that it may be heated to a red heat; this vessel, generally known as "the balloon," adds considerably to the temperature and, therefore, to the drying power of the stove. Over the furnace and balloon, and around the sides of the stove, are built ranges of skeleton shelves of iron, on which is placed the bricks of barytes taken from the tops of the drying flues, and where they are allowed to remain until they get properly dry, which takes several days; when dry they are removed from the stove, and the barytes is ground up in mills to a fine powder, and then packed for sale.

"Floated barytes" is, or should be, a fine quality prepared by a process of levigation; in other respects it does not differ from ordinary barytes.

PROPERTIES AND COMPOSITION OF BARYTES.

Barytes is a fine and rather heavy white powder, having a specific gravity of 4.5 to 4.75. It has a more or less crystalline structure, which causes it to have a slight gritty feel, no matter how fine it may be ground. It is quite insoluble in all acids and alkalies, a property, or rather properties, which distinguishes it from other white pigments. It is quite unaffected by any injurious influences, such as affect white lead, and, therefore, as a pigment it is the most permanent white known; for this reason it has been called, and sold, as constant white, permanent white, &c.

In body or covering power it is much inferior to either white lead or china clay, but it is better than zinc white. It mixes very well with oil, of which it takes about 7 per cent. to grind into a stiff paste; as an oil paint it is satisfactory in use, but for water colours it is not so good, seeming to lose some of its body on mixing with water.

Barytes consists essentially of barium sulphate, Ba S O, but commercial barytes generally contains traces of oxide of iron, silica, &c. The following analyses will show the general composition of barytes:

It is largely used for mixing with other pigments, especially with white lead, many commercial samples of which contain from

20 to 25 per cent. of barytes; its presence in such is easily detected by its insolubility in acids and its tinging the Bunsen flame green.

ASSAY AND ANALYSIS OF BARYTES.

Barytes should be assayed for colour and covering power in the usual way. There being no cheaper pigment, barytes is never adulterated, but it is used as an adulterant in other pigments. It is distinguished by its high specific gravity, being heavier than any other white pigment except white lead; and by its insolubility in acids, which distinguishes it from all other white pigments except china clay. Barytes can be detected by moistening a little with hydrochloric acid, and holding it on a platinum wire in the lower part of a Bunsen flame, when, if barytes is present, the latter will become tinted with a pale yellow green colour. It is not always easy to see this colour, as sometimes it only comes in flashes, while at others it is more persistent; much depends upon the amount of acid used. With a little care the presence of barytes in any pigment may be detected by this test.

Blanc fixe or artificial barytes is obtained as a fine white precipitate when sulphuric acid or a sulphate is added to a solution of a barium salt, as, for example, the sulphocyanide with aluminium sulphate; the reaction is shown in the equation

Generally the chloride is used when it is desired to make blanc fixe.

The sulphate precipitated is allowed to settle, the aqueous layer is run off, fresh water run in to wash the precipitate free from acid, &c., and the precipitate collected on filters. It now forms a pasty mass containing from 70 to 75 per cent. of barium sulphate, and from 25 to 30 per cent. of water, and in this form is largely sold to paper stainers, cotton finishers, and others. It is rarely dried for use as a pigment in oil and water-colour painting. Blanc fixe is, when dried, a very fine white powder, possessing a great degree of colouring power and body, in which respects it very much excels the natural barytes; hence it

is preferred by users to the natural variety, and is employed as a base for many of the large pigments prepared from the coal-tar colours (see Chapter IX.), as also in the preparation of zinc sulphide whites, being generally, in such pigments, precipitated simultaneously with the actual colouring principle. Its chemical properties are identical with those of barytes, given above. It is much more expensive than ordinary barytes, because the natural barytes is the source from which all other barium salts are derived, and the cost of making first the chloride and then the sulphide from this adds to the cost of the latter. Barium salts, such as the acetate, sulphocyanide, &c., are used in the preparation of mordants, such as acetate of chrome, sulphocyanide of alumina, &c., by double decomposition with sulphates of chrome and alumina, in which case barium sulphate is also formed as a bye-product; this may be used as blanc fixe, proIvided it is well washed.

GYPSUM, TERRA ALBA, MINERAL WHITE.

Gypsum is a mineral found in great abundance in many parts of the world. In this country it is found at Chellaston, near Derby, Aston-on-Trent, and a few other places in Derbyshire, at Newark-on-Trent in Nottinghamshire, Fauld in Staffordshire, and at Netherfield in Sussex. At these places it occurs in large quantities. In smaller quantities it is found in other places. In France there are large deposits, more especially in the district round Paris; it is also found in Germany, America, Canada, and other countries.

Gypsum is the sulphate of calcium, CaSO4; but, unlike the corresponding barium sulphate, gypsum contains 2 molecules of water of crystallisation, so that it has the formula, Ca SO4, 2 H2O; this water of crystallisation confers on it some important properties which will be briefly noticed presently. The composition of gypsum is

Sulphuric anhydride, SO3,
Calcium oxide, Ca O,.
Water, H2O,

46.51 per cent.
32.56
20.93 ""

Gypsum is found in several forms. The most useful form is that in large amorphous, crystalline masses of a white or nearly white colour and more or less opaque; this is the variety used for a pigment. Often it occurs more or less coloured in a variegated manner; this variety is known as alabaster, Derbyshire spar, and is used for making ornaments. Satin spar is a

variety of gypsum occurring in large fibrous silky-looking pieces; this variety is also used as a pigment. Selenite is a variety which occurs in large transparent flat pieces, which have the property of cleavage very highly developed; this is chiefly used for optical purposes. In all its forms gypsum is a soft mineral, easily scratched with the finger nail and easily ground into a white powder. Its specific gravity varies from 2-28 to 2-33.

For use as a pigment gypsum is ground up in the same way as barytes, when it is obtained in the form of a soft white powder of a very good colour known as terra alba, mineral white, satin white, &c.

It can also be prepared artificially. It is a bye-product in some operations, as in preparing acetate of alumina from sulphate of alumina and acetate of lime, or, generally, whenever acetate of lime or other soluble calcium salts are precipitated by sulphates

In preparing many coal-tar colours gypsum is obtained, and this bye-product could be used in preparing the so-called aniline lakes (see Chapter IX.).

Properties of Gypsum.-The colour of gypsum is generally very good, and in tone is a bluish-white, rather bluer than barytes, but not so blue as white lead; occasionally samples may have a yellow tint, which is due to the presence of oxide of iron, which, however, can be eliminated by treatment with acids, as in the case of bleaching barytes. It is much lighter than either white lead or barytes, but is rather heavier than china clay or zinc white. Its body is not as good as that of white lead, but it is at least equal to barytes in this respect. It mixes well with water, but is rather transparent when mixed with oil. Being neutral in its properties, it can be mixed with all other pigments without affecting them, or being affected by them in any way. On exposure to light and air it is unaffected, being one of the most permanent pigments known. It is used very largely by paper stainers and makers of paper hangings, who prefer it to barytes on account of its having more body when used for that class of work. It is used in the finishing of cotton goods, in paper making, and for a variety of other purposes where a cheap white pigment is required.

Gypsum is slightly soluble in water, about 1 part in 500; this solution will give a precipitate of calcium oxalate on addition of ammonium oxalate and a precipitate of barium sulphate on addition of barium chloride. It is more soluble in hydrochloric acid; long boiling with dilute hydrochloric acid will dissolve it without effervescence, and the solution will show the presence of both calcium and sulphuric acid on the application of the usual tests for those bodies. Gypsum is also soluble in solutions of ammonium salts.

At a temperature of about 300° F. gypsum loses its water of crystallisation and forms a white powder, which has the property of combining with water and setting into a hard mass; this property is a very important one, the manufacture of the white powder being carried on on a large scale, and the product sold under the name of plaster of Paris for various ornamental and useful purposes.

ASSAY AND ANALYSIS OF GYPSUM. Gypsum should be assayed in the manner given in Chapter X. for tint, brilliancy, covering power, and other properties appertaining to a pigment. A chemical analysis is rarely required and it can be made in the usual way; oxide of iron, silica, barytes, and whiting being the substances most likely to be added or present in a sample of gypsum. Generally, the gypsum offered commercially is nearly pure, containing about 78.5 per cent. of calcium sulphate, the rest being water of crystallisation.

SATIN WHITE.

6

Satin white is a pigment consisting essentially of a mixture of calcium sulphate (Ca SO,) and aluminium hydroxide (Al, H. O). It is very largely employed by paper stainers and others on account of its fineness, freedom from grit, good colour, and opacity, which gives it good covering and staining power.

It is prepared by slaking quicklime (16 parts) with water to a thick cream; dissolving alumina sulphate (34 parts) in water; heating the two, and then mixing them, allowing the mixture to stand for a few hours, then filtering, washing, and drying the precipitate or residue. Care must be taken to employ a good quality of lime, which should be free from grit and much insoluble matter; hence chalk lime is better than limestone lime. Another method of preparing it is to take 118 lbs. of freshly-slaked lime, adding to this 350 lbs. of alumina sulphate and 10 lbs. of alum; the mixture is then worked in a pug mill