that the lead which has been corroded still retains the form of the blue lead, but is more bulky in volume, is white or greyish in tint, and opaque. The corrosions are not of a uniform character throughout the whole of the stack; in some places they are porcellaneous and flaky, are firm to handle, do not break up, and give the best quality of white lead; in other parts of the stack the corrosions are soft, easily crumble to a fine powder or dust when handled, and do not give a good quality of white lead. In some places the lead may be discoloured owing to a variety of causes, such as the presence of tarry matter in the acid (especially when crude pyroligneous acid is used), by droppings of coloured water from the layer of tan on to the lead, &c. In chemical composition the corrosions will vary; in some places they will approximate closely to the normal composition of white lead, 2 Pb C Og, Pb H, O, in others more nearly to that of 3 Pb C Og, Pb H, O2, while in others they consist of the normal carbonate, Pb C Og.

As the stack is being pulled to pieces the corrosions are carried to the grinding rooms. The method of treating the white lead varies in different works, but the following may be taken as a good example of the usual manner of working :The corrosions are first passed through a pair of rolls; these break up the masses, the white lead crumbles to powder, while the unchanged blue lead is flattened out into thin sheets. The crushed materials are then sieved, which separates the white from the blue lead; the latter is sent to the melting pot where it is melted and re-cast for use in building another stack. The white lead is sent into tanks full of water, where it is thoroughly agitated, and the small traces of acetate of lead which the corroded lead contains washed out of it. While still wet the white lead is ground as fine as possible under edge runners or between rollers, and then dried, when it is ready for sale. As the grinding must be thorough, the lead is passed through several sets of grinding mills.

Grinding white lead is a source of danger to the workpeople, for the fine dust flies about the room in which it is done and is breathed by the workpeople, who, sooner or later, suffer from lead poisoning; much of this danger is avoided by grinding the lead in a wet condition only, when the particles of lead are practically too heavy to fly about.

The greatest risk now arises in the packing of the ground lead, as the workmen frequently get some on their hands and eat their meals without previously washing their hands. Lead poisoning may be prevented by drinking water acidulated with

sulphuric acid, whereby the lead absorbed into the system is converted into the harmless sulphate of lead. The great trouble is that the workmen will not take sufficient care to make use of these precautions.

It was stated above that in the early or Dutch modification of this process, horse-dung was used as the source of the heat and carbonic acid necessary to carry on the process; while with dung the process is quicker (only taking from 8 to 9 weeks), yet it is not so good as the English method with tan, the product is not quite so regular in composition, and it is more liable to discolouration owing to the evolution of sulphuretted hydrogen from the decomposing dung, and to its combination with lead to form the objectionable black sulphide.

The theory of the process of white-lead making by the Dutch process, which at present is most favoured by chemists, and was substantiated by some experiments carried out by Hochstetter, is due to Liebig. The first action which goes on in the stack is to convert the blue lead into basic acetate of lead; this is brought about by the heat of the fermenting tan, or dung, causing the evolution of acetic acid from the liquid in the pots, which, attacking the lead, causes the production of the normal acetate of lead, thus

The normal lead acetate, under the influence of water and heat, parts with some of its acetic acid and passes into the basic acetate, thus

The acetic acid is ready to attack a further quantity of blue lead. The basic acetate is now attacked by the carbonic acid evolved by the fermenting tan, the acetic acid it contains is liberated and its place taken by the carbonic acid, and white lead is formed, thus―

3. 2 Pb 2 C2H3O2 + 2CO2 = 2 Pb CO3, Pb H2 O2 + 4 H C2 H3 O2

Normal lead

acetate.

Carbonic
acid.

White lead.

Acetic acid.

Although the reactions shown in the above equations are those usually accepted as representing the formation of white lead from blue lead in the stack process, yet they are probably

not quite correct; the evolution of hydrogen in the first step in the process is rather improbable; a better explanation would be the following:-(1) By the action of moisture and oxygen on the lead there is formed lead hydroxide, thus

1. Pb + H2O + 0 =

Pb Hg 02
Lead hydroxide.

Then this being acted on by the acetic acid forms the normal or neutral acetate and water, thus

= Pb 2 C2 H3 O2

2. Pb H2 O2 + 2H C2 H3 O2 + H2O The normal acetate now combines with lead hydroxide to form basic lead acetate, thus

3. Pb 2 C2 H3 O2 + 2 Pb H, O2

=

Pb 2 C2 H3 02, 2 Pb H2 Og

Basic lead acetate.

This is now acted upon by the carbonic acid with the formation of white lead and normal acetate, thus—

4. 3 [Pb 2 C2 H3 02, 2 Pb H2 O2] + 4CO2

2

=

3 Pb 2 C2 H3 02

Normal acetate.

+ 2 [2 Pb CO3, Pb H2 O2] + 4 H2O

White lead.

The normal acetate thus reproduced then forms more tribasic acetate by the reaction shown in equation 3. It is again decomposed by the carbonic acid, as shown in equation 4, so that a continuous cycle of changes is set up; the lead being oxidised to lead hydroxide, and this converted into white lead, the basic carbonate, pari passu with its formation.

As a rule, nearly all the blue lead is converted into white lead, one ton of lead producing one and a quarter ton of white lead, the amount varying from time to time according to the degree of perfection with which the corrosion has proceeded.

The great fault of the Dutch process is the great length of time required (8 to 12 weeks), the great amount of capital it takes to construct a stack of lead, and the loss of interest which takes place on the capital while the lead is in process of making. Then there is always a risk, owing to some defect, of producing a useless and imperfectly corroded lead, which has to be sent to the smelting furnace and again reduced to blue lead. Hence it is that inventors have turned their attention to devising other methods of producing white lead which shall be free from the defects of the Dutch process; so far, however, no such method has been discovered.

White lead, as made by the process described above, is a basic carbonate of lead having the composition

therefore having the formula 2 Pb C O3, Pb H2 O2.

2

As will be seen hereafter, when the properties of white lead as a pigment come to be more fully considered, the carbonate is the substance to which white lead owes its colour and body; while the hydroxide with which it is associated, by chemically combining with the oil used to convert the white lead into a paint, imparts to the white lead great covering properties.

Numerous samples of white lead, of both high and low qualities, have been analysed by many chemists, some of which will be given later on. These analyses show that when the composition of any sample varies greatly from the figures above given, it is more or less defective. Generally, an increased proportion of carbonate, while causing the colour to be better, reduces the covering power; on the other hand, an increase in the amount of the hydroxide causes a loss of body and opacity. If the sample contain any monoxide then the tint becomes more yellow or greyish. Some of these points will be touched upon when considering the other processes for the production of white lead. In the meantime the success or non-success of any process depends upon the approximation of the white lead produced to the composition above given.

2nd GROUP.-CHAMBER METHODS.

In one sense the Dutch method just described is a chamber method, but it has been classed as a separate group because while being made in a chamber, it differs materially from those now to be described. In these processes the operation of whitelead making is carried on in large chambers in which metallic lead is placed, and into which currents of carbonic acid gas, acetic or other acid vapours, are passed, together with air and steam. The different methods are distinguished one from another by the construction of the chambers, the method of admitting the

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acid gases, &c., and in other points. Although many such have been invented and will be found described in the Patent Records, yet very few are in actual operation for the production of white lead; and of those which have become obsolete very little is known beyond the, often very scanty, description which is to be found in the specification of the patent which protected the process.

In all the accounts which have appeared describing these processes, and which are evidently copied from one source, the chamber process is given as "the German method;" but it is a matter of doubt whether the process was invented in Germany or not. The author is inclined to consider it to be of English origin, partly because it is described in a patent taken out in 1749 by Sir James Creed, who makes no mention of having taken it from a foreign source. This was the first patent to describe a chamber process, and since then many have been patented; but the author must refer readers to the Patent Records for an account of these or to a series of articles which appeared in The Chemical Trade Journal in October and November, 1890.

1. Creed or German Process.-This process was the first in which chambers were used in conjunction with lead and acid gases. In detail it is carried out as follows:-A chamber of brickwork is built of any convenient size and with few openings, the usual ones are a door to enter into the chamber for the purpose of filling it, and an opening in the roof for ventilation; sometimes a window or two for the purpose of observation is added. The chamber has a number of shelves, on which are placed sheets or gratings of lead; it is immaterial which are adopted, although the gratings expose more surface to the action of the various gases which are used. When all the lead is placed on the shelves, the doors are closed, and currents of aqueous vapour, air, carbonic acid, and acetic acid at once admitted into the chamber. After a period varying from four to five weeks, the white lead will have been formed; it is collected and treated as in the Dutch process. The chemical action which proceeds is supposed to be the same as that which takes place in the old stack method. The acetic acid acts upon the lead, forming neutral acetate of lead; this, under the action of the aqueous vapour, is transformed into basic acetate of lead, and this, in its turn, is changed by the carbonic acid into basic carbonate of lead or white lead.

The quality of the product is usually very good, not, perhaps, quite equal to that produced by the Dutch method, but better than that produced by the precipitation processes. It is, how