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and 14 show respectively the longitudinal and transverse sections. From these drawings it will be seen that it is a low oven, open only in front, over which is constructed a hood and chimney to carry off the products of combustion, &c., from the furnace. The bed usually measures about 11 feet by 12 feet 4 inches, and is divided (as shown) into three divisions—the central one measures 8 feet 4 inches wide, and constitutes the bed or hearth of the furnace, while the two side divisions measure about 2 feet each, and form the fireplaces of the oven, as a
rule, they are not fitted with firebars; the partitions between the fireplaces and hearth are formed of firebrick; the bed of the furnace is made to slope from the back to the front, usually the back is about 7 inches higher than the front, while it also slopes slightly from the side to the centre. In the front of the furnace are three doors—the two side ones are for feeding the fires, while the centre one serves for introducing and extracting the material, and for working the charge while in the furnace; it is placed a little higher than the two side doors so that a draught is generated through the latter and out of the centre door; in the top of this door an opening is left so that the products of combustion, &c., can pass out and up the chimney.
This furnace is open to improvement, and an improved form is shown in Fig. 15, from which it will be seen that this form of furnace has firebars fitted to the fireplaces.
The operation of drossing is carried out in the following manner :-22 cwts. of lead, which is the quantity usually dealt with in one charge, are placed in the furnace, which is now
raised to a dull red heat, just enough to melt the lead, the molten lead being prevented from flowing out of the furnace by the construction of a dam, formed of pieces of dross or “ leanings” from previous workings, across the front of the hearth; the melted lead rapidly becomes coated with a layer of
oxide, the formation of which is hastened by rabbling the lead and pushing the oxide as it is formed to the back of the furnace, the object being to always have a fresh surface of lead exposed to the oxidising action of the air which passes through the furnace. The workmen by a peculiar splashing action while rabbling expedite this oxidation very much; at intervals pigs of lead are thrown into the furnace. This drossing takes about 10 to 12 hours, at the end of which time the dam across the front of the furnace is broken down, and the unmelted lead allowed to run out, while the “dross” or “casing," as it is called, is taken out to be worked for the next stage. The furnace is now ready for another charge.
The “dross" or "casing" has a rather bright yellow colour, and is coarse in texture ; it consists essentially of the monoxide of lead, Pb O, but still contains some unoxidised lead. It is
now ground and levigated with water; the oxide grinds to the form of a fine powder, while the lead is simply flattened out, and by sieving can easily be removed; it is sent back again into the furnace, while the ground oxide is washed by a stream of water into settling tanks, where it settles out in the form of a paste, which is ready for use in the next stage.
One point of importance in the drossing stage is to see that the temperature is carefully regulated, so that, while it is above the melting point of the lead and therefore in a molten state, yet it is below the melting point of the casing; as the margin is not great, considerable care has to be taken to avoid overstepping the limit. If the casing is allowed to melt it passes into litharge and this cannot be converted into red lead. The dross or casing is also known as massicot.
2nd Stage, Colouring. The next operation consists in heating the dross obtained in the first stage, either in the same oven or in another, which only differs from the drossing oven in a few minor details. The colouring oven is heated to a low red heat, care being taken to ensure a large supply of air. The operation takes about 48 hours, and the mass is frequently rabbled during that period; after it has been in about 12 hours a sample is taken out and its colour examined; this sampling is repeated at the end of each twelfth hour and near the end of the operation more frequently When the red lead has attained the correct colour, the fires are drawn and the furnace allowed to cool down; when cold, the red lead is drawn from the oven, ground as finely as possible, and sent into the market.
The change which takes place in the transformation of the metallic lead into red lead is shown in the following equations1st Stage.
Pb, 04 Lead monoxide plus oxygen forms lead peroxide. Theoretically, 100 lbs. of lead should yield 110.36 lbs. of red lead, practically, about 108 lbs. of red lead are obtained, which is a very near approach to the theoretical amount. The best red lead for painters' use is made from pure lead, as the presence of impurities in the metal has a material and injurious influence on the colour of the product; iron, in particular, causes the colour to be dark. For glass-makers' red lead a pure product is absolutely necessary, as an impure lead causes the glass to be coloured, not white as it should be.
Burton's Process.-Although the only process at present
worked for the preparation of red lead is the one described above, yet in 1862 Burton patented a process for making red lead from sulphate of lead, in which 1 equivalent or 1.894 parts of lead sulphate are mixed with 1 equivalent or 0.665 part of sodium carbonate and l equivalent or 0.143 part of sodium nitrate. The mixture is heated to a dull red heat with an excess of nitre; the fused mass is lixiviated with water, whereby the red lead formed is separated from the alkaline salts, and this is washed and dried.
PROPERTIES AND COMPOSITION OF RED LEAD. - Red lead is a heavy, bright red powder of an orange hue, its specific gravity being 8.53. Heat turns it to a dark brownishred, but the colour is restored on cooling. Acids act on red lead. Nitric acid and glacial acetic acid first dissolve out the monoxide, leaving the dark puce oxide; on further boiling, this gradually dissolves and colourless solutions of the nitrate or acetate are formed. Hydrochloric acid when heated with red lead decomposes it with the evolution of chlorine and the formation of the chloride, which settles as the solution cools in the form of transparent needles, a very characteristic reaction of lead. Sulphuric acid boiled with red lead forms the sulphate, with the evolution of oxygen.
Red lead is a combination of the two oxides of lead, the monoxide, Pb , and the puce or dioxide, Pb 0; it is generally considered that they are present in the proportion of two equivalents of the first to one of the second, red lead, therefore, having the formula Pbą 04, the percentage composition being
There is reason for believing that Pbg 04 does not accurately represent the true composition of red lead; for, although the proportions of the two oxides is about that given in the above analysis, it is probable that the whole of the monoxide present is not combined with the dioxide as red lead, but that some of it is in the free condition ; this free oxide is not distinguishable from the combined oxides by treatment with acids, but, by treating with a 10 to 12 per cent. solution of lead nitrate, it is possible to extract 16 to 31 per cent. of free oxide, while the red lead obtained contains 25.4 to 25.7 per cent. of dioxide.*
* Lowe, Dingl. polytech. Journ., vol. 271, pp. 472-477 ; Woodman, Chemical Trade Journ., April 24, 1897.
The formula of red lead would then be Pb, Os, which is that assigned to it by Phillips and other authorities. Percy* gives the following analysis of red lead :
Lead monoxide, Pb 0,
80.54 per cent.
which corresponds to the formula 4 Pb O, Pb Oor Pb, 08.
Both Pb, 04 and Pb, 0, are known; the former is much easier to prepare than the latter, and the latter can only be made by repeated oxidation of the monoxide. A little free monoxide is desirable in red lead, as then the colour is not so readily liable to spoil by over-oxidation.
As a pigment red lead is very useful, it mixes very well with linseed oil, and takes from 8 to 9 per cent. of it to grind into a stiff paste. It exerts a powerful drying action on the oil; hence, paint containing red lead dries very quickly; on this account, also, red lead mixed with linseed oil is largely used as a lute and packing for steam pipes and joints of all kinds. It possesses good covering and colouring power, and is capable of resisting all ordinary atmospheric influences, although it is liable to be discoloured by sulphuretted hydrogen as is the case with all lead pigments. It may be mixed with nearly all pigments, the only exceptions being those containing sulphur, such as ultramarine, cadmium yellow, &c.
ASSAY AND ANALYSIS OF RED LEAD.—Red lead should be assayed for colour, fineness, and body in the usual way. It is rarely adulterated; but if so, it is usually by the oxide of iron reds. The quantity of red lead in such an adulterated sample can be ascertained by taking 2 grammes and boiling with nitric acid until it is thoroughly decomposed; the insoluble matter can be filtered off and its amount ascertained by weighing it; to the solution, which is colourless if the red lead be
but yellow if there is any iron present, a little dilute sulphuric acid is added, and a precipitate of sulphate of lead obtained ; this is filtered off, washed, dried, and weighed in the usual manner. The weight multiplied by 0.955 gives the amount of red lead in the sample. The solution from the lead sulphate can be tested for iron, &c., by the usual tests.
Percy, Metallurgy of Lead.