Ultramarine was also made the subject of a very interesting paper, by Herbert J. L. Rawlins, read before the Society of Chemical Industry, in December 1887.
After referring to the native form, lapis lazuli, Rawlins goes on to observe that “analysis could give no clue as to the cause of the blue colour. To prepare it artificially became a great object, and the efforts in this direction were stimulated by the offer of prizes, amongst which was one of 6000 francs, offered by the ‘Société d’Encouragement’ of France, to be awarded to the discoverer of a method of making ultramarine, provided it did not cost more than 90s. per lb. How strange it seems to think of this in these days when the value has fallen to less than half that price per cwt.!
“As early as 1814, two German chemists, Tessärt and Kuhlmann, had observed the formation of a blue product in soda kilns and calcination kilns, but Guimet, in 1828, first discovered how it was produced, and gained the 6000 francs prize. He did not, however, publish his method, and grew immensely rich, although the price sank to about 16s. per lb. In 1828 he was producing at the rate of 120,000 lb. annually.
“About the same time, or, as is positively asserted by some, even prior to Guimet, Gmelin made the same discovery and published his researches in full, thus perhaps laying the foundation stone of the present supremacy of Germany in this manufacture.
“In spite of the valuable discoveries of Hoffmann, Unger, and others, our knowledge of the chemical constitution of ultramarine is very limited and uncertain, many different theories having been advanced regarding the cause of the blue colour.
“According to Wilkins, ultramarine is composed of two portions, one of which consists of two silicates of alumina with sulphite and sulphide of sodium, and is constant in its composition; the other being a mixture of variable quantities of sand, clay and oxide of iron, with sulphuric acid. The blue colouring principle he considers to be a compound of sodium sulphite and sulphide. Another ingenious theorist, Stein, in two papers published in the Jahresberichte in 1871 and 1872, concludes that blue ultramarine contains sulphurous, and not thiosulphuric acid, that neither sulphites nor thiosulphates are necessary to its composition, and that it owes its colour to the presence of black sulphide of sodium, which is formed at high temperatures by the action of sulphide of sodium on alumina—admitting, therefore, that it is not a chemical compound, but merely a mechanical mixture, the blue colour of which is due to the bodies composing it.
“Brunner considers ultramarine to be a compound of aluminium silicate, with sodium sulphate and sulphide; while Brünlin regards it as a double silicate of aluminium and sodium, in combination with pentasulphide of sodium. Green ultramarine he considers to be the same double silicate in combination with bisulphide of sodium.
“Again, according to Ritter, ultramarine contains a double silicate, not only associated with polysulphide, but also with thiosulphate of soda; and Schülzenberger, on the other hand, considers that it is a mixture of a double silicate with sulphite and monosulphide of sodium.
“Endemann considers that the blue colour is due to a ‘colour nucleus,’ consisting of unchanging proportions of aluminium, sodium, oxygen and sulphur, in each variety of ultramarine the proportion being different, while the rest of the sodium and aluminium and the whole of the silica merely act as a vehicle necessary to the preparation and existence of the colour. He considers that this ‘colour nucleus,’ in the case of white ultramarine, which he calls the ‘mother-substance in the manufacture of blue ultramarine,’ has the formula AlNa4O2S2. By the action on two molecules of this of sulphurous acid gas, Na2O is removed, and green ultramarine Al2Na6O3S4 is formed, which then, by the action of oxygen, which forms sodium sulphate, passes into the pure green compound, having the formula Al2Na4O3S3. In the ‘indirect process’ of manufacture, green ultramarine is converted into blue by being burned with sulphur. By this means Endemann considers that more sodium and sulphur are removed, and blue ultramarine Al2Na2O3S3 is formed. He considers that the other portion, not included in the ‘colour nucleus,’ differs in different samples. In one which he mentions it has about the composition 3Al2O3.5Na2O.16SiO2.
“But of all chemists who have worked on this subject, none has done more to increase our knowledge of ‘the blue marvel of inorganic chemistry,’ as he himself has called it, than Reinhold Hoffmann. His position of manager of the Marienberg Ultramarine Works, near Benscheim, in the Grand Duchy of Hesse, renders his acquaintance with the manufacture perfect, and his untiring researches on the subject have been well rewarded by results both interesting and valuable. He considers ultramarine to be a double silicate of sodium and aluminium, together with bisulphide of sodium, the variety poor in silica, characterised by its paleness and purity of tint, and easy decomposition by acids, having the formula 4(Al2Na2Si2O8) + Na2S4; while that rich in silica, characterised by its dark and somewhat reddish tint, and more difficult decomposition by acids, has the formula 2(Al2Na2Si3O10) + Na2S4. He also considers it very doubtful whether green ultramarine is really a chemical compound, and indeed it is now generally considered that the colour is only due to small traces of sodium salts in very intimate mechanical mixture with the blue variety, for by heating the green body for some time at 160° with water in closed tubes, it is converted into the blue product, and small traces of sodium compounds are found in solution in the water; and further, on heating blue ultramarine strongly with sodium sulphate and charcoal—that is, acting upon it with sodium sulphide—the green variety is formed.