United States Patent 19 3,938,978 Hummel 45 * Feb
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United States Patent 19 3,938,978 Hummel 45 * Feb. 17, 1976 54) METHOD OF MAKING CRYSTALLIZED 3,834,911 9/1974 Hammel.................................. 65/33 GLASS 3,846,099 l l 1974 Simmons................................. 65/33 3,854.99 l 21974 Pirooz..................................... 65/33 75 Inventor: Merritt J. Hummel, Lower Burrell, 3,856,497 l2/1974 Hummel ................................. 65/33 Pa. (73) Assignee: PPG Industries, Inc., Pittsburgh, Pa. Prinary Evanliner-S. Leon Bashore Assistant Evanliner-Frank W. Miga * Notice: The portion of the term of this Attorney, Agent, or Firm-Dennis G. Millman; William patent subsequent to Dec. 24, 1991, J. Uhl has been disclaimed. 22 Filed: Aug. 26, 1974 57 ABSTRACT 21 Appl. No.: 500,415 Crystallized glass has hitherto been made by the use of Related U.S. Application Data the mineral petalite as the source of lithium values. 63 Continuation-in-part of Ser. No. 343.981, March 22, Petalite is in short supply, and when attempts are 1973, Pat. No. 3856,497, which is : made to use lithium carbonate or a similar lithium salt continuation-in-part of Ser. No. 239,335, March 29, in place of petalite in the glass batch, unwanted micro 1972, abandoned, cracking develops during a subsequent crystallizing heat treatment with certain glass compositions. Such (52) U.S. Cl................................ 65/33; 106/39.7 microcracking can be avoided by using a purposefully 51 int. Cl........................................... C03B 29/00 small addition to the petalite-free batch of an alkali 58 Field of Search......................... 65133; 106139.7 metal salt, wherein the alkali metal is selected from the class consisting of potassium, rubidium and (56 References Cited cesium. UNITED STATES PATENTS 6 Claims, No Drawings 3,489,577 1 / 1970 Tashiro et al....................... 65133 X 3,938,978 1. 2 The resultant glass would generally be weaker than METHOD OF MAKING CRYSTALLIZED GLASS normal crystallized glass, and because of the tendency of the microcracks to catch dirt, the cracked crystal CROSS REFERENCE TO RELATED lized glass sheets or plates stained easily and were diffi APPLICATIONS cult to clean, making them commercially unacceptable. This application is a continuation-in-part of copend Unfortunately, the prior art did not afford any clear ing application Ser. No. 343,981, filed Mar. 22, 1973, indication of what to do to overcome the problem. U.S. Pat. No. 3,856,497 which in turn is a continuation It is known in the prior art that sources other than in-part of earlier filed application Ser. No. 239,335, petalite can be used in the glass batch as a source of filed Mar. 29, 1972, now abandoned. 10 lithium values, for example, U.S. Pat. Nos. 3,489,577 to Tashiro et al., 3,352,656 and 3,352,698 and British BACKGROUND OF THE INVENTION Pat. No. 1,028,871, all to McMillan et al., and U.S. Pat. 1. Field of the Invention No. 3,006,775 to Chen, disclose the use of lithium This invention relates to a method of making crystal carbonate in the glass batch. These same references lized glass, and in particular, to a method of making 5 also disclose the optional incorporation of a potassium such glass when starting with lithium carbonate or the salt into the glass batch. Surprisingly, there is no disclo like, rather than the mineral petalite, as the source of . sure in these references of a microcracking problem or lithium values. In one aspect, the invention concerns of the effect that potassium has in eliminating the prob avoiding microcracking of the glass surface during the lem. heat treatment used to crystallize the glass by the pur Therefore, it appears that the prior art has not recog poseful addition of potassium, rubidium or cesium val nized the problem of glass surface microcracking which S. occurs in certain crystallized glass compositions during 2. Description of the Prior Art the heat treatment. This invention has uncovered the Glass-ceramic articles produced by devitrification or problem and provided a solution. crystallization of glass are well known in the art. In U.S. 25 Pat. No. 3,625,718 to Petticrew, there are disclosed SUMMARY OF THE INVENTION glass compositions which are particularly suited for In accordance with this invention, it has been found crystallization. Such compositions comprise on a per that the problem of glass surface microcracking which centage by weight basis from 64 to 74 percent SiO, occurs during the crystallizing heat treatment of glass from 15 to 23 percent AlO, from 3.3 to 4.8 percent 30 of certain compositions which has been made by start LiO, from 1 to 3.8 percent ZnO, from 1.2 to 2.4 per ing with a glass batch containing lithium carbonate or cent TiO, from 0 to 2 percent ZrO, and wherein the the like as a source of lithium values, rather than petal named ingredients constitute at least 90 percent by ite, may be overcome by purposefully adding a potas weight of the total glass composition. Additional com sium, rubidum and/or cesium salt to bring the potas ponents such as AsOs and SbOs sometimes end up in 35 sium, rubidium and/or cesium content of the glass the final glass composition because they are added to (measured as the oxide) to at least 0.15 percent by the glass batch to act as fining agents. weight based on total weight of the glass composition. Such glass compositions, as disclosed above, can be Glasses in which microcracking is a problem contain an obtained by suitably melting a mixture of sand, petalite, appreciable quantity of ZnO, that is, about 1 to 4 per hydrated alumina, mixed zinc and zirconium silicates, 40 cent by weight, based on total weight of the glass com arsenic trioxide, antimony trioxide, zirconium oxide, position. Particular glasses have the following composi titanium dioxide and a suitable quantity of appropriate tion: cullet. After the batch has been suitably melted, fined, homogenized and pressed into plates, the composition can be heat treated to crystallize or devitrify the glass. 45 Component Percent by Weight SiO, 60 - 75 At the end of the heat treating period, the crystalline AlO 15 - 25 phase constitutes about 98 percent of the composition LiO 3 - 6 ZnO 1 - 4.0 with the remainder being a glassy phase. TiO, 1.0 - 6.0 Crystallized glass articles offer significant advantages ZrO. 0 - 3.0 over ordinary amorphous glass articles in that crystal 50 F- 0 - 0.5 lized glass has a high modulus of rupture and a very low linear coefficient of thermal expansion, generally below 20 x 107 per C., which makes these glasses wherein the above ingredients constitute at least 90 useful as cooking surfaces for kitchen ranges, hot plates percent by weight, and preferably about 95 percent by and the like. 55 weight of the glass composition. As has been mentioned, it has been customary to use The method of the invention of making a crystallized the mineral petalite as the source of lithium values in glass article substantially free of microcracking is ac making the glass of the kind indicated above. Petalite complished as follows: consists principally of lithium aluminum silicates, a a. preparing a glass batch consisting essentially of: typical analysis being 4.29 percent Li,0, 16.15 percent 60 i. a silica source, AlO, 77.1 percent SiO, 0.49 percent NaO, 0.31 ii. an alumina source, percent KO and 0.026 percent Fe2O3. Petalite is in iii. a lithium family glass batch ingredient consist very short supply, being available substantially. only ing of a lithium salt, and a zinc salt in an amount from Rhodesia. When efforts were made to produce sufficient to provide a crystallized glass article crystallized glass of the kind indicated above, but with 65 from said glass batch having a ZnO content, per the use of lithium carbonate and the like in place of the cent by weight on an oxide weight basis of 1 to petalite, microcracks developed in the glass surface 4.0, characterized by the presence of micro during the heat treatment used to crystallize the glass. cracks in the surface, 3,938,978 3 4 b. adding to said glass batch an alkali metal salt oxides of arsenic and/or antimony act as fining agents wherein the alkali metal is a member of the group in the making of glass compositions. Preferably, the consisting of potassium, rubidium and cesium, said ranges for the batch contents are selected such that the alkali metal salt being present in an amount suffi composition in the crystallizable glass is as follows: cient to provide a glass article upon cooling a melt 5 of said glass batch having a content of an oxide of Component Percent by Weight the alkali metal of said salt, percent by weight on SiO. 60- 75 an oxide basis of 0.15 to 2.50, A.O. 15 - 25 C. melting the glass batch containing the ingredients LiO 3.0 - 6.0 10 ZnO 1 - 4.0 set forth in (a) and (b) to form a glass melt, TiO, E.0 - 6.0 d. cooling the glass melt and subsequently therewith ZrO. 0 - 3.0 F- 0- 0.50 e. forming a glass article containing silica, alumina, RO, wherein R is selected from the lithia and the required amounts of ZnO and RO, class consisting of potassium, where R is potassium, rubidum and cesium men rubidium and cesium 0.15 - 2.5 tioned in (a) and (b), 15 f heat treating the glass article at a temperature and for a time sufficient to form crystals throughout the Melting of the batch ingredients may be done in a glass article to form a crystallized glass article hav furnace or a pot and preferably in a tank adapted for ing surfaces thereof which are substantially free of continuous forming.