White Lead-Antimony Pigments.
Lead antimonite and antimonate are both heavy, white powders which can be used as pigments. They are dearer than white lead, to which they are inferior in covering power, and which they do not exceed in permanence.
Lead Antimonite is obtained by heating 5 parts of finely powdered antimony with 20 parts of sulphuric acid until a dry, white mass of antimony sulphate is left. This is fused with soda ash, the melt is extracted with water, and lead antimonite obtained by precipitating with lead acetate.
Lead Antimonate is formed by introducing in small quantities at a time a mixture of 1 part of finely powdered stibnite (antimony trisulphide) with 5 parts of sodium nitrate into a red-hot crucible, boiling the mass with water and precipitating the solution with lead acetate.
CHAPTER VIII.
ENAMEL WHITE.
Barium sulphate, known as permanent white, enamel white, blanc fixe, barytes white, is the only white pigment which is absolutely unaltered by exposure to the atmosphere. Lead pigments are discoloured in the course of time, and in the end turn black; bismuth white behaves in the same manner; zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention from the colour manufacturer, especially as it can be made by a very simple and cheap method. When sulphuric acid or a soluble sulphate is added to the solution of a barium salt, all the barium is at once precipitated in the form of barium sulphate.
When quite pure, barium sulphate forms an extremely soft, brilliantly white powder, which offers complete resistance to the action of the atmosphere, and also of strong acids and alkalis. It is extremely insoluble, and can be precipitated from the most dilute solutions, and is then obtained in so fine a state of division that it cannot be filtered from the liquid; it passes through the closest filter together with the liquid. When the barium solution is heated to boiling before precipitation, the precipitate is somewhat coarser, and can be filtered off without difficulty.
Barium sulphate occurs ready formed in nature as the mineral barytes or heavy spar. Finely ground barytes may be used alone as a pigment, but more commonly is used for reducing white lead, for which purpose it is particularly applicable on account of its high specific gravity. This admixture must be regarded as diminishing the quality of the pigment, because ground barytes has far less covering power than white lead. Artificial barium sulphate is in a state of division which cannot be reached by grinding barytes, consequently it considerably surpasses the latter in covering power.
The raw material for the manufacture of enamel white is either barytes or witherite (barium carbonate); the latter, however, occurs so rarely, in comparison with barytes, that the greater quantity of all barium compounds is obtained from barytes.
If witherite is obtainable in large quantity, enamel white can be prepared from it by dissolving in hydrochloric acid and precipitating the solution of barium chloride so obtained by sulphuric acid. If the witherite is very pure, the process may be simplified by treating the mineral directly with sulphuric acid, and separating the enamel white by a process of levigation from the impurities. In this case it is, however, necessary to add a small quantity of hydrochloric acid to the sulphuric acid, for the latter forms on the surface of the witherite, at the commencement of the reaction, a thin layer of barium sulphate, which is quite sufficient to prevent the further action of the acid on the witherite lying below. The hydrochloric acid forms barium chloride, which is at once decomposed by the sulphuric acid into barium sulphate and free hydrochloric acid; this again dissolves a fresh quantity of witherite, and this process is repeated until the mineral is completely and quickly dissolved.
Enamel white is, however, generally prepared from barytes, which is ground into a very fine powder and converted into barium sulphide by heating with coal (see pages [41] and [42]). Hydrochloric acid acting on the sulphide produces barium chloride and sulphuretted hydrogen.
The covering power of a pigment is greater the finer its state of division, so that it would appear advisable to precipitate a weak solution of barium chloride by sulphuric acid at the ordinary temperature. When the barium sulphate has been completely precipitated, a solution of pure hydrochloric acid remains, which ought to be utilised; but when very dilute barium chloride solution is used, the hydrochloric acid is so dilute as to be useless. The barium chloride is, therefore, given in practice such a strength that it has a specific gravity of about 1·198; when the barium sulphate has been precipitated from this solution the residual hydrochloric acid has a specific gravity of 1·043.
Water of considerable purity must be used to dissolve the barium chloride. Experience has shown that water which contains appreciable quantities of organic matter does not give a pure white product. The presence of sulphate of lime in the water, which precipitates barium sulphate, need not be regarded, because the barium sulphate is so finely divided that it remains suspended in the liquid, and is carried down on precipitation of the enamel white by sulphuric acid. Carbonate of lime in the water causes the separation of barium carbonate; this may be avoided by slightly acidifying the barium chloride solution, thus converting the calcium carbonate into chloride.
According to C. A. F. Meissner, artificial barytes, suitable for use in oil paints, is obtained by precipitating barium salts by soluble sulphates in place of sulphuric acid, then quickly heating the washed and dried precipitate in a muffle to a red heat and throwing into cold water.
As has been already stated, enamel white is the most permanent pigment that exists; it appears destined in course of time to replace white lead and all other white pigments, especially as its cost is generally lower than that of the other white pigments. It costs, for example, only half as much as white lead. At present, the principal uses of enamel white are found in paper staining; it is not used to any extent in oil paints. On account of its permanence, it should be used in the place of white lead and white zinc. It also appears particularly suitable for obtaining pale shades; it can be mixed with any other pigment in any quantity without altering it in the least. This is, of course, only true when the enamel white is completely pure, and when it has been freed from every trace of hydrochloric acid by careful washing.
Lithopone.
A white pigment is obtained, according to Orr’s process, by lixiviating crude barium sulphide, obtained by igniting barytes with coal, with water and dividing the solution into two parts. Zinc chloride is added to the first portion, then zinc sulphate, and finally the second portion of barium sulphide solution. The white precipitate obtained by this process contains one equivalent of barium sulphate to two equivalents of zinc sulphide. It is collected, quickly dried, heated in retorts to redness, and, whilst still hot, thrown into cold water, by which its density and therefore covering power are increased. The pigment is finally washed and ground. It is a good white, but when mixed with lead pigments discolours them by reason of the sulphide it contains.
CHAPTER IX.
WASHING APPARATUS.
In all colour works operating on a large scale, special apparatus is used, in which are carried out the washing, pressing and drying of the pigments obtained as precipitates. Only pigments prepared in small quantities are filtered through filter paper. The treatment of enamel white and white lead requires the use of apparatus for this purpose in a special degree. We insert a short description here.
The preparation of enamel white takes place most conveniently in tubs provided with a stirring apparatus. When the precipitate of barium sulphate has once settled to the bottom, it is very difficult to again mix it up with water by means of a hand stirrer, an operation which must be often performed in washing. If vessels be used provided with a suitable mechanical stirrer, the precipitates are rapidly and thoroughly washed. We have already stated that vessels with a stirrer capable of being raised out of the liquid were specially suitable for washing white lead. Such an arrangement can with advantage be used in washing all precipitates. Many of the mineral pigments have to be freed from admixed salts by washing. A description follows of an effective washing apparatus with movable stirrer. In the cylindrical vessel ([Fig. 21]), which may be of any size, is a vertical iron shaft rotating upon a pin in the bottom of the vessel. On this axis is a horizontal wooden crosspiece, the under surface of which is studded with brushes; the disc, which this crosspiece carries, is united by means of two bars with a second, in which is cut a screw moving on the screwed shaft. The shaft is rotated by means of the cog-wheels shown in the illustration. The handle fastened to the upper disc enables the crosspiece carrying the brushes to be raised or lowered. When the handle is held fast, this crosspiece rises or falls according to the direction of rotation of the axis. The pipe shown at the side supplies the water for washing the precipitates.
Fig. 21.
In the preparation of colours liquids have often to be brought into the precipitating vessel which would attack the iron of the stirring apparatus, so that it is advisable to make the connecting rods between the two discs of such a length that the screws may be above the vessel. All iron parts of the apparatus dipping into the liquid should be protected by asphaltum varnish.
When a specially heavy precipitate is to be washed, such as enamel white, chrome yellow or white lead, the stirrer is raised as high as possible before the commencement of the operation. When the precipitate has formed and settled, the liquid is run off, the water tap opened, and the stirrer slowly brought down to the precipitate; the brushes fastened to the crosspiece stir up first the top of the precipitate, then the next portions, and so on until the whole of the precipitate has been stirred up into the liquid. When this has been accomplished, the stirrer is kept going for some time, so that the water may take up as much of the soluble materials as possible; it is then raised out of the vessel, in which the precipitate again settles.
As a rule, two or three washings of permanent white, in an apparatus of the construction described, are sufficient to render it quite free from acid. Washing must be continued until the wash water leaves blue litmus paper quite unchanged. When dry, precipitated barium sulphate is a very soft powder of great whiteness, which, on account of the fineness of its particles, can be readily ground with binding materials.
Enamel white loses, in a remarkable manner when completely dry, a great portion of its covering power and of its valuable property of being easily ground with oil or size to a homogeneous paste. It is not known whether this alteration is caused by a molecular change of the barium sulphate, as is not altogether improbable. In order to preserve the valuable properties of enamel white it is not, as a rule, completely dried, but is brought into the market in the form of pulp, which is obtained by bringing the washed precipitate into strong linen bags and allowing the water to drop through. This object is more quickly accomplished when the last wash water is drawn off only to such a point that, when the precipitate is again mixed up by means of the stirrer, a thin paste is formed. This paste is run into a centrifugal machine, of which [Fig. 22] is an illustration. In the drum, B, provided with an outflow, R, the smaller removable drum, C, with perforated walls, is caused to rotate by gearing, G, to which it is attached by the screw, V. The drum, C, is lined by a tight linen bag.
Fig. 22.
When the drum is in rapid motion, the thick liquid in the washing vessel is run in. In consequence of the centrifugal force due to the rapid rotation, the whole mass is at once thrown on the sides of the drum, the liquid penetrates the fine openings, is caught in the outer vessel, and runs away to a receiver, in which it is kept until the finest particles of the precipitate, which will penetrate even the closest fabric, have settled. The operation is continued until the excess of water has been separated, when the bag containing the precipitate is lifted out of the drum. Centrifugalised enamel white is a fairly stiff, white paste, which should be packed in sacks lined with oiled paper to prevent drying.
Fig. 23.
Filter Presses.
In recent years the use of filter presses for separating liquids from precipitates and for washing the latter has become general. A filter press consists, as shown in [Fig. 23], of a number of frames, between which are perforated plates and sheets of filter cloth, and which can be pressed tight together by a screw. A powerful pump forces the liquid containing the precipitate into the hollow spaces of the frames (known as chambers), in which the solid body remains, whilst the liquid goes through the filter cloth. By afterwards pumping fresh water through, the solid remaining in the chambers is soon completely washed.
CHAPTER X.
ZINC WHITE.
Zinc oxide, known as a pigment under the name of zinc white, is one of the most important white pigments. Although not absolutely permanent, yet it has, in common with enamel white, the valuable property of preserving its whiteness in air containing sulphuretted hydrogen. Its low price has brought it into general use.
Although zinc white is a most important pigment, it is very seldom made in colour works, because on account of its origin it is a product of metallurgical processes. Zinc white in chemical composition is pure zinc oxide; it is formed when zinc vapour burns in air. In zinc-smelting works zinc white is obtained by putting zinc in tubes which are heated to whiteness; the zinc vapours burn when they come in contact with the air, and the zinc oxide is caught by special arrangements.
The retorts used for this purpose are similar to those employed in the manufacture of coal gas. From 8 to 18 of these retorts are arranged in a furnace, in two rows, one above the other. In the lid of the retorts is an opening, which serves for charging and for carrying off the zinc vapours. When the operation is commenced, the retorts being heated to a white heat, two zinc plates are brought into each, the metal is soon volatilised, and the vapours pass through the above-mentioned openings. A current of air, heated to 300° C., is blown in to meet the zinc vapours, which take fire and burn with a blinding white flame, producing a very fine white powder, which is carried by the current of air through a series of chambers in which it deposits.
Fig. 24.
C. Freitag recommends for the production of zinc white the use of retorts of oval section, A, [Fig. 24]. These retorts, containing crude zinc, are heated to a white heat, and then a mixture of generator gas from coke and air is introduced by B and the pipe running through B. The zinc burns completely in the flame of the generator gas, which contains excess of oxygen. A product of faultless nature is said to be obtained in this way.
In zinc works zinc white is always made in the manner which has been described. It may also be obtained as a by-product in another metallurgical operation, the desilverisation of lead by Parkes’ process. In this process an alloy of silver and zinc is obtained. By sending a current of superheated steam over the molten alloy the zinc decomposes the steam, hydrogen and zinc oxide being formed. The zinc white is carried by the current of gas into chambers, in which it deposits.
The zinc white obtained by burning zinc is, as has been said, a very fine pure white powder, which can at once be used for paint without further preparation. The price of zinc white is rather higher than that of white lead, but the difference is counterbalanced by the greater covering power of the zinc white. Ten parts by weight of zinc white completely cover a surface for which 13 parts of white lead are required.
Whilst white lead cannot be mixed with many pigments, such as those which contain sulphur, zinc white may be mixed with all without fear of alteration. Zinc white is even better than enamel white for producing pale pigments from lakes; it has a lower specific gravity than enamel white, so that the mixture with the light lake can be more easily made.
Zinc grey, which is produced by some works, is zinc oxide discoloured by metallic zinc. Pure zinc white always has a pure white colour; if it is tinged with grey it is contaminated by metallic zinc, whilst a brownish hue denotes the presence of cadmium oxide. The latter impurity will be rarely met with in commercial samples, since cadmium is worth much more than zinc. Zinc oxide is used by the colour maker in the preparation of Rinmann’s green; it is also used, as stated above, as an addition to other pigments.
Griffith’s Zinc White.—This pigment, which is equal in covering power to white lead, consists of zinc oxysulphide. It is obtained by precipitating a zinc solution with a solution of barium sulphide, washing, drying, igniting and grinding the precipitate. As it contains sulphur, it should not be mixed with copper or lead pigments.
Tungsten White (Lead Tungstate), PbWO₄, is obtained as a heavy powder by precipitating a solution of sodium tungstate with lead acetate and treating the precipitate, which consists of basic lead tungstate, with dilute acetic acid, by which lead oxide is dissolved and a salt of the above composition left. This white pigment is dearer than other lead pigments, and has no special advantages over them; it is, therefore, seldom used.