the contents turned out, and the badly-burnt pieces carefully separated; the good portions are ready to be finished. The changes which go on during the heating of the mixture are both curious and interesting. The mixture when first put into the crucibles is of a greyish colour, but during the process of burning it passes through quite a series of colour-changesbrown, green, blue, violet, red, and white. The brown appears with the blue flames, due to the burning of the sulphur; it is a fine chocolate brown, but is very unstable; on exposure to the air it enters into combustion. Many efforts have been made to preserve it, but these have been fruitless. The green, which is the next change, begins to form when the sulphur has ceased to burn; like the brown it is unstable, as the substance burns on exposure to the air. Following the green comes the blue, which is formed when the temperature has reached about 700° C., or a bright red heat; when the temperature gets higher the colour changes to a violet. With still higher temperatures, first a red, then a white variety is formed. These changes are due to oxidation; when the white ultramarine is heated with reducing agents, such as carbon, the colours are re-formed in the reverse order to that in which they first appeared. The form of furnace to be used in burning the ultramarine is not a matter of importance, the operation can be effected in a reverberatory furnace, in a muffle furnace, in earthenware pots, in ovens, or in any convenient apparatus. Finishing Ultramarine. By whatever process the pigment is prepared it comes from the furnaces in the form of a gritty, somewhat cindery-looking, blue mass containing a large quantity of soluble sodium salts, and in this condition is unserviceable for use as a pigment. To fit it for this purpose the crude ultramarine has to undergo a finishing process, which has for its object to purify the colour and to develop the hidden beauty of the pigment. The process of finishing is essentially one of washing and levigation. The crude ultramarine is thrown into grinding mills where it is ground with water, this grinding being done as thoroughly as possible, as on it depends to a very large extent the excellence of the pigment as regards colouring power and fineness. After the grinding, the wet ultramarine is run into large tubs, where it is treated with hot water, or even boiled with water, so as to make sure that all the soluble contents of the crude ultramarine are dissolved out. The ultramarine is now allowed to settle and the liquor run off; this contains sodium sulphate which may be recovered by evaporation and used in making new batches of ultramarine. Then clean water is again run on to the pigment, which, after being thoroughly stirred up, is again allowed to settle and the water again poured off; this washing is repeated several times. The wet ultramarine is now ground in grinding mills specially constructed for grinding wet materials very finely; such mills will be found described in another Chapter. This grinding is important and takes several hours; the length of time depends upon the use to which the ultramarine is to be put. The finer qualities, which are used in calico-printing and letterpress and lithographic printing, and must be very fine, require the longer grinding; they are sold under the name of calico-printers' ultramarine; painters do not require so fine a quality, and for this the wet ultramarine is not subjected to lengthy grinding. Another method of separating the different qualities of ultramarine is by levigation, which forms an essential part of the process. The wet ultramarine as it comes from the grinding mills is run into large tubs of water, in which it is thoroughly stirred and then allowed to settle for two hours; this allows the coarser particles to subside, while the finer particles still remain in suspension, and are run into other tubs, where they are allowed to settle. The coarse particles in the first tub are run into the mills again to be re-ground with another batch of crude ultramarine. The particles which settle in the second tubs are collected, dried at a gentle heat, and sent into the market. In the water of the second tub there still remains some fine ultramarine; this is run into a third tub, where it is allowed to settle, and, after drying, is sold as a fine quality. Frequently, there still remains in the last waters some very fine ultramarine, even when the tubs have been allowed to stand for a month to settle; by adding a little lime water, which causes an aggregation of the particles, this can be collected by filtering. Before being sold the dry ultramarine is, in many works, subjected to a process of sieving, which separates the coarser particles and yields the pigment in the form of an impalpable powder; the finer qualities should have a buttery feel when rubbed between the fingers. The shade of the finished ultramarine depends upon several factors, such as the proportions of the constituents used in the mixings, the perfection of the burning operations, and the fineness to which the pigment has been ground; as it is impossible to regulate each of these factors with mathematical accuracy, it follows that the shade of the finished colour must vary from time to time; and as this variation is objectionable the makers overcome it by having a number of standard or type colours or shades, to which standard they bring up all batches by a process of blending and mixing different shades together so as to obtain the marketable brands. Wet Methods of Making Ultramarine.-Many attempts have been made to prepare ultramarine by wet processes, but mostly without any success. Knapp's process, given in the Jour. für Praktisch. Chem., 1885, p. 375, consists in first roasting a mixture of kaolin, sodium carbonate, and sulphur to such a temperature that the roasted mass has a brown colour, at which point it is maintained until the kaolin is completely decomposed; after cooling, the mass is digested in a solution of sodium persulphide. The defect of this process consists in the small margin there is between success and failure; if the colour of the roasted mass be allowed to pass beyond the brown, the colour of the finished ultramarine begins to deteriorate; and if it becomes red, then no blue is produced when the mass is digested with the persulphide under these conditions the process can hardly become a commercial success. The colour of the finished product is not quite equal to that of ultramarine made by the dry methods; it is, however, not much inferior. PROPERTIES OF ULTRAMARINE.-Ultramarine is one of the most important pigments at the command of the painter. As a pigment it is perfectly permanent when exposed under all ordinary conditions, being perfectly fast to light and air, the only destructive agents being acid vapours which rapidly decolorise it. It can be mixed with all the ordinary vehicles used by painters and with most other pigments without being changed thereby or itself causing any change. The only exceptions are those pigments containing lead or copper, which, owing to their forming black sulphides with sulphur, are liable to become discoloured when mixed with ultramarine; the rate of change of such mixtures as ultramarine with chrome-yellow or emeraldgreen is very variable; sometimes the mixture will change colour very soon, at other times the mixture will keep its colour for a considerable time; much depends upon the quality of the pigments and the care with which they have been made. Ultramarine is distinguished by its pale but pure tone and by its tint of blue being quite different from that of all other blue pigments. The soda ultramarines are of a violet-blue shade, the variety rich in silica having the darkest and deepest tint; the sulphate ultramarine is of a pale greenish-blue tint and is the palest blue pigment made, resembling blue verditer in tint. The most characteristic property of ultramarines is their being readily acted upon by acids; the colour is discharged and the pigment decomposed, sulphuretted hydrogen being evolved and sulphur deposited. All acids have this property, even weak organic acids, such as acetic acid, tartaric acid, &c., this distinguishes ultramarine from all other blue pigments; on the other hand, it prevents the use of ultramarine wherever there is the least chance of its coming into contact with acid influences, which are, sooner or later, sure to destroy the colour. Of the varieties of ultramarine, the sulphate is the most readily decomposed, while the highly silicated soda variety is the most stable of the soda ultramarines. Boiled in strong nitric acid, there is, first, a decoloration, and then a deposition of sulphur; afterwards the sulphur is dissolved and a residue of gelatinous silica is left behind. Alkalies have no action on ultramarine. When boiled in alum, ultramarines take a more violet tone; the sulphate variety is the most readily changed, while the highly silicated soda ultramarine resists the action most; the latter variety is therefore used by papermakers, because, owing to their having to use alum or sulphate of alumina in sizing their papers, they require an ultramarine which will not change much, if anything, under the influence of those bodies. Heat has no action on ultramarine. COMPOSITION OF ULTRAMARINE.- Ultramarines are compounds of silica, Si O2, alumina, Al, O, soda, Na, O, sulphur, S, and sulphur oxide, SO3. The last, although present in almost every sample of ultramarine, is not an essential constituent of the colour. The following are some analyses of ultramarines, mostly by the author, which will show the average composition of these important pigments:— ANALYSES OF ULTRAMARINES. Soap- Calico- Silica, Si O2, Sulphur trioxide, SO3, 49.685 40.647 40.885 45'420 38.52 100 000 100 000 100 000 100 000 99.44 The soap-makers', calico-printers', and paper-makers' ultramarines are of English make, the others of Continental make. The analyses of paper-makers' and soap-makers' ultramarines show the difference between the two varieties of soda ultramarines; the first named is rich in silica, while the other is poor in silica; the soap-makers' and calico-printers' samples are evidently identical in composition, but the latter is much finer than the former. The analysis of green ultramarine shows the difference between the green and blue ultramarines. CONSTITUTION OF ULTRAMARINE.-One of the problems chemists have endeavoured to solve has been how the various constituents of ultramarine are combined together, but it is still unsolved, and will probably remain so for some time to come; the difficulty of solving it seems to be the inability to effect the substitution of particular groups of elements in it in the same manner as can be done in organic chemistry, where, even in complex molecules, the power of replacing one group by another enables one to form some conception as to the actual constitution of the compound. It is true that the sodium in ultramarine can be replaced by silver and other metals so as to form varieties of ultramarine, and that the sulphur can be replaced by selenium or tellurium; but these replacements throw no light on the problem, for they are simply replacements of one element by another, not of groups of elements. Many chemists, e.g., Wilkins, Hoffmann, Unger, Endeman, and Elmer, have worked on this question. Hoffmann's theory of the constitution of ultramarine is, perhaps, nearest the truth. Hoffmann was head of the Marienberg Ultramarine Works and did much to throw light on this subject; he considered ultramarine to be a double silicate of alumina and soda combined with bisulphide of sodium. The formula assigned to the soda ultramarine poor in silica was 4 (Al, Na, Si, Og) + Na, S4, and to that rich in silica, 2 (Al, Na, Si, 010) + Na, S4. Endeman considers that the ultramarines contain a colournucleus (an oxysulphide of alumina and soda) disseminated through a double silicate of alumina and soda. The colour-nucleus of white ultramarine (which may be regarded as the parent body) has the formula Al Na4 O2 S2; the action of sulphur upon this is to remove soda and to form green ultramarine, which contains the nucleus, Al, Na, S2 O; this, by oxidation, can be converted into Al, Na, S, O, which has a jet green colour; by burning with sulphur, this is converted into the nucleus of the blue variety, which has the formula Al, Na, S, O3. The base through which the colour-nucleus is distributed is of 3 3 3 |
