with as much water as will make a thin paste, the ingredients being kept mixed for an hour or so; the pigment is then filtered, washed, and dried.

Satin white may be distinguished from gypsum or mineral white by its greater solubility in acids and by the solutions giving a white, gelatinous precipitate of alumina hydroxide on the addition of ammonia. In its other properties it resembles gypsum, except that it is rather more opaque, is whiter in colour, and has greater colouring and covering powers.

STRONTIAN WHITE.

49

Strontian White.-Sulphate of strontium, Sr S O occurs naturally as the mineral celestine in small quantities in many places; it is used on a limited scale as a pigment, being prepared for this purpose in the same way as barytes is made, for which it is often sent out as a substitute. It can also be made by adding a solution of sulphuric acid or a sulphate to a solution of a strontium salt. Prepared in this way it enters into the composition of some patent white pigments.

As a pigment it is quite equal, if not superior, to barytes in some points, and in all respects its properties are identical. If it could be obtained in larger quantities it would be a formidable rival to both barytes and gypsum.

Strontium sulphate may be distinguished from barytes by its lower specific gravity and by its giving a crimson colour to the Bunsen flame; this latter test will also distinguish it from gypsum, from which it is also distinguished by its less solubility in water and acid and its higher specific gravity.

WHITING, PARIS WHITE, SPANISH WHITE,

ENGLISH WHITE.

3.

The body sold under all these names, of which the first is by far the most common, is the carbonate of calcium, Ca C O This compound is one of the most important and abundant of rock-formers; it occurs in large masses, forming, in many cases, mountain ranges of no mean size, and, in a variety of forms, only surpassed in number by those of quartz. It not only occurs as a rock-former, but it is found as a mineral, that is as a body of definite form and composition, in great abundance and variety of shape. One of the rocks made of calcium carbonate is chalk, which, in this country, is largely developed both north and south of London, especially in the

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WHITING, PARIS WHITE, SPANISH WHITE, ENGLISH WHITE. county of Sussex, the coast line of which is celebrated for its chalk cliffs. It is also found in the north of France, in the district between Paris and the English Channel. In England it is extensively quarried for a variety of purposes, one of which is the manufacture of whiting; and English whiting has a world-wide reputation for quality. Chalk is essentially a rock of organic origin, and it owes its formation to the operations of certain minute organisms known as Foraminifera; these bodies have the property of secreting a shell of calcium carbonate, and chalk is the remains of a deposit of the shells which has been formed after the death of the animal that inhabited them. Chalk is very nearly pure calcium carbonate, but it contains traces of silica, oxide of iron, alumina, &c. Besides the traces in which silica is found distributed throughout the main mass of the chalk, it also occurs in large masses of very irregular shapes known as flint.

For use as a pigment, the chalk is quarried, ground under water in edge-runner mills to a coarse powder, then passed to flatstone mills, where it is ground very fine; from which mills it is run into tanks, in which the coarser and heavier grit and sand settles, while the finer chalk passes on to other tanks in which it settles. When the settling tanks or pits are full, the chalk or whiting is dug out and dried. It is first taken to a hot room, on the floor of which it is thrown in heaps. Under the floor are a number of flues connected with a furnace fed from the outside of the chamber; sometimes the flues do not pass under the floor of the chamber, but round three sides of it, much in the same manner as in the drying-room used in making barytes. In this room it remains about twelve hours, when the great bulk of the water it contains will have evaporated away. The partially dry mass is now cut into masses of a cubical shape, and taken to a large drying-room, constructed similarly to the one described under barytes. In this hot chamber it is placed on iron racks, and kept there until it is thoroughly dry, which will take from 24 to 48 hours, according to the degree of dryness of the whiting, and to the temperature of the drying stove; the drying is also facilitated if a current of air is sent through the drying-room during the process, as is done in some works. The temperature of the drying stove should not be too high, and in arranging the whiting on the racks it is advisable not to place it in any position where it is liable to be subjected to extreme heat. After being dried, the whiting is ground in a flatstone mill before it is sent out for use.

It may be stated that one reason for avoiding over-heating

during the process of drying is to prevent the conversion of the carbonate of lime into quicklime. One of the properties of carbonate of lime is that when heated it loses its carbonic acid, and passes into the state of oxide, which has some powerful alkaline properties undesirable for some purposes to which whiting is put.

Paris white is a finer quality of whiting, prepared from chalk in the same way, but the grinding is more thoroughly done. Spanish white is an old name given to Paris white sold in a cylindrical form, prepared by moulding the wet material into that form, and allowing it to dry in the open air.

COMPOSITION AND PROPERTIES OF WHITING.— Whiting is a dull white powder of an amorphous character, and soft to the feel; it is moderately heavy, the specific gravity being about 2.5 to 2.8. It consists chiefly of calcium carbonate, but it may alsc contain traces of silica, water, &c. The following analysis of an ordinary commercial sample will show the average composition of whiting:

in water containing It is also soluble in

It is quite insoluble in pure water, but carbonic acid gas in solution it is soluble. acids such as acetic, hydrochloric, nitric, &c., with effervescence due to the evolution of carbonic acid gas. Sulphuric acid decomposes it with effervescence, carbonic acid being evolved, and insoluble calcium sulphate formed. There are but few white pigments which are soluble in acids with effervescence; white lead and whiting are the most common, while the carbonates of barium, strontium, and magnesium are occasionally met with.

Heat decomposes it with the evolution of carbonic acid gas, CO2, and the separation of calcium oxide, quicklime, Ca O; this property is taken advantage of in preparing lime for making cements, mortars, &c.

As a pigment it is mostly used as a body colour in distemper work, colouring walls, ceilings, &c., using water as a vehicle. It is not used as an oil colour, for it is subject to the defect that, when mixed with oil, it loses its white colour and turns a dirty grey; mixed with about 18 per cent. of linseed oil, it forms the useful article known as putty.

It has a tendency to be somewhat alkaline in its properties. In this respect much depends upon the care which has been exercised in drying the whiting during the process of manufac

ture; over-heating is apt to cause the formation of small quantities of quicklime, which increases the alkaline properties of the whiting. It is not safe, therefore, to mix it with such pigments as Prussian blue, chrome yellow, verdigris, emerald green, &c., which are more or less affected by alkalies. Most other pigments-ultramarine, yellow ochre, oxide reds, &c., are affected by whiting in any way.

It is quite permanent when used as a pigment, and is not at all acted on by any of the ordinary atmospheric agents which are often destructive to pigments.

ASSAY AND ANALYSIS OF WHITING.-The assay and analysis of whiting is a matter which is rarely required, the pigment is so cheap that it is not subject to any adulteration. When, however, an assay is required, it can be done in the usual way.

Whiting is distinguished by the following tests :-It is soluble in dilute hydrochloric acid with much effervescence; there may be left undissolved small traces of silica in a gelatinous form, but no considerable amount of white powdery insoluble residue I will be found. On adding to this solution ammonium chloride and ammonia, no precipitate of iron or alumina should be obtained, or, at most, a very slight one. On further adding ammonium sulphide, no precipitate should be obtained. Addition of ammonium oxalate to the same solution throws down a white precipitate of calcium oxalate. The filtrate from this should not give with sodium phosphate more than a mere trace of a precipitate of magnesium phosphate.

The quantity of calcium carbonate or whiting which may have been added to any pigment may be ascertained by throwing it down as calcium oxalate, as described above, and then filtering off this precipitate; but it is best to allow it to stand for some time before filtering, so as to give time for the precipitation to become complete. The precipitate is well washed, then dried, placed in a weighed crucible, burned, and weighed in the usual manner. The weight of the residual calcium carbonate gives the weight of the whiting in the sample taken.

MAGNESITE

is the form in which magnesium carbonate, Mg CO2, occurs in many localities, closely resembling the more crystalline varieties of calcium carbonate in appearance. Ground up and levigated with water it has been offered, from time to time, as a pigment, but it has not come into general use. It is rather heavier than

whiting, its specific gravity being about 3.05. Its colour is good, and it forms a dense white powder; in body and covering power it is quite equal to barytes. It works well in both oil

It is

and water, and mixes fairly well with other pigments, although it has a slight tendency to possess alkaline properties. quite permanent, and resists exposure to light and air.

Magnesite may be distinguished by the following tests :It is soluble in dilute hydrochloric acid, or in sulphuric acid with effervescence. This solution gives no precipitate with ammonium chloride and ammonia, ammonium sulphide or ammonium oxalate, but gives a copious white, somewhat crystalline, precipitate of ammonium magnesium phosphate on addition of sodium phosphate to an ammoniacal solution; this precipitate, on ignition, is converted into magnesium pyrophosphate.

CHINA CLAY.

China clay or, as it is sometimes called, kaolin, which is its Chinese name, is a natural product, and only requires levigating and drying to prepare it for use as a pigment. It owes its name to its use in the manufacture of the white and finer varieties of pottery. Prior to its discovery in this country the pottery in common use had a brown colour, due to the use of clays highly contaminated with iron, and much of the best kinds of white pottery were imported from China; hence the use of the term, "china," for white pottery. When the existence of the deposits of china clay in Cornwall was found out, and its use in making pottery became general, the goods so made were denominated 66 china," and the material "china clay." As a pigment it is not used to the extent its many good qualities entitle it to.

ORIGIN AND COMPOSITION OF CHINA CLAY.In England and other countries where china clay is found it is invariably associated with granite rocks, and is evidently a decomposition product from them. The exact cause which has led to the decomposition of the granite rocks in those places where china clay is found, and to its not undergoing this decomposition in others, where it is equally well developed, is a matter which at the present moment is one of uncertainty.

Granitic rocks are formed by the aggregation of three minerals in various proportions; these three minerals are quartz, felspar, and mica. Quartz is the special form of silica (oxide of silicon, Si O2) which occurs in these granitic rocks; it is absolutely unchangeable by any amount of exposure to the atmosphere.