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Patent No. 1,623,301 for Foval Heat-Resistant Glass - Page 2 Home  > Insulators  > Patents  > Page 2

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1,623,301
As a specific example the following is a suitable batch: Pounds. Sand 750 Boric acid 262 Cryolite 98 Nitre 45 Feldspar 10 Fluorspar 3 Talc 28 Lepidolite 20 Hydrate of aluminum 6 Glass made from this batch will have a chemical composition substantially as follows: Per cent. Silica 74.38 Boric acid 14.70 Sodium oxide 5.96 Aluminum oxide 3.45 Calcium oxide .22 Magnesium oxide .72 Potassium oxide .45 Lithium, rubidium, and cæsium oxides .12 The batch is heated to a suitable melting temperature, for example, 2600° to 2800° Fahrenheit. The melt will be turbid, due to the colloidal condition of the alumina and possibly some of the other ingredients. The silica, boric oxide and alkali oxides probably form a true solution. The cryolite (Na2AlF6) breaks up forming, with oxygen, sodium oxide and aluminum oxide, the fluorine being volatilized in a great measure at least. The sodium oxide goes into solution but the aluminum oxide will remain in a suspended or colloidal state for a time. Probably an aluminum fluoride is formed which breaks down eventually but leaves the alumina is such condition that it does not immediately dissolve. I understand that cryolite has been used as a flux in the manufacture of boro-silicate glasses but so far as I am aware it has not been employed for producing a colloidal condition in the melt by preventing the complete solution of the alumina. As soon as the ingredients have been melted, and while the material is still turbid, preferably before planing has ceased, the application of heat to the batch is discontinued so as to preserve in the glass in the colloidal condition produced in the melt. The glass will be translucent in appearance and will be considerably tougher than the ordinary heat resisting glass of the boro-silicate type. Subjected to a deflection test, the fracture of the glass does not occur until some time after the glass begins to yield under a constant load. When ordinary glass is subject to a deflection test and the result plotted the breaking points occurs in the curve, that is, the point at which the glass begins to yield	without additional load, the yield up to this point having been substantially proportional to the increase in load. With the glass of my invention a considerable distance intervenes on the curve between the yield point and the breaking point. This indicates lack of brittleness, that is to say, toughness. Other physical characteristics by actual tests of the glass are as follows: Coefficient of expansion (linear) .0000043 millimeters per centigrade degree, between 20° centigrade and 300° centigrade. Tensile strength 9 kilograms per square millimeter. Young's modulus of elasticity by deflection 6200 kilograms per square millimeter. Density 2.29. Stability .0015 grams per hundred square centimeters. Thermal endurance calculated by Winkelmann & Schott's equation (Hovestadt, Jena Glass, MacMillan 1902, p. 229), 7.5. It may be said that the turbid or colloidal condition of the glass whereby the increased toughness is obtained, is dependent upon three factors: (1) the character of the batch, it being essential that the batch contain an ingredient which will remain for a time in a colloidal suspension, that is to say, not completely dissolved; (2) a melting temperature which is not too high, too high a temperature bringing about the dissolving of the suspended material; and (3) the duration of the heating period, it being necessary to stop the application of heat before the melt becomes clear, that is, before all the ingredients are completely dissolved. There is an interrelation between these three factors. It is necessary, practically, to use ingredients which will melt easily and at a relatively low temperature and which, when melted, will produce a turbid condition that will last for an appreciable time. If the turbidity disappears quickly, that is, if the ingredients all go into solution very soon after melting, it may be possible to preserve in the glass the colloidal condition of the melt but the necessity of calculating the temperatures and the length of the melting period, so that the melt will not become clear, would, under such circumstances, be so great as to make the operation impractical,. With the manufacture of the glass carried on as above describe, the melt will remain turbid long enough so that the proper character if glass can be obtained without inconvenience or excessive watchfulness. I claim: 1. A heat resisting glass containing silica, boric oxide, an alkali oxide and aluminum oxide, the latter being in colloidal condition. 2. A heat resisting glass containing silica 65% to 80%, by weight, boric acid 10% to 20%, alkali oxides 5% to 10% and aluminum oxide 2% to 10%, the latter being in a colloidal condition.