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Patent No. 1,623,301 for Foval Heat-Resistant Glass - Page 1
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1,623,301: 1 of 3

Patented Apr. 5, 1927. 1,623,301

No Drawing. Application filed May 9, 1922. Serial No. 559,609.

My invention relates to the production of a stable, translucent, heat resisting glass of the boro-silicate type suitable for use in oven ware, laboratory vessels, or other utensils or articles which must be capable of withstanding high temperatures without fracture.

Glass of this character has usually contained silica 65% to 85% (by weight), boric oxide 10% to 20%, alkali oxides such as sodium and potassium 5% to 10%, and also small amounts of aluminum, calcium and magnesium oxides.

Heretofore the efforts to produce a satisfactory glass for oven ware or other utensils or articles subject to high temperature have proceeded on the theory that the requisite thermal endurance must be obtained by reducing the coefficient of expansion to a minimum. Fracture of glass subjected to rapid heating or cooling occurs because of the inequality in change of temperature as between one part of the body and the other, for example, as between the surface of the body and the interior portions thereof, this unequal change in temperature setting up internal strains that the body may be incapable of withstanding. By reducing the expansivity of the glass, that is, by minimizing the difference in expansion between the highly heated and the less heated portions of the glass body the liability to fracture is, obviously diminished.

I have discovered, however, that a stable glass (stability referring to relative nonsolubility in water) of the boro-silicate type can be obtained by increasing the toughness of the glass (non-brittleness) as well as by decreasing its coefficient of expansion and, in fact, that it is possible, by the novel method to be hereinafter described, to produce a very serviceable oven glass, for example, in which the coefficient of expansion is, relatively speaking, high, by a composition of the batch and manipulation of the melt which will increase the toughness of the glass sufficiently to compensate for the higher expansivity. I have discovered further that the increased toughness is to be obtained by producing in the melting operation a colloidal suspension of certain ingredients and arresting the heating
operation before the suspended particles are fully dissolved, as will occur in most cases, at least, if the application of heat is continued for a considerable period of time. It is the colloidal condition of certain of the ingredients of the glass that gives the glass its increased toughness. This colloidal condition manifests itself in the finished glass as a lack of transparency. The glass of my invention is relatively opaque, that is to say, translucent, while the heat resisting boro-silicate glasses, as heretofore made, have been transparent or nearly so. Moreover, the glass in accordance with my invention has this advantage over other heat resisting glasses, that it can be more conveniently and cheaply produced; the batch is melted at a lower temperature; the control of the manufacture simplified and facilitated because less care and accuracy in the melting operation is required; the glass produced is softer and more easily worked.

My invention, therefore, has for its objects, first, the production of a stable, heat resisting, boro-silicate glass of novel character and properties; and, second, a new and improved method of producing such glass.

In carrying out my invention a batch is made up comprising, preferably, sand, boric acid, cryolite, and substances containing aluminum, sodium and potassium, and also magnesium and calcium. To the batch are preferably added, also, substances containing lithium, rubidium and cæsium. While it is not essential to use all of the ingredients named there is an advantage is having the batch as complex as indicated. The tendency to crystallize is minimized and apparently the maintenance of the colloidal suspension of certain ingredients is facilitated. The ingredients of the batch are preferably proportioned so that the glass contains: silica 65% to 80%, boric oxide 10% to 20%, alumina 2% to 10%, alkali oxides (of sodium, potassium, lithium, rubidium and cæsium) 5% to 10%, and oxides of the second group of the periodic system (magnesium and calcium) 0.5% to 2%.