White Antimony Pigments.
Antimony forms a number of white compounds which can be made by a simple and inexpensive process, and might, therefore, be used as pigments. Two antimony compounds in particular are so used—antimony trioxide and oxychloride (powder of algaroth).
Antimony Trioxide occurs ready formed in nature as white antimony or antimony bloom. It is very simply prepared by burning the metal in air, when it forms soft needles with a silvery lustre. It is only necessary to heat melted antimony to a little above its melting point in a crucible placed in a slanting position, when the metal takes fire and burns with a blue flame. Nitric acid converts metallic antimony very quickly into antimony oxide with a copious evolution of brown fumes.
Antimony trioxide may be more cheaply prepared from antimony sulphide, artificial or natural (stibnite), by finely powdering, moistening with water, and gently warming on plates. Oxidation takes place, the sulphur is converted into sulphur dioxide and the antimony to trioxide. The heating should not be carried too far, or the antimony takes up further oxygen and forms the tetroxide.
Antimony Oxychloride, or Powder of Algaroth, is obtained by dissolving stibnite in strong hydrochloric acid, the operation being performed under a chimney to carry off the sulphuretted hydrogen. The solution of antimony trichloride, when the impurities have settled, is poured into a large vessel of water. At once a pure white precipitate is formed, which quickly sinks; it is washed with water until the washings have no acid reaction. Washing should not be continued too long or the oxychloride will be further decomposed; by washing with hot water it is almost entirely changed to antimony trioxide. The precipitate, after washing with cold water, has generally the composition expressed by the formula SbOCl.
Antimony oxide and oxychloride are both very crystalline powders, and have in consequence small covering power.
Bismuth White is not used as a painters’ pigment; it has no advantage over the white pigments previously described, and is much more expensive. Its only use as a pigment is for the preparation of white cosmetics, and even for this purpose zinc white is now frequently used; it is cheaper and quite as satisfactory.
Bismuth white is prepared by treating metallic bismuth with fuming nitric acid. The white precipitate at first formed completely dissolves in an excess of acid, and when the solution is poured into a large quantity of water, basic bismuth nitrate—bismuth white—separates.
Pure bismuth white is a soft, heavy powder, brilliantly white; it must be preserved in air-tight vessels as soon as it is dry, otherwise it acquires a yellowish tinge. Bismuth compounds are, if possible, more susceptible to the action of sulphuretted hydrogen than lead compounds. The yellow colouration, turning to black in the course of time, is due to black bismuth sulphide.
Tin White is used for earthenware enamels. It is obtained by treating granulated metallic tin with very strong fuming nitric acid. The heavy, white powder which is formed is separated from the undissolved tin by floating. Tin white has no application as a pigment; when mixed with glazes it gives them a handsome, milky appearance.
Manganese White.—When large quantities of a solution of impure manganese chloride are at hand, such as are produced in the preparation of chlorine, manganese carbonate may be obtained. A small quantity of soda solution is first added and the liquid left for several days so that the oxide of iron may separate. When this is the case an addition of soda gives a pure white precipitate.
Magnesia White or Mineral White is obtained, according to T. H. Cobley, by mixing a solution of magnesium sulphate with calcium chloride, adding 10 per cent. of aluminium chloride to the mixture and stirring in slaked lime so long as a precipitate is formed. A cheaper process is to precipitate mixed solutions of magnesium and aluminium sulphates by slaked lime.
Annaline.—A white pigment is recommended for use under this name; it consists of dead-burnt gypsum, which has been converted into a fine powder by grinding and levigating. (Dead-burnt gypsum has been so strongly heated that it is not able to again unite with water.)
To obtain paler shades of certain colours, additions are made of natural pigments, such as chalk, which has been converted into a very fine powder by levigation. An addition of chalk to a heavy mineral pigment, such as chrome yellow, is not advisable; it would, besides, seriously diminish its covering power.
In the preceding pages, a large number of white pigments has been enumerated. It would be easily possible to increase the list, but the result would be of no practical interest, for other pigments, neither in respect of quality nor price, can compete with the cheaper white pigments. Although white lead has at the present time an enormous use, it is to be hoped that this pigment, of good colour but little permanence, may be replaced entirely in the course of time by zinc white, and for some purposes by enamel white.
White pigments, in addition to their use alone, are employed to produce tints, which are obtained by mixing deeper colours with the white pigment. By adding the proper quantity of white to a colour it is possible to produce all paler shades of that colour. For example, the different varieties of the red lakes which are found in commerce are obtained by mixing white pigments with the deep red lakes.
The particular white pigment to be employed in these mixtures depends on the nature of the colour and on its specific gravity. It should always be remembered that white lead will not increase the permanence of a colour, since it will be discoloured in a short time by the action of the atmosphere. In the manufacture of fine colours for artists lead pigments should be absolutely excluded.
To produce paler shades of colours which contain lead and therefore have a high specific gravity, e.g., chrome yellow, white lead may be used; for other colours, barytes or zinc white should be employed. In the case of lakes enamel white is too heavy; zinc white or magnesia is suitable.
CHAPTER XI.
YELLOW MINERAL PIGMENTS.
As was the case with white pigments, so with yellow: of the large number known very few are in extensive use. In former times the number of yellow pigments employed in painting was far greater than at present; several, formerly in general use, have dropped out, partially or entirely, owing either to their poisonous character or to their replacement by others, deeper and more handsome. Especially since the discovery of cadmium yellow and the development of the manufacture of chrome yellows, many colours once in general use have properly fallen into disuse.
Again, unfortunately, the most important of the yellow mineral pigments contain lead, and have little stability; but a series of yellow colours free from lead is known, and though some of them are inferior in shade to the lead pigments they surpass the latter in permanence.
In addition to lead compounds, pigments derived from barium, zinc, antimony and cadmium are in general use. The yellow lead pigments were formerly preferred, and at present, so far as concerns beauty, they must be regarded as superior to other mineral yellows. The endeavour to provide the artist with permanent colours has resulted in the use of others, perhaps less brilliant, but very lasting. Under the name of chrome yellow a single pigment consisting of lead chromate was formerly understood; to-day, under this title is comprised a series of pigments containing zinc or barium in place of lead, but all commercially known as chrome yellow.
CHAPTER XII.
CHROME YELLOWS.
The chrome yellows are the lead, zinc or barium salts of chromic acid. In describing the different methods used in making these colours, lead chrome yellow will be taken first, since this is the one most largely used.