U. S. Department of Commerce Research Paper RP1881 National Bureau of Standards Volume 40, May 1948 Part of the Journal of Research of the National Bureau of Standards Changes in the Indices of Refraction and Liquidus of a Barium Crown Glass Produced by the Partial Substitu­ tion of Some Oxides 1 By Edgar H. Hamiltont Oscar H. Grauert Zeno Zabawsky, * and C. H. Hahner Optical glasses with high indices of refraction and Abbe values are very desirable for wide-angle lenses. In order to determine the range of composit ions in which glasses of this type eould be produced, oxides of Li , Be, Ca, B, La, or Th, were substituted for BaO or Si02 in a three- 0 1' fou r-component base glass . Substitut ions were made on a mole-for-mole basis. The indices of refraction fol' tlle C, D, F, a nd G' lines and t he liquidus were de­ termined for each gla s. Experimental glasses wit h indices of refraction (n D) and Abbe values fl' om l.600 to l.714 and 62.2 to 52.7, respectively, " 'ere made in mall platinum crucibles. 1. Introduction systematic data arc available on the C'fl'ect of these oxides 011 the range of compositions in which glass Previous to 1880, optical glasses could be divided can be produced. In this investigation ODe of the into a few typf'S such as crown, crown flint, and above oxides was substituted on a mole-for-mole flint glasses with indices of refraction (n) n,nd basis for part of the barium oxide or silica in a base Abbe value (v) mnging from 1. 50 to 1.92 and 70 glass containing the following: to 20, respectively. The II value of the glasses decreased as the index of refraction increa cd so that, when v is plotted agn,inst n, the points fall Percent on a curved line. Abbe and Schott introduced a number of new tjpes of glasses containing boron Moles Weigh t and barium oxides with v values higher than for the old types of glasses with the same UD. R ecently Si02 ______ -------- 54.0 35. 5 B20 3 _____________ 14.0 10.7 G. W. M orey 2 [1] developed a number of new BaO ____ __________ gln,sses in which the ratio of Abbe value to index 32.0 53. 8 of retraction is much higher than for the glasses of Abbe and Schott. This paper gives the composi­ T he substitutions WE're made in steps in order tion, optical and other properties of some other to determine the shapes of the index-of-refraction­ glasses, which also have a high ratio of II value to composition and liquidus-composition curves. index of refraction. These glasses may find a use When glasses with desirable propertiE's resulted, in optical instruments where a large flat fi eld these glasses were USE'd as base glasses for fUl'ther with a minimum of aberration is desired. substi tu tions. The oxides that gave promise of being useful in making such glasses were B20 a, BeO, CaO, La20 a, II. Experimental Procedure Th02, and Li20 . Although glasses containing such o -ides have bf'en described in the litemture [1], few All melts were made in platinum crucibles 90 mm deep by 36 mm in diameter and stirred with ' Now with the Armstrong Cork Co., Lancaster, Pa. I Presented at the Forty·Eighth Annual Meeting, the American Ceramic platinum-lO-p81'cent-rhodium stirrers. Although Society. Buffalo, N. Y., May 1, 1946 (Glass Division No. 13). the melts were stirred, striae-free glass could con­ 2 Figures in brackets indicate the literature references at the end of the paper. sistently be obtained only by crushing, mixing, Barium Crown Glasses 361 and remelting each glass.3 A few compositions tion by their measured indices of refraction than required r~melting a second time to obtain glasses by chemical analysis. For all but one set of suffi ciently free from striae to permit reliable index duplicate glasses the measured indices of refrac­ of refraction measurements to be made. tion (n D ) agreed within O.OOOL The presence of striae in optical glass is a very The liquidus temperatw-e of each glass was serious obstacle to the production of useful glasses. determined by a temperature gradient method [2]. As the striae consist of local inhomogeneities in The crystals formed in many of the glasses were composition, any measurement on the glass that very small, making it extremely difficult to de­ includes striae will be different in general from termine the exact location in the specimen at measurements on the rest of the glass. Even the which devitrification ceases. Complete identifica­ portion of the glass that do es not contain striae tion of the crystal phases was not attempted. will have a different composition, because the The appearance of a new crystalline phase at the striae will have removed di sproportionate amounts liquidus was determined by petrographic examina­ of its constituents. tion. The crystallographic evidence indicates Furthermore, on annealing the glass, differences that the discontinuities found in the liquidus in the coefficients of expansion between the glass curves are accompanied by changes in the primary and striae will cause lo cal strains and thus give phase at the liquidus. rise to differences in index of refraction for different parts of the glass. III. Results and Discussions Striae have always "haunted" the optical-glass 1. Substitution of Beryllium Oxide for maker and have been chiefly responsible for the Barium Oxide slow development of new optical glasses. E ven The substitution of beryllium oxide for barium at present, unless the utmost care is exercised in oxide produced glasses with lower indices of re­ procedures and techniques, striae will be found fraction and higher Abbe values than the base in most glasses. glass (table 1). No indices of refraction are After the final remelting, the glass was poured given for melt 469, containing 12 mole percent of into a 3-in. diameter steel mold. As soon as it beryllium oxide, as this glass could not be cooled solidified, it was transferred to a heated, covered without devitrification. clay box and placed in an electric annealing fur­ A minimum point is 'ndicated on the liquidus­ nace. The glasses were cooled through their composition CUTve, figure 1, near the 6-mole per­ annealing ranges at approximately 5 deg C per cent beryllium-oxide composition, and there is a hour. definite change in slope of the index-of-refraction­ The indices of refraction of the glasses were composition curves near this composition. Glasses determined for the G, D , F, and Gf lines by the co ntaining 6-mole percent or less of beryllium Pulfrich method.4 The maximum error of a oxide produced anisotropic crystals at the liquidus; single determination should not exceed ± 0.0001 . the 8-mole percent glass, isotropic crystals; the For the glasses that were analyzed, it was found lO-mole-percent glass, a mixture of isotropic and that except for a few glasses . the difference be­ anisotropic crystals; and the 12-mole-percent glass, tween the analyzed and batch compositions did anisotropic crystals. not exceed 0.3 mole percent. The calculated Abbe values (v) have been plotted on this and compositions are given in the tables, as many of succeeding figures. Discontinuities are usually the glasses have not been analyzed. In general it found on the Abbe-value-composition curves at is believed that the calculated compositions were approximately the same compositions as on the more accurate than those determined by chemical other curves for the same series of glasses. Be­ analysis, particularly as duplicate glasses listed in cause of the large probable error in calculating the table indicated far less difference in composi- Abbe values, discontinuities on the Abbe-value­ composition curves were not considered significant. 3 Alter most of the glasses had been made, it was found that striae-free glass could be made without remelting provided it was melted and stirred in a 2 . Substitution of Beryllium Oxide for Silica platinum crucible of the proper proportions. In a platinum crucible 3 in. in diameter by 3).2 in. deep, 2-pound melts of striae-free glass have been made The substitution of beryllium oxide for silica­ consistently. • Determinations by 1. Malitsky of this Bureau. produced glasses with higher indices of refraction 362 Journal of Research T A RLE 1 . Series 1. Substitution of beryllium oxide for barium oxide Glass number Composition 680 691 69.1 696 700 169 --------------,---I----~--- --.------------------------ N ole % wt % }vIole % wt % 1'vlole % wt % }vIole % wt % Mo{e % wt % Alole % wt % SiO, ________ ._____ ___________ 54 35.5 54 37. 6 54 38.8 54 40. 0 54 4L4 54 42. 8 I' B ,O,. ___ ._____________________ 14 10.7 14 11. 3 14 11. 7 14 12.0 14 12.4 14 12.8 B aO _________________________ 32 53.8 28 49.9 26 47. 7 24 45.5 22 43.0 20 40. " B eO _________________________ _ 0.0 4 1.2 6 1.8 8 2.5 lO 3.2 12 '1.0 I IND EX OF RE-FRACTIO:>I 71 C ___________________________ _ 1.6236 1.6145 1. 6100 1.6043 1.5969 (I) 11 0 ___________________________ _ 1.6268 1.6176 1.6130 1.6072 1.5999 (I) 111<' ___________________________ _ 1.6343 1.6249 1.6202 ['6142 1.6067 (I) 11 0' ___________________________ _ 1.6404 1.6307 1.6209 1.6198 1. 61 22 (I) 11 ______ ------------. 58.5 59.7 60.:3 61.1 61.