|The Chemistry of Paints and Painting is a free textbook on chemical aspects of painting. See the editorial for more information....|
|Home Pigments Blue Pigments Artificial Ultramarine|
|Search the VIAS Library | Index|
In the year 1814 a blue coloration, subsequently proved to be due to ultramarine, was noticed in the soda (black-ash) furnaces of St. Gobain. About fourteen years afterwards a method of making the same blue substance at will was discovered by Christian Gmelin, and by Guimet; this method was founded in part upon chemical analyses of natural ultramarine, and in part upon a study of the conditions under which the above-named blue coloration occurred. By successive improvements in its manufacture artificial ultramarine is now produced at a cost of no more than a few pence per pound. It is chiefly made in Germany and in France.
The raw materials employed in the preparation of ultramarine are kaolin, or China-clay, silica, sodium sulphate, sodium carbonate, sulphur, charcoal and rosin. Some makers omit the sodium sulphate, others the rosin, while calcined alum is occasionally substituted for the kaolin. These materials are heated together in closed crucibles in a furnace, and slowly cooled. A greenish porous cake is the product: this is powdered and gently roasted, after the addition of a little sulphur, for some hours. The material is again powdered, and then washed and dried: further calcination is sometimes required to develop the proper blue colour.
In preparing artificial ultramarine for use as an artists' pigment it must be very finely ground, and very thoroughly washed with water free from lime. The grinding not only improves the colour, but renders working with the paint less difficult to manage; the washing removes soluble sulphates and certain sulphur-compounds, which are liable to discolour some of the pigments (those containing lead or copper) with which the ultramarine may afterwards be associated in a picture.
The hue of artificial ultramarine is commonly of a less pure quality than that of the natural pigment, verging somewhat towards a purple. But its range of hue is considerable, from a greenish-blue to a decided violet. The greenish-blue and blue varieties are not affected in hue by admixture with zinc white, but the varieties which incline towards violet become remarkably enfeebled in richness of colour by this admixture, such weakening being out of all proportion with the dilution of tint which would be expected to ensue from this addition of white. However, other white substances do not produce this curious result.
Weak acetic acid, and a saturated cold solution of alum, which are without immediate action upon natural ultramarine, generally change the hue, and always ultimately decolourize the artificial product. Those kinds which have a somewhat violet tinge resist the destructive action of the above reagents longer than the pure blue and greenish-blue varieties. Neither sulphuretted hydrogen, nor caustic lime or other alkaline substance, affects the colour of artificial ultramarine.
Although the colour of ultramarine is certainly due to a substance containing sulphur, the precise chemical composition of this blue substance has not yet been determined. Some chemists are of opinion that it is a compound of aluminium, sodium, sulphur, and oxygen; others regard it as a sulphide of aluminium. There is little doubt, however, that it contains sulphur in two conditions. Some curious derivatives of blue ultramarine, of various colours, have been obtained, in which it is believed that the sodium of the original compound has been replaced by other metals; such is the yellow 'silver-ultramarine,' prepared by keeping blue ultramarine in a solution of silver nitrate. These bodies are, however, useless as pigments. The function of the silica in ultramarine is not known, although it forms from 30 to 45 percent of the total weight of all the varieties, and although it has been found by experience that ultramarines rich in silica resist the action of alum better than those which are poor in this constituent: these silicious ultramarines are sometimes sold under the name of 'Oriental Blue.'
Artificial ultramarine, when properly prepared, is permanent both in water and oils. When thin washes on paper appear to lose strength as they dry, or soon afterwards, the change is due to the chemical action of the alum, or other aluminium compound, present in the size of the paper. If an ultramarine should discolour emerald green, chrome yellow, Naples yellow (true), or flake white, it probably contains free sulphur, or has been insufficiently washed.
In order to test the purity of ultramarine, an easy plan, useful so far as it goes, is this: Boil a small quantity of the sample with distilled water in a wide test-tube for five minutes. Pour the liquid on to a wetted Swedish filter-paper fitted in a funnel. Divide the clear filtrate which runs through into 2 parts - to one add a few drops of basic acetate of lead; to the other a few drops of barium chloride solution. No darkening should occur in the first case, nor any white cloudiness in the second. Besides the white adulterants, gypsum and heavy spar, both chessylite and Prussian blue have been found in artificial ultramarine. If chessylite be present, its presence may be detected by warming the sample with ammonia solution, when a blue solution will be obtained. Ultramarine containing Prussian blue acquires a brown hue when warmed with caustic soda solution, while it does not completely lose its blue colour when treated with dilute hydrochloric or sulphuric acid.
When an acid (such as hydrochloric or oxalic) acts upon ultramarine, it disengages both sulphur and sulphuretted hydrogen; a good deal of silica is also, in most cases, then separated in a gelatinous form. The sulphur separated as such may amount to as much as 10 percent of the weight of the pigment taken, or it may be less than a half percent The sulphur disengaged as sulphuretted hydrogen through the action of an acid varies between a half percent and 6 percent As these variations have no relation to the depth of colour in the several samples, it is evident that a great proportion of this sulphur does not form an essential part of the blue pigment itself. Other things being equal, it is well to select samples of ultramarine which contain as little as possible of sulphur in any state.
The ultramarines known as 'Guimet's' and 'Heu-mann's' are of fine quality. Large quantities of this pigment are manufactured in Germany and France, smaller amounts in Belgium and England. The green, violet, lilac, purple and red ultramarines of commerce are permanent pigments of some artistic value. They are or may be produced in the manufacture of the blue variety, the green being its precursor and the others being formed by further heating or treatment of the blue kind. The final product is a greyish-white body.
There are two easily applied tests which, taken together, enable one to estimate the relative values of a set of samples of ultramarine in powder. These tests have distinct objects in view. In one, resistance to alteration is determined - in the other, the colouring power. The quantity operated on is only 5 centigrams, so the best plan is to begin by weighing out, with the aid of a sensitive balance, two portions of .05 gram a-piece from each sample. One of each of these portions is thoroughly mixed, by means of an ivory spatula, on highly glazed white paper, with 2 grams of pure kaolin: the tints of the several kinds are then compared. For the other test we require a number of small precipitating glasses or large test-tubes, and a supply of a saturated solution of potash alum in distilled water. The five centigrams of each sample of ultramarine are put into the duly labelled glasses and a measured quantity (say 50 cubic centimetres) of the alum solution is poured on with constant stirring. The change or loss of colour is duly noted at intervals of time, some samples opposing a resistance of days to the destructive influence of the alum, while others are injured by a contact of a few hours, or, in some extreme cases, of a few minutes.
It is scarcely necessary to add that the contents of each test-glass should be thoroughly stirred at regular intervals. To avoid this necessity, and at the same time to improve the accuracy of the results, a slight modification of the method may be introduced. Ten grams of the purest agar-agar are dissolved by the aid of heat in a litre of the alum solution. Just before this liquid has become a jelly by cooling, a measured portion is poured upon the necessary quantity of ultramarine: the mixture after thorough agitation is allowed to set, so that the pigment remains suspended throughout the mass; any changes of colour can be easily recognised.
|Home Pigments Blue Pigments Artificial Ultramarine|