M. F. Capitaine, who, acting upon a suggestion made by Liebig, some twenty years since, has recently taken up the subject of the manufacture of soluble glass, and silicate of potash, from farine fossile (an infusorial earth), has published an account of his researches in ‘Dingler’s Polytechnic Journal.’[334]
[334] See ‘The Journal of the Society of Arts’ for January 11th, 1878.
Although M. Capitaine does not think that the farine will be able to compete in cheapness with flint (where this latter is abundant) for the preparation of the alkaline silicates, he states that it possesses the advantage over flint of being much more soluble, and of yielding a far more neutral glass; added to which the production of the silicate is said to be effected with much less trouble than when flint is employed. An important condition is, that the farine must be first well calcined, since if the least trace of organic matter be left in it, the resulting solution will have a yellowish or brownish tint, which will make it unsaleable.
“The lyes being prepared partly with caustic soda, and partly with carbonate of soda, had densities ranging from 1·22 to 1·24 which were found to be most advantageous. A reservoir furnished with mechanical agitators, was about two thirds filled with lye, and the necessary quantity of calcined farine added, the stirring being kept up continually. The proportion of farine is easily calculated on the datum, that one part of hydrate of soda dissolves about 2·8 parts of chemically pure farine, the quality of which varies but little. Lye of the
density indicated produces a rather light solution, which presents little resistance to the agitators. If steam is afterwards introduced the solution becomes very rapid, when the pressure reaches about three atmospheres, and at the end of about three hours the silica is completely dissolved.
“For the preparation of silicate of potash for surgical purposes the farine fossile is said to be peculiarly adapted. In this case the boiling must be continued for one or two hours longer than in the case of soluble glass, with an addition of 10 to 15 per cent. of farine.”
Uses. &c. Soluble glass, in solution, has been used to render textile fabrics less combustible, as a varnish to protect stone, and as a vehicle in fresco-painting, The soda compound (silicate of sodium) is largely used as a dung-substitute in calico-printing, and by soap manufacturers in place of the resinates formerly in use. 10 or 12 tons are produced weekly in the district of South Lancashire. The potassa compound (silicate of potassium) has been recommended as a remedy for gouty concretions by Mr Ure.—Dose, 10 to 15 gr., in 6 or 8 fl. oz. of water twice a day. See Dunging, Varnish, &c.
Glass, Toughened. Syn. Verre Trempé. M. de la Bastie’s process for converting ordinary, into toughened, tempered, or hardened glass, may in general terms be said to consist in heating the glass to a certain temperature, and then plunging it into an oleaginous bath. For the process, however, to be successful, the observance of a number of minute details is essential; if these be neglected failure is certain to ensue. Thus it is found, that if the glass be insufficiently heated it will, when immersed in the bath, fail to be affected by it, and will consequently experience no alteration in properties. Again, if overheated, it will then get out of shape; or, further it may be heated to the right temperature, and yet be spoilt as it is being transferred to the bath. Moreover, the exact composition of the bath itself, and its temperature constitute very important conditions, the most trifling departure from which may give rise to unsatisfactory results. All these obstacles appear to have been overcome by M. de la Bastie, who has designed plant in the shape of furnaces and baths, by means of which the tempering process can be carried out, without chance of failure. When the glass is brought to the required temperature, all that is necessary is that they should be plunged into the bath, and instantly withdrawn. The cost of the operation is stated to be very small.
“The process as carried out at New York is thus described:—The glass after being run from the furnaces and moulded as usual, instead of being put into annealing pans, is immersed in a hot bath consisting of three parts of flaxseed oil, and one part of tallow. The bath stands at about 320°; and after remaining in this the ware is removed to a second, and similar bath, by which it is cooled down to about 200°. Finally the pieces are immersed in a water bath, and then dipped into a quantity of ordinary refined burning oil. They are then cleaned, ready for packing, with plaster of Paris powder. The work is but in its infancy, and but one small furnace is used in the experiments. Improvements will doubtless be made, by which the cleaning can be done more rapidly than by the powdered plaster, probably some chemical being used for the purpose. It is supposed that the oil works into the pores of the hot glass, and thus toughens it. Great care has to be exercised in the final cooling by water, as too long a contact with the air in changing from one bath to another, makes the ware crack. Articles cooled entirely in oil retain the oil on the surface, but are thus rendered stronger than otherwise.
This new process is very much employed in the manufacture of lamp chimneys, though they have the disadvantage of flying into small pieces, and with violence when they do break, which sometimes does occur.”[335]