can give off lead fume more readily than those named is because the oxide formed on the surface is a dry powder and not in the form of slag. Hence, when the bath is stirred, some of the dry oxide is broken up and may rise into the air. When a bath of molten lead is not stirred at all, it can be heated to over 740° C. without finding oxide in the air aspirated—a temperature not obtained under ordinary working conditions.
[A] In these experiments air was aspirated through an iron funnel having an area of 113 square inches (12 inches diameter), placed at a height of 1¹⁄₂ inches above the molten metal, and connected to an iron tube 3 feet in length and ¹⁄₂ inch in diameter. Inside the iron tube was a glass tube, one end reaching own to the top of the funnel and the other connected with a tube containing pure loose asbestos wool, and continued down to a tightly stoppered bottle holding dilute sulphuric acid. Another glass tube connected this bottle with an aspirator. The asbestos tube was weighed before and after each test, and the asbestos then treated with nitric acid, and the lead determined volumetrically. In none of the tests made was lead found in the bottle containing sulphuric acid.
Were there nothing else to consider but escape of lead fume from a pot or bath of molten metal, obviously hooding over of the bath and removal of the fume from the atmosphere of the workroom would be unnecessary until this temperature was reached. Usually, however, the bath is kept standing exposed to the air, and the oxide which forms on the surface has to be skimmed off periodically, and whenever the ladle is emptied a small cloud of dust arises. Or at times, in certain processes, chemical interaction takes place in the bath, as in the dipping of hollow-ware articles previously cleaned in hydrochloric acid, with evolution of fume of volatile chloride of lead. Any vessel, therefore, of molten metallic lead in which skimming is necessary, or in which chemical action gives rise to fume, requires a hood and exhaust shaft, even although the temperature is little, if at all, above the melting-point—unless, indeed, a separate exhaust can be arranged for the removal of the dust immediately above the point where the skimmings are deposited.
Of many samples of dust collected in workrooms where there are baths of molten lead, it is impossible to say definitely how much of the lead present is due to fume, and how much to dust. Thus, a person tempering the tangs of files was attacked by plumbism, and a sample of dust collected from an electric pendent directly over the pot, at a height of 4 feet from the ground, was found to contain 15·6 per cent. of metallic lead. Similarly, a sample taken above a bath for tempering railway springs contained 48·1 per cent. metallic lead[1]. And, again, a sample collected from the top of the magazine of a linotype machine contained 8·18 per cent. Such analyses point to the necessity of enclosing, as far as possible, the sources of danger—either the fume or the dust, or both. Determination of the melting-point of the molten mass will often help in deciding whether there is risk of fume from the pot, and, if there is not (as in the sample of dust from the linotype machine referred to), will direct attention to the sources of dust in the room. Proceeding on these lines, S. R. Bennett[2], using a thermo-electric pyrometer which had been previously standardized and its rate of error ascertained, and checking the results in some cases by a mercury-in-glass thermometer (the bulb of which was protected by metal tubing), determined the temperature of the various pots and baths of molten lead used in the Sheffield district. As was anticipated, temporary cessation of work, stirring up of metal, recoking of furnaces, and other causes, produced fluctuations of temperatures from minute to minute in the same pot, and in its different parts. The compensated pyrometer used gave for file-hardening pots a maximum of 850° C., and a minimum of 760° C., the average mean working temperature being about 800° C. The variations of temperature of lead used for tempering tangs of files and rasps was found to be high, and largely unrestricted from a practical standpoint. The maximum was 735° C., and the minimum 520° C., the average mean working temperature being 650° to 700° C., varying more than this within a few hours in the same pot. Spring tempering is carried out at some comparatively constant temperature between a maximum of nearly 600° C. and a minimum of 410° C., depending on the kind of steel and the purpose for which the steel is to be employed. Generally, the temperature required rises as the percentage of carbon in the steel is diminished. As these baths are larger than file-hardening pots, the temperature range is higher at the bottom than at the top unless well stirred up. Some lead pots are set in one side of a flue, and the temperature in the mass is then greater on the furnace side. From further observation of these pots during experiments, he was inclined to believe that the lead did not volatilize directly into the atmosphere, as heated water does, but that the particles of coke, fused oil, etc., which rise from the surface, act as carriers of the rapidly oxidized lead particles which cling to them.
Similar experiments were carried out in letterpress printing works. The average temperature was 370° C. in the stereo pots, and in the linotype pots at work 303° C. Scrap lead melting-pots when hottest registered 424° C., but registered as low as 310° C., according to the amount of scrap added, the state of the fire underneath, etc. The best practical working temperature depends largely on the composition of the metal used. That at some factories is the same for stereo drums as for lino pots—viz., 81·6 per cent. lead, 16·3 per cent. antimony, and 2·0 per cent. tin, added to harden the lead. On the other hand, some printers use a higher percentage of antimony in the lino than in the stereo metal. Lead melts at 325° C., and antimony at 630° C., but by adding antimony to lead up to 14 per cent. the melting-point is reduced at an almost uniform rate to 247° C., after which further addition of antimony raises the melting-point. This explains why temperatures as low as 290° C. are practicable for linotype pots. The molten eutectic has a specific gravity of about 10·5, whereas the cubic crystals average 6·5 only; therefore in these pots the latter float on the top, and excess of antimony is to be expected in the skimmings or on the surface.