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Pyrex glass formula patent No. 1,304,623 - Page 2 Home  > Insulators  > Patents  > Page 2

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(3) Tenacity, modulus of elasticity density and specific heat of such values that in connection with their expansivity and conductivity the glasses have a high co-efficient of thermal endurance. This factor, which is the power to undergo sudden cooling without fracture, is expressed by Winkelmann and Schott (Ann. d. Phys. u. Chem. 51-730 (1894); also Hovestadt, Jena Glass, published by MacMillan 1902, p. 228), as follows;-- in which--     F = co-efficient of thermal endurance,     P = tensile strength,     a = the linear expansivity,     E = Young's modulus of elasticity,     K = heat conductivity,     S = density,     c = specific heat.     If the cubic expansivity is used in the formula, instead of the linear expansivity the result would be F/3, and these values of F/3 are used for comparing glasses. The values of F/3 calculated by Winkelmann and Schott for glasses of various compositions range between 1.17 and 484.     In calculating F/3 Winkelmann and Schott use a figure for the tensile strength which is undoubtedly too low, their tensile strength determination having been made by a method which they themselves recognized tended to give low results.     Using factors of tensile strength for our glasses, such as would be given by them under the test methods of Winkelmann and Schott, the cubic co-efficient of thermal endurance (F/3) of all of our glass is greater than 6. In specifying thermal endurance in our claims, we will base them on such factors. Using the actual tensile strength of our glasses the factor is still higher.     (4) High stability against chemical attack. As illustrative of this, it may be stated that a glass of composition B2, after being subjected to the solvent action of distilled water at 80° C. for 48 hours (see for procedure, Walker, Journal of the American Chemical Society, vol. 27, p. 865, 1905) has only .0001 to .0005 grams of matter dissolved per 100 sq. cm. exposed to the solvent action, while all of the compositions here given have a stability better than .002 grams per 100 sq. cm.

(5) Good workability. This includes the ability to properly melt the glass at the temperatures readily obtainable in glass furnaces. It also includes the property of remaining amorphous, and of being sufficiently plastic at a point below the furnace temperature to permit its working in the ordinary manner by blowing, pressing, etc. This latter property may be expressed numerically by the temperature on the centigrade scale, at which a thread of the glass 1 mm. in diameter and 23 cm. in length suspended vertically and heated through the upper 9 cm. elongates of its own weight at the rate of 1 mm. per minute. A glass doing this at the temperature of 800° C. is said to have a hardness equal to, (or no greater than) 800° C. Glasses A, B1, and B2, have a hardness of about 800° C., D of about 862° C., while C and E have a hardness above 862° C., and below 900° C.     (6) The glasses are all colorless and are transparent.     (7) By high silica contents (i.e. not less than 70%). This tends to low expansivity and good stability, but tends to render the glass hard.     It will be noted that in all of the above formulæ the percentage of alumina is very low, as is desirable in order to reduce the hardness of the glass. Alumina has been heretofore used in sodium-boro-silicate glasses in larger percentages in order to obtain ability to resist chemical attack and prevent crystallization, but we find it possible to still achieve these desirable results while reducing the alumina contents, and to increase as stated, in a suitable mixture, the silica contents to 70 per cent. or over, and that the expansion with such percentages is less than would be inferred from the hitherto known properties of silica. From our investigations we have discovered that with these high percentages of silica, the unit expansion factor of the silica is apparently less than the unit factor for less percentages of silica, or in other words that when the percentage of silica is made sufficiently great the factor by which its percentage is to be multiplied to obtain the thermal expansion due to the silica is reduced. If the silica content is increased to above 84 per cent., owing to the fact that with high silica contents suitably combined, but little alumina is required to give requisite resistance to chemical attack, and freedom from tendency to crystallize. In composition D, the antimony, and composition E, the lithia, serves to soften the glass without decreasing the resistance to chemical attack as would the amount of soda of potash requisite to soften the glass to the same extent.