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

Fig. 15.

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

how 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 equations— Pb O

1st Stage.

2nd Stage.

=

Pb +
Lead plus oxygen forms lead monoxide.

=

3 Pb O + 0 Pb3 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 1 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 O, and the puce or dioxide, Pb O2; 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 Pb, O4, the percentage composition being

There is reason for believing that Pb, 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, O,, which is that assigned to it by Phillips and other authorities. Percy* gives the following analysis of red lead:

which corresponds to the formula 4 Pb O, Pb O,, or Pb, O

4

Both Pb, 0, and Pb, O, 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 pure, 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.

ORANGE LEAD.

This pigment is identical in composition with red lead, but is rather paler in colour and lighter in weight. To make it, white lead is placed in a furnace similar to a red lead furnace, and heated to a low red heat for from 24 to 48 hours, or until the mass has acquired the desired red tint. During this operation the white lead loses its carbonic acid and water, while it takes up oxygen from the air which passes through the furnace. The change is shown in the following equation :—

2 Pb CO3 Pb H2 O2 + O

=

Pb3 04 + H2O + 2 CO2.

In washing white lead a scum collects on the top of the washing waters; this is collected and made into orange lead, and gives a brighter and more bulky product than dry white lead. Orange lead has a slightly paler colour than red lead, is more voluminous and of lower specific gravity, which is about 6.95 In its composition and properties it is identical with red lead, and it is used for very similar purposes.

RED OXIDE, INDIAN RED, AND IRON REDS.

Ferric oxide, Fe, Og, the red oxide of iron, is the basis of a very large number of pigments which are sold under the names of rouge, light red, Indian red, red oxide, Venetian red, purple oxide, scarlet red, &c., which are all red pigments of varying shades of colour. In the hydrated form, ferric oxide also forms the colouring principle of the ochres, siennas, and umbers. The red oxides are valued very highly as pigments, on account of their generally fine colour and their permanence.

Ferric oxide occurs naturally in a variety of forms in the minerals hæmatite, specular iron ore, limonite, &c., in which it is nearly chemically pure. As a rule, they are too dark in colour, and too hard to be used as pigments; but, occasionally, deposits of oxide of iron are found of sufficient brilliance to be used as a pigment-e.g., the Warton oxide named below.* Indian red was originally found native, but is now mostly of artificial manufacture. Such substances as red ochre, yellow ochre, red raddle, umber, &c., owe their colour to oxide of iron, but they contain many other bodies.

These pigments are prepared both from natural and artificial Before entering into the details of their preparation a

sources.

* At Bovey Tracey, in Devonshire, a micaceous iron ore, commonly known as "shining ore," is found, and largely employed for making paint. It exists in the granite formation, in lodes running in an east and west direction; they are small, varying from 2 feet to 6 inches in width. The ore is mined by driving adits or levels into the hill sides. The granite in which the ore is found is that of the eastern fringe of the Dartmoor range.