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CHAPTER II.

WHITE PIGMENTS.

THE white pigments are a very important group of painters' "colours," probably the most important, as while the red, blue, green, &c., pigments are used simply or almost entirely as colouring pigments, the white pigments are used in two ways1st, as "body colours," i.e., to give body or covering power to paint; 2nd, as "colouring pigments." Thus, in making a red paint, white lead or barytes is added to give the necessary body and vermilionette is used to colour the paint. On account of this dual feature of the white pigments they merit a more detailed account of each individual member of the group than is necessary for other pigments.

The white pigments are a fairly numerous group of bodies derived entirely from inorganic sources. Many white bodies are known which could be used as pigments, but are not so used on account of expense, &c. The following list comprises all that are used either on a large or small scale :

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White Lead, basic carbonate of lead, 2 Pb CO2 Pb H2O2; this pigment is also sold under a variety of other names.

Lead Sulphate, Pb S O4; many pigments sold under various fancy names consist essentially of this body combined with other white pigments.

Lead Oxychloride, Pb, O Cl, Pattinson's white lead.

Zinc White, zinc oxide, Zn O.

Zinc Sulphide, Zn S; this body combined with barytes, &c., is largely used as a white pigment.

Barium Sulphate, Ba SO,, barytes.

Barium Carbonate, Ba CO3.

Calcium Sulphate, Ca S O,, gypsum.

Calcium Carbonate, Ca CO, whiting.

Calcium Oxide, Ca O, quicklime.

Strontium Sulphate, Sr SO4.

Strontium Carbonate, Sr CO,.

Magnesium Carbonate, Mg CO2, magnesite.

China Clay, hydrated silicate of alumina.

French Chalk, silicate of magnesia.

Of these, the most important are white lead, lead sulphate, zinc white, zinc sulphide, barytes, gypsum, calcium carbonate, and china clay.

WHITE LEAD.

White lead has been known and used as a pigment for centuries; the Romans and Greeks used the native carbonate of lead or 66 cerusse, as it was then called, from which the mineralogical name cerussite has arisen. This natural pigment is found only in comparatively small quantities, and it is no wonder that a process for the artificial production of white lead was soon found out and adopted, with the result that natural cerussite is not now used as a pigment.

It is not known when white lead was first made, who made it, or to what country it owes its birth. The oldest known method is that commonly called the "Dutch method," from the supposition that it was invented in Holland; it is described as the Dutch process in an English patent granted in 1787, and there is no doubt but that it is the process referred to in three earlier patents granted in 1622, 1635, and 1745, in which it is spoken of as an old process. Evidently white lead has been made for several centuries. During all this period there has been but little change made in the Dutch process. But in the interval inventors have not been idle, for there is no other pigment which has attracted so much attention at their hands as white lead; and the number of processes and modifications of processes which have been devised, is almost innumerable. With all this invention, the ancient Dutch process still retains its pre-eminence as the best process for the manufacture of white lead.

MANUFACTURE OF WHITE LEAD.

White lead, the basic carbonate of lead, is manufactured by a variety of methods. It is not easy to classify these processes into groups, as they not unfrequently pass one into the other imperceptibly. The author suggests the following classification, which is based on the principles which appear to underlie the various methods adopted, or which have been proposed and used on a limited scale :

1st Group.-Stack method.

2nd Group.-Chamber methods.

3rd Group.-Precipitation processes based on the action of carbonic acid gas on various lead salts.

4th Group.-Precipitation methods based on the action of alkaline carbonates on various lead salts.

5th Group.-Miscellaneous methods. Most of these are now obsolete and are only of historical interest.

1st GROUP.-STACK METHOD.

Only one process is included in this group, the old Dutch or stack process. No process which is now in use or which has been proposed can claim the antiquity that this process can, and, notwithstanding all the labours of chemists and white lead

makers to supersede it, for reasons which will be pointed out presently, it still remains the best process for the manufacture of white lead. Tradition assigns its discovery to the Dutch and to a person named Stratingh in particular. It must be at least 300 years old. Since 1787 this process has been carried on without much alteration in its details. The Dutch method is used in all

Fig. 3.

parts of the world for the manufacture of white lead, and there is but little variation in the details of the process and in the construction of the plant used in different countries.

The plant used in the stack process is shown in Figs. 2 to 5. A shed of brick work, Fig. 2, is built, the size of which varies a little, but averages 16 feet long by 13 feet wide and 20 feet high; this may have either a lean-to roof, as shown in the figure; or, as in some works, two of these sheds are built back to back, with a single-ridge roof between them.

In some places parts of the structure are built below the level of the ground, but there is no advantage to be gained by so doing: A large white-lead works will have a number of these sheds, so as to keep the workmen fully occupied with filling and emptying them.

These pots vary

A number of earthenware pots are provided. in size at different works, but an average size is 8 inches high by

4 inches in diameter. In shape they resemble crucibles (see Fig. 3), but have a shelf inside, as shown. In the bottom of these pots is placed some weak acetic acid or vinegar; this diluted acid contains about 2 to 3 per cent. of actual acetic acid. On the shelf inside the pot is placed a roll of thin sheet lead (Fig. 4),

made from a strip of lead 2 feet long by 4 inches broad. In a stack of ordinary dimensions some 11,000 to 12,000 of these pots will be used, and they will contain about 800 to 900 gallons of weak acid.

The stack is built up as follows:-First, a layer of ashes, upon which is placed a layer of spent tan of about 3 feet in thickness. In the older Dutch method horse-dung was used, but this is open to some disadvantages which will be pointed out presently; the use of tan was introduced in England so that this modification of the Dutch process is sometimes spoken of as the English method. This layer of tan is pressed down very firmly and is evenly spread; on it is placed a layer of the pots, which layer is kept at a distance of about 6 inches from the sides of the shed. In some works the outside rows of pots are made of larger size than the others, so as to act as supports for a layer of flooring boards. In other places the pots are all of one size and wooden supports for the boards are provided.

On the top of the pots is placed a layer of lead buckles or gratings (Fig. 5). These are placed face to face in a layer of about 3 to 5 inches thick; above these comes the layer of flooring boards, a space of about 6 inches being left between them. On the top of the boards another layer of tan, then a layer of pots, then a layer of gratings, then another layer of boards, and so on until the stack is completely built up. The number of sets of layers varies from seven to eleven. The doorway through which the filling is done is closed as the work progresses by boarding, but a small space is left at the top through which the progress of the operation can be observed, and fresh additions of material made as required to allow for sinking of the tan, &c.

The quantity of lead used varies considerably, or from about 3 tons to 7 tons in a layer of materials, so that in a large stack there may be something like 85 tons of lead.

In stacks of very large area it is usual to construct chimneys throughout the mass, whereby the steam which is produced during the operation is carried off; in stacks of small area, these chimneys are not required, as the space around the side walls of the shed affords a sufficient outlet.

When the stack is built up it is left for a period of about three months. During this period the stack gets quite hot (140° F.) through the fermentation of the tan which sets in; large quantities of carbonic acid gas are given off, and the acetic acid is converted into vapour. The "blue lead" is gradually

converted into "white lead." At the end of three months the stack is pulled to pieces. As the boards are removed it is found