Glass, Toughened

Syn. Verre Trempé. M. de la Bastie’s proce for converting ordinary, into toughened, tempered, or hardened gla , may in general terms be said to consist in heating the gla to a certain temperature, and then plunging it into an oleaginous bath. For the proce , however, to be succe ful, the observance of a number of minute details is e ential; if these be neglected failure is certain to ensue. Thus it is found, that if the gla 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 proce can be carried out, without chance of failure. When the gla is brought to the required temperature, all that is nece ary is that they should be plunged into the bath, and instantly withdrawn. The cost of the operation is stated to be very small. “The proce as carried out at New York is thus described:—The gla 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 doubtle 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 gla , 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 proce 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] [335] Supplement to ‘Ure’s Dictionary of Arts, Manufactures, ., 1878. The results so far obtained when gla is subjected to M. de la Bastie’s proce are variable. In some cases the articles subjected to it po e great toughne , and the gla bears a blow without experiencing any fracture. In other instances, however, a slight fall or blow shivers it to atoms. When the toughened gla under any circumstances breaks, it po e es a disadvantage over ordinary broken gla , in distributing itself into a great number of small, sharply angular fragments. Another proce for toughening gla , which has been patented by Herr F. Siemens, consists in heating, and then pre ing, and suddenly cooling the gla to be hardened; but when the articles are such as are usually moulded, the hardening and tempering are accomplished at the same time as the pre ing; thus the molten gla is run into suitable moulds, and while still highly heated, is squeezed, the moulds effecting the nece ary cooling, a proceeding which renders the employment of the oleaginous bath unnece ary. Mr Bauer’s method for toughening gla consists in heating ordinary gla plates so strongly that they begin to bend from softening, and then plunging them into a liquid paraffin bath having a temperature of 200°. Toughened gla is liable to rupture under circumstances that have not yet been accounted for. M. de la Bastie conceives that the fragile nature of gla is due to the weakne of the cohesion of its particles, and that if this cohesive power can be increased, the strength of the material will be improved in proportion. M. de la Bastie first tried to obtain this end by forcibly compre ing the gla while in a plastic or fluid condition, but without succe ; and it was only after various experiments that he was enabled to harden the gla , by dipping it into oil or any other liquid that permitted of being heated to a temperature considerably above that of water.