The Chemical Gazette 321, 1.3.1856
By C. Stölzel.
The method of producing ultramarine artificially has, since, its discovery, been repeatedly the subject of investigation; both scientifically and technically; and the production of artificial ultramarine can now no longer be regarded as a secret. Nevertheless the scientific questions involved in the process have never been sufficiently answered. Hitherto the influence of one or the other body in the formation has been alone discussed, and the whole theory of the formation has never been established by accurate chemical formulæ. This is explained by the various processes which go on at the same time, by the easy decomposability of the ultramarine when dissolved up for analysis, and by our insufficient acquaintance with the compounds of sulphur, and of the polythionates with the alkalies.
*Kaiserslautern in Rhenish Bavaria.By various investigations which I have had occasion to make in the different stages of the ultramarine manufacture, I have been repeatedly led to the question of the cause of the blue colour. By the kindness of the manufactory of this place*, I have obtained specimens not only of the finished colours, but also of the intermediate products of red and green ultramarine. I have from this been led to make accurate analyses of the different kinds of ultramarine, and to try the action of various reagents on them.
I. Analyses of Blue and Green Ultramarine.
a. Qualitctive Determination.
I found in green and blue ultramarine, prepared with sulphate of soda, besides silica, alumina, soda, and sulphur, which were the chief constituents, no inconsiderable quantities of iron, lime, and chlorine, with traces of magnesia, potash, and phosphoric acid. It is still a matter of dispute whether the iron is essential. It comes from the materials used in the preparation, especially from the alumina and charcoal (often coal); and in almost all the analyses of ultramarine, natural or artificial, it has been found to be a never-failing constituent. All the ultramarines I have investigated contained iron, though sometimes in very small quantities. Some processes require the direct addition of copperas in the manufacture, and Elsner showed that with Gmelin's process the desired colour could only be obtained when ferruginous materials were employed. Brunner used, with the exception of wood-charcoal, materials which were free from iron; and he obtained a preparation equal to that prepared with ferruginous materials. Certain manufacturers not only put no iron in, but take care that the alumina is free from iron; and it is rather a forced assumption that a small quantity of iron, which may be accidentally present, should be essential to a good colour. With the investigation of this question I am at present engaged.
With a view to obtain more beautiful tints, or greater resistance to acids, or a greater fineness for printing and painting, many manufacturers make special additions, either before or after the firing, besides such which must be regarded as adulterations, and which must by no means be neglected in the analysis. Instead of coal or charcoal, resin is sometimes added to the mixture, and may even occasionally be detected in the prepared colour.
In a specimen of ultramarine which I examined I found traces of fatty matter, which appeared to have had nitre mixed with it.
b. Quantitative Analysis.
1. Blue Ultramarine.
The determination of the individual constituents was made in several portions. The first portion was used for the determination of the silica, sulphuric acid, alumina, oxide of iron, lime, and soda. Its solution in hydrochloric acid was attended with teh development of sulphuretted hydrogen, and the separation of sulphur and silica. After separating the latter, the analysis of the other constituents was made in the usual manner.
In a second portion the iron was determined. As its quantity is very small in comparison wit hthat of the alumina, the usual modes of separation cannot be employed. The quantity of iron in the mixed precipitates of iron and alumina was therefore determined by means of permanganate of potash.
A third portion was used to determine the sulphur, which is not in the form of sulphuric acid. A portion was dreched with hot aqua regia in a capacious vessel, quickly corked, and digested till the sulphuretted hydrogen liberated was fully oxidized, and the sulphur separated had disappeared, and the separated silica had become quite gelatinous. The silica was separated, and the sulphuric acid precipitated as sulphate of baryta.
The chlorine was determined in a fourth portion by the usual methods.
The composition of blue ultramarine from this manufactory was as follows:
Al2O3 ... 31.18 p. c.
Fe ... 0.50
CaO ... 0.44
Na ... 11.10
SiO3 ... 38.11
SO3 ... 3.54
S ... 4.52
Cl ... 0.91
MgO, Kao, PO3 ... traces
-------
90.30
O ... 9.70
-------
100.00
(Fe2O3 ... 0.71)
(NaO ... 14.96)
-------
97.08
O ... 9.70
-------
100.00
Assuming that the iron and sodium were contained in the ultramarine as such, there is a deficiency of 10 per cent.; and assuming them to be contained as oxides, there is a deficiency of 8 per cent. I made therefore special search for other substances which might have escaped detection, such as resin, fat, carbonic acid. But none of these were present. An escape of sulphuretted hydrogen in estimating the sulphur might cause an error, but many analyses of the same substance gave me constant results.
It is scarcely to be doubted that the deficit is made up by the oxygen, which may be variously combined with the sodium, iron, or sulphur; but the analysis gives no clue to the mode in which this union takes place.
Green Ultramarine.
In the same manner an analysis was made of a green ultramarine to which a prize was awarded at the Munich Industrial Exhibition. In the first firing of the ultramarine, a product more or less green is obtained, and this is only converted into blue by another firing with sulphur. If the firing be effected in pots, the interior is of a particularly beautiful green (at times also red), and these pieces can then be picked out and worked up as a green. This would find many an application if the conditions for its production were more under our command. While the various analyses of blue ultramarine differ materially, the analyses of green which I have made agree essentially with those of Elsner.
Stölzel.
Al2O3 ... 30.11 p. c.
Fe ... 0.49
CaO ... 0.45
Na ... 19.09
SiO3 ... 37.46
SO3 ... 0.76
S ... 6.08
Cl ... 0.37
MgO, Kao, PO3 ... traces
-------
94.81
O ... 5.19
-------
100.00
(Fe2O3 ... 0.7 p.c.)
(NaO ... 25.73 p.c.)
-------
101.66
Elsner
Al2O3 ... 30.00
Fe ... 0.90
Na ... 25.50 NaO
SiO3 ... 39.90
SO3 ... 0.40
S ... 4.60
-------
101.30
It would hence appear that the green ultramarine is a determinate chemical compound, while in the blue, many constituents, such as silica and alumina, vary; so that the chemical compound which causes the colour is mixed with an excess of one or the other of these constituents.
It will be seen, on comparing the analyses of green and of blue ultramarine, that in the change from green to blue, while the other constituents remain the same, the absolute quantity of sulphur and sodium decreases; at the same time the relative quantity of sulphur to sodium is increased, and there is an increase in the sulphuric acid and oxygen. An increase in this last body is essential for blue ultramarine, and all technical processes require a certain free access of air.
If the firing of ultramarine be made in pots of porous clay, the ocntents are never equally coloured. Two, and sometimes three and four stages, are to be distinguished, and the intermediate stages are distinctly marked. These stages, though objectionable in practice, are important for a theory of the process. Sometimes in the interior there is a red core, easily changed by air and water; then a green mass, which passes through a bluish-green to a beautiful blue; and when the heat has been too strong, this is covered by a white layer. That the addition of oxygen plays an important part in the formation of blue is seen by the fact, that when the pot is emptied the green often changes suddenly into blue. The ultramarine mass has become a complete pyrophorus by the sulphide of sodium contained in it, and burns on coming into the air with the formation of sulphurous acid. In the so-called “fine firing,” which is done to convert the raw product into the commercial mass of the finest shade, the oxygen of the air is allowed to act, for the raw ultramarine mixed with sulphur is heated, and the latter burnt off at as low a temperature as possible under a slow access of air.
From a consideration of the above analysis, it is clear that part of the oxygen must be in combination with sulphur, but not as sulphuric acid. Even assuming that the whole of the sodium present is in the form of soda, which is by no means the case, there is still a deficiency of 3 per cent, which must be oxygen, and there is nothing but the sulphur with which it can be combined. When sulphide of sodium is acted upon by air, or when sulphur is heated with a sulphate, as is the case in the formation of ultramarine, some lower oxygen compound of sulphur than sulphuric acid must be formed. The presence of any such compounds has however been hitherto entirely ignored.
II. Action of various Reagens on Ultramarine.
As these experiments were made with ultramarine from this manufactory, it is probable that that prepared by other methods may exhibit slight deviations.
1. When ultramarine was heated to redness in a platinum crucible over a Berzelius lamp, it became lighter coloured; and when the platinum crucible, enclosed in a clay one, was heated in a charcoal fire, it became quite white. The residue, heated with hydrochloric acid, evolved no sulphuretted hydrogen, but much sulphurous acid. When ultramarine was heated in a platinum tray, in a combustion-tube closed at one end, a small deposit of sulphur was formed at first in the colder part, and afterwards a few drops of sulphuric acid were condensed there. Hence the fireproof silica exercises a decomposing action at a high temperature, and hence ultramarine cannot well be used in many processes of the porcelain manufactory.
Green ultramarine, treated similarly, passed into a dark blue colour with a tinge of green, which was so constant that it was not affected by many hours' violent firing.
2. Blue ultramarine, heated in oxygen, was changed into white. When heated with saltpetre, the colour is at first heightened; but with an increase of the saltpetre the mass is completely decolorized. This heightening of the colour is no real addition to its value as a colour, for when the mass was washed out a residue remained equal in quality and quantity to that originally employed. No colour is so deceitful in external appearance as ultramarine, and the shade of ultramarine gives no clue to its real value. By fine levigation the shade is brought down several numbers. The practical way to esti mate its value is to mix it with 8 or 10 times its weight of fine China clay, and fix, from the shade produced, its real value.
By fusing ultramarine with chlorate of potash at a low temperature, no change is produced; at a higher temperature the mass is changed into a beautiful rose colour.
When green ultramarine was acted on by these reagents, the result was much the same as with the blue.
3. When blue ultramarine was heated in a current of dry sulphurous acid, the colour vanished gradually, and became ultimately white.
Hydrogen passed over ultramarine gently heated, acted more nº. The colour became gradually paler, and sulphur and sulphuretted hydrogen were evolved in quantity. At a stronger heat the mass was changed into a gray colour, which, treated with acid, evolved sulphuretted hydrogen, and heated in the oxidizing flame of the blowpipe, passed though a beautiful green to a blue.
Green ultramarine afforded, when similarly heated, ultimate products of the same appearance, though the transition was somewhat different.
4. Strong acids attack ultramarine most energetically, and this is a great hindrance to its extended use. Potash- and soda-ley have no action even if boiled; but when fused with potash or soda, blue ultramarine passes through various stages of green or red to a red. These intermediary stages are however very evanescent. By gently heating ultramarine with small pieces of potassium, some beautiful vermilion- and purple-red colours were produced, which were also evanescent.
The results obtained may be thus expressed: —
1. Blue ultramarine, heated with exclusion of air, showed a variable degree of resistance to fire. At a high temperature blue ultramarine lost its colour, and left a mass which evolved sulphurous acid on treatment with hydrochloric acid; blue ultramarine, obtained by heating green ultramarine, remained unchanged, and evolved sulphuretted hydrogen when treated with hydrochloric acid.
2. Oxidizing and reducing agents destroyed the colours of both ultramarines at a high temperature; solid potash and soda at a lower temperature, and strong acids and chlorine in the cold.
3. Hydrogen, when passed over blue ultramarine heated, liberated sulphuretted hydrogen, but not with green. Both left an argillaceous gray mass, which heated in the oxidizing flame of the blow-pipe, passed through a green to a blue colour.
4. Solid potash and soda, and more perceptibly potassium and sodium, changed both ultramarines, when gently heated, partially into red ultramarine.
5. Green ultramarine, when not acted upon by strong reagents, had always a tendency to pass at a high temperature into blue.
These investigations I am still continuing.
— Liebig's Annalen, January 1856.
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