Abstract:
Optical, fluorine containing barium borosilicate glasses having an improved varying partial dispersion in the optical positional range of lanthanum crown and lanthanum flint glasses wherein the ratio of SiO 2  with respect to SiO 2  and B 2  O 3  is at least 0.62 and the content of fluorine in said glass is between 2 and 7 grams per 100 cc of said glass.

Description:
This is a continuation of application Ser. No. 402,805, filed Oct. 2, 1973, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a novel class of optical glasses, and more particularly, to such glasses which exhibit good stability with respect to changes in climatic conditions, low staining susceptibility and can be ground, polished and pressed like ordinary silicate glasses. These glasses have the same optical position as lanthanum crown glasses (LaC) and lanthanum flint glasses (LaF), but exhibit a higher partial dispersion P g ,F than prior glasses. 
     Normal commercial glasses, in the optical positional range of the LaC and LaF glasses, have a partial dispersion P g ,F =  (n g  - n F )/n F  - n C ) which is considerably reduced, by comparison, with the normal partial dispersion, as calculated from the Abbe equation 
     
         P.sub.g,F = 0.6438 - 0.001682v.sub.d 
    
     and set forth, for example, in the Jenaer Glaswerk Schott &amp; Gen. catalogue for optical glass no. 3050/1966/II 5. 
     Prior art glasses within the optical positional range of LaC and LaF glasses are characterized by the passage of a straight line through each of the following limiting values (as illustrated in FIG. 1): 
     
         ______________________________________n.sub.d = 1.642     v.sub.d = 58.0 to n.sub.d = 1.699                        v.sub.d = 46.0n.sub.d = 1.642     v.sub.d = 58.0 to n.sub.d = 1.652                        v.sub.d = 58.5n.sub.d = 1.652     v.sub.d = 58.5 to n.sub.d = 1.702                        v.sub.d = 54.6n.sub.d = 1.702     v.sub.d = 54.6 to n.sub.d = 1.742                        v.sub.d = 44.0n.sub.d = 1.742     v.sub.d = 44.0 to n.sub.d = 1.699                        v.sub.d = 46.0______________________________________ 
    
     Each of these prior art glasses exhibit particularly low P g ,F values. 
     Glasses having the lowest P g ,F values and most extreme optical positions (e.g. n d  =1.697; v d  =55.4) consist essentially of B 2  O 3 , CaO, La 2  O 3 , ZrO 2  and less than 10 percent by weight of SiO 2 . In addition to these ingredients, these glasses can also contain MgO, SrO, PbO, ZnO, CdO, TiO 2 , Al 2  O 3 , Nb 2  O 5 , and WO 3  for varying the n d  and v d  values and for improving the crystallization stability and chemical resistivity of the glasses. 
     In prior art glasses, when the SiO 2  content was raised to above 10 percent by weight, with a corresponding reduction in the amount of B 2  O 3  present, the optical positional range became smaller and hence an increase in the crystallization tendency and a lowering of the v d  value with a given n d  value of the glass occurred. One attempt to avoid the tendency of the glass to crystallize was to partially replace CaO and La 2  O 3  with BaO and ThO 2 , respectively. However, this attempt was not successful because the v d  was considerably low and the P g ,F values were also considerably below the aforementioned normal partial dispersion value. A need therefore has existed for optical glasses having the same optical position as LaC and LaF glasses but with higher partial dispersion P g ,F than prior known glasses of the same optical position. 
     OBJECTS OF THE INVENTION 
     It is therefore a significant object of this invention to provide optical glasses having an improved varying partial dispersion in the optical positional range of lanthanum crown glasses and lanthanum flint glasses. 
     Another object of this invention is to provide optical glasses having a very good stability with respect to changes in climatic conditions. 
     Still another object of this invention is the provision of optical glass that can be ground, polished and pressed like ordinary silicate glasses. 
     It is a further object of this invention to provide optical glasses which exhibit a low susceptibility to staining. 
     A still further object of this invention is the provision of a composition for the production of optical glasses having an enlarged optical glass range without having the drawback of an appreciable reduction in the P g ,F value. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, the invention relates to optical glasses containing barium borosilicate and having a varying partial dispersion P g ,F [(n g  - n f )/(n F  - n C )] in the optical positional range of LaC and LaF glasses wherein the amount of SiO 2  present in the glass satisfies the following condition: ##EQU1## is at least 0.62 and the content of fluorine is at least 7 grams per 100 cc of glass. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The compositions of the glasses according to the invention consist essentially of the following materials: 
     
         ______________________________________                          EquivalentOxide % by wt.   or      Oxides                          %______________________________________SiO.sub.2 20-26           26-31                      Si.sub.1/2 O                            42-53        60-66B.sub.2 O.sub.3 2-8.0                B.sub.2/3 O                            5-20BaO   20-38                BaO   9-18MgO   0-6             25-40                      MgO   0-5          10-22CaO   0-18                 CaO   0-10SrO   0-18                 SrO   0-10Ba.sub.2 O.sub.3 20-32                Ba.sub.2/3 O                            10-20Y.sub.2 O.sub.3 0-25           20-32 Y.sub.2/3 O                            0-20        10-23ThO.sub.2 0-18                 Th.sub.1/2 O                            0-12______________________________________ 
    
     The above compositions further comprise at least 2 weight percent or 5 equivalent percent and preferably from 2 to 7 weight or 5 to 20 equivalent percent of F 2  -O. Thus, in a fictitious purely oxidic glass of the above composition range, so much oxygen is to be replaced by 2 fluorine atoms that the increased weight amounts to 2 to 7 grams per 100 grams of oxidic glass. In an analagous manner, F 2  -O is from 5 to 20 equivalent percent which means that 5 to 20 percent of the oxygen are replaced by an equivalent amount of fluorine. 
     The fluorine content of the glass can be introduced into each compound which is normally employed in the glass-melting art. For example, the fluorine can be introduced by using such compounds as BaF 2 , LaF 2 , ThF 4 , BaSiF 6 , etc. 
     In order to adjust the required n d  and v d  values, up to 12 equivalent percent of one or more of the following ingredients can be used: 
     divalent components, e.g., ZnO, PbO and CdO; 
     tetravalent components, e.g., TiO 2  and ZrO 2  ; 
     pentavalent components, e.g., Nb 2  O 5  and Ta 2  O 5  ; and 
     hexavalent components, e.g., WO 3  and MO 3 . 
     The total content of the above-identified highly refracting, weakly or moderately dispersing constituents including La 2  O 3 , ThO 2  BaO, SrO, CaO, ZnO, Ta 2  O 5 , Nb 2  O 5  and ZrO 2 , is to amount to at least 29 equivalent percent or 60 weight percent, Y 2  O 3  can be used as a substitute for La 2  O 3 , either is whole or in part. Preferably, up to 25 percent by weight of La 2  O 3  are replaced by Y 2  O 3 . 
     Similarly, in order to adjust the required optical positions and improve the crystallization stability of the optical glasses of this invention having the components BaO, La 2  O 3  and ThO 2 , portions of the latter components can be replaced by one or more of the following highly refracting, weakly or moderately dispersing components: 
     
         ______________________________________          weight percent______________________________________CaO              0 - 18SrO              0 - 18ZnO              0 - 10Y.sub.2 O.sub.3  0 - 25ZrO.sub.2        0 -  7Ta.sub.2 O.sub.5 0 - 12Nb.sub.2 O.sub.5 0 - 12______________________________________ 
    
     When more strongly dispersing components, e.g., MgO, PbO, CdO, Al 2  O 3 , TiO 2  and WO 3 , are added to the glasses in order to adjust a required optical position and for crystallization stability, generally up to 8 equivalent percent is utilized. 
     From 0 to 2 equivalent percent of As 2  O 3 , Sb 2  O 3 , NaCl, NH 4  NO 3 , alkali silicofluorides, etc. can be used as refining agents in the practice of this invention. A preferred refining agent is As 2  O 3  which is preferably used in an amount of 0.3 weight percent of the total composition. 
     Represented in Tables 1 and 2 are examples of the optical glasses in accordance with the invention. The various ingredients are set forth in terms of percentage by weight as well as equivalent percent. 
     
                                           TABLE 1__________________________________________________________________________[Compositions in Equivalent Percent]         1   2    3    4    5    6    7    8    9    10__________________________________________________________________________Si.sub.1/2 O  50.8             50.2 44.2 49.4 50.2 51.6 50.6 50.8 51.3 50.1B.sub.2/3 O   14.6             14.4 19.3 14.3 14.4 14.9 14.5 14.5 14.6 14.2MgO           --  --   --   3.1  --   --   --   --   --   --CaO           --  --   --   --   --   --   --   --   --   --SrO           --  --   --   --   3.5  --   --   --   --   --BaO           15.3             13.4 10.9 14.3 12.9 13.1 14.0 15.4 14.7 13.6PbO           --  --   --   --   --   --   --   2.0  3.2  --ZnO           --  --   --   --   --   --   --   --   --   5.1CaO           --  --   --   --   --   --   --   --   --   --La.sub. 2/3 O 16.2             16.9 18.3 15.9 16.0 15.2 13.8 14.2 13.1 14.0Y.sub.2/3 O   --  --   --   --   --   --   --   --   --   --Al.sub.2/3 O  --  --   --   --   --   --   --   --   --   --Zr.sub.1/2 O  --  --   --   --   --   --   --   --   --   --Ti.sub.1/2 O  --  --   --   --   --   --   --   --   --   --Tb.sub.1/2 O  3.1 5.1  7.3  3.0  3.0  5.2  7.1  3.1  3.1  3.0Nb.sub.2/5 O  --  --   --   --   --   --   --   --   --   --Ta.sub.2/5 O  --  --   --   --   --   --   --   --   --   --W.sub.1/3 O   --  --   --   --   --   --   --   --   --   --total         100.0             100.0                  100.0                       100.0                            100.0                                 100.0                                      100.0                                           100.0                                                100.0                                                     100.0F.sub.2 --O   15.3             13.4 10.9 15.2 15.5 13.1 14.1 9.3  9.4  9.3As.sub.2 O.sub.3 (refining agent)         0.3 0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3 ##STR1##     0.78             0.78 0.70 0.77 0.78 0.77 0.78 0.78 0.78 0.78highly retractive weakly         34.6             35.4 36.5 33.2 35.4 33.5 34.9 32.7 42.2 35.7dispersing substancesstrongly disp. substances         0   0    0    3.1  0    0    0    6.8  3.2  0         11  12   13   14   15   16   17   18   19__________________________________________________________________________Si.sub.1/2 O  43.7             51.2 50.2 48.6 43.2 50.3 49.4 50.8 49.6B.sub.2/3 O   19.1             14.4 14.0 13.6 18.8 14.5 14.5 14.5 14.3MgO           --  --   --   --   --   --   --   --   --CaO           --  --   --   --   --   --   --   --   --SrO           --  --   --   --   --   --   --   --   --BaO           10.8             15.6 13.9 13.0 10.6 15.3 14.4 14.4 15.1PbO           --  --   --   --   --   --   --   0.3  --ZnO           --  --   --   --   --   --   --   0.3  --CaO           --  --   --   --   --   --   --   --   --La.sub.2/3 O  15.8             7.5  15.6 15.7 15.6 16.2 16.1 16.2 15.8Y.sub.2/3 O   3.4 8.2  --   --   --   --   --   --   --Al.sub.2/3 O  --  --   --   --   --   --   --   --   --Zr.sub.1/2 O  --  --   6.3  --   --   --   --   0.5  --Ti.sub.1/2 O  --  --   --   7.9  --   --   --   --   3.1Th.sub.1/2 O  7.2 3.1  --   1.2  7.1  2.1  3.1  3.1  2.1Nb.sub.2/5 O  --  --   --   --   4.7  --   2.5  --   --Ta.sub.2/5 O  --  --   --   --   --   --   --   --   --W.sub.1/3 O   --  --   --   --   --   1.6  --   --   --total         100.0             100.0                  100.0                       100.0                            100.0                                 100.0                                      100.0                                           100.0                                                100.0F.sub.2 -O    6.2 9.4  9.0  9.0  6.2  15.2 14.4 14.4 15.0As.sub.2 O.sub.3 (refining agent)         0.3 0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3 ##STR2##     0.70             0.78 0.78 0.78 0.70 0.78 0.77 0.78 0.78highly refractive weakly         37.2             34.4 35.9 29.9 38.0 33.6 36.1 34.6 33.0dispersing substancesstrongly disp. substances         0   0    0    7.9  0    1.6  0    0.3  3.1         20  21   22   23   24   25   26   27   28__________________________________________________________________________Si.sub.1/2 O  49.8             49.4 50.9 43.0 41.4 42.9 41.9 44.6 41.5B.sub.2/3 O   13.9             14.3 14.5 18.8 19.0 18.8 18.3 19.5 18.1MgO           --  --   --   --   --   --   --   --   --CaO           --  --   --   --   --   --   --   --   --SrO           --  --   5.1  --   --   --   --   --   --BaO           13.8             13.2 10.2 10.5 10.7 10.6 10.2 11.0 10.2PbO           --  --   --   --   --   0.6  --   --   --ZnO           --  --   --   --   --   --   --   --   --CaO           --  --   --   --   --   --   --   5.2  --La.sub.2/3 O  15.5             16.0 16.3 17.9 18.1 17.8 13.5 12.3 11.6Y.sub.2/3 O   --  --   --   --   --   --   --   --   --Al.sub.2/3 O  --  --   --   1.0  --   --   --   --   --Zr.sub.1/2 O  5.9 --   --   0.5  --   --   --   --   --Ti.sub.1/2 O  --  5.0  --   0.8  --   3.2  --   --   --Th.sub.1/2 O  1.1 2.1  3.0  7.1  5.7  6.1  6.9  7.3  6.9Nb.sub.2/5 O  --  --   --   --   4.9  --   9.1  --   --Ta.sub.2/5 O  --  --   --   --   --   --   --   --   --W.sub.1/3 O   --  --   --   --   --   --   --   --   --total         100.0             100.0                  100.0                       100.0                            100.0                                 100.0                                      100.0                                           100.0                                                100.0F.sub.2 --O   8.9 9.2  9.3  10.5 10.7 10.6 6.0  11.0 10.2As.sub.2 O.sub.3 (refining agent)         0.3 0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3 ##STR3##     0.78             0.77 0.78 0.70 0.68 0.69 0.70 0.70 0.70highly refractive weak         36.3             31.3 34.6 36.0 39.4 34.5 39.7 30.6 40.4dispersing substancesstrongly disp. subst.         0   5.0  0    2.2  0    3.2  0    5.2  0__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________TABLE 2 [Compositions in Weight Percent]        1    2    3    4    5    6    7    8    9    10__________________________________________________________________________SiO.sub.2    23.9 23.5 20.8 23.8 23.8 24.9 23.6 23.2 23.2 24.2B.sub.2 O.sub.3        5.3  5.2  7.0  5.3  5.3  5.5  5.2  5.1  5.1  5.3MgO          --   --   --   2.0  --   --   --   --   --   --CaO          --   --   --        --   --   --   --   --   --SrO          --   --   --   --   5.7       --   --   --   --BaO          36.7 32.0 26.7 34.9 31.3 2.2  33.4 35.4 33.4 33.0PbO          --   --   --   --   --   --   --   6.8  10.8 --ZnO          --   --   --   --   --   --   --   --   --   6.6CdO          --   --   --   --   &#39;    --   --   --   --   --La.sub.2 O.sub.3        27.7 28.6 31.1 27.7 27.6 26.4 23.1 3.4  21.4 24.5Y.sub.2 O.sub.3        --   --   --   --   --   --   --   --   --   --Al.sub.2 O.sub.3        --   --   --   --   &#39;    --   --   --   --   --ZrO.sub.2    --   --   --   --   --   --   --   --   --   --TiO.sub.2    --   --   --   --   --   --   --   --   --   --ThO.sub.2    6.4  10.5 15.0 6.3  6.3  11.0 14.7 6.1  6.1  6.4Nb.sub.2 O.sub.5        --   --   --   --   --   --   --   --   --   --Ta.sub.2 O.sub.5        --   --   --   --   --   --   --   --   --   --WO.sub.3     --   --   --   --   --   --   --   --   --   --F.sub.2 -O   5.3  4.6  3.7  5.4  5.3  4.6  4.8  2.9  2.9  3.1total        105.3             104.6                  103.7                       105.4                            105.3                                 104.6                                      104.8                                           102.9                                                102.9                                                     103.1As.sub.2 O.sub.3 (refining agent)        0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3highly refractive weaklydispersing substances        0.8  17.3 72.2 68.9 71.9 69.6 71.2 64.9 60.9 70.5strongly disp. subst.        0    0    0    2.0  0    0    0    6.8  10.8 0m.sub.d      7.6560             1.6764                  1.6906                       1.64683                            1.65726                                 1.66801                                      1.67060                                           1.68076                                                1.68369                                                     1.66480y.sub.d      58.08             56.32                  55.47                       57.95                            57.36                                 56.54                                      56.81                                           51.22                                                49.19                                                     55.49n.sub.g -n.sub.6n.sub.f -n.sub.C        0.5491             0.5502                  0.5504                       0.5405                            0.5500                                 0.5518                                      0.5498                                           0.5590                                                0.5600                                                     0.5508n.sub.g -n.sub.cn.sub.f -n.sub.C        1.0067             1.0102                  1.0104                       1.0091                            1.0109                                 1.0119                                      1.0098                                           1.0216                                                1.2059                                                     1.0110__________________________________________________________________________        11   12   13   14   15   16   17   18   19__________________________________________________________________________NiO.sub.2    20.8 25.3 24.9 24.7 20.8 23.8 23.4 23.8 23.8B.sub.2 O.sub.3        7.0  5.5  5.4  5.5  7.0  5.3  5.3  5.3  5.3MgO          --   --   --   --   --   --   --   --   --CHO          --   --   --   --   --   --   --   --   --SiO          --   --   --   --   --   --   --   --   --BaO          26.1 39.0 35.2 33.1 26.1 37.0 35.0 34.0 37.1PbO          --   --   --   --   --   --   --   1.1  --ZnO          --   --   --   --   --   --   --   0.5  --CnO          --   --   --   --   --   --   --   --   --Na.sub.2 O.sub.3        27.1 13.3 28.1 28.8 27.1 27.6 27.7 27.6 27.5Y.sub.2 O.sub.3        4.0  10.1 --   --   --   --   --   --   --Al.sub.2 O.sub.3        --   --   --   --   --   --   --   --   --ZPO.sub.2    --   --   6.4  --   --   --   --   0.5  --TiO.sub.2    --   --   --   5.3  --   --   --   --   2.0ThO.sub.2    15.0 6.8  --   2.6  15.0 4.4  6.5  6.3  4.3Nb.sub.2 O.sub.5        --   --   --   --   4.0  --   2.1  --   --Ta.sub.2 O.sub.5        --   --   --   --   --   --   --   --   --WO.sub.3     --   --   --   --   --   1.9  --   --   --F.sub.2 -O   2.2  3.2  3.3  3.2  2.3  5.3  5.0  5.0  5.3total        102.2             103.2                  103.3                       103.2                            102.3                                 105.3                                      105.0                                           105.0                                                105.3As.sub.2 O.sub.3 (refining agent)highly refractive weak        72.2 69.2 69.7 61.5 72.2 69.0 71.3 69.8 68.9dispersing substancesstrongly disp. subst.        0    0    0    5.3  0    1.9  0    1.1  2.0h.sub.d      1.70571             1.65876                  1.67777                       1.69932                            1.21580                                 1.66717                                      1.66350                                           1.65717                                                1.67727v.sub.d      53.87             56.53                  53.54                       47.06                            50.41                                 55.33                                      54.53                                           56.84                                                52.44D.sub.g -D.sub.f        0.5512             0.5500                  0.5521                       0.5666                            0.5564                                 0.5529                                      0.5531                                           0.5500                                                0.5572N.sub.f -D.sub.cN.sub.g -N.sub.gN.sub.f -N.sub.c        1.0112             4.0099                  1.0126                       1.0314                            1.0189                                 1.0137                                      1.0139                                           1.0298                                                1.0192__________________________________________________________________________        20   21   22   23   24   25   26   27   28   29__________________________________________________________________________SiO.sub.2    24.4 24.5 24.9 20.4 19.7 20.3 20.6 20.7 20.7 21.0B.sub.2 O.sub.3        5.3  5.5  5.5  6.9  7.0  6.9  6.9  7.0  7.0  6.1MgO          --   --   --   --   --   --   --   --   --   --C.sub.2 O    --   --   --   --   --   --   --   --   --   --SrO          --   --   8.6  --   --   --   --   --   --   --BaO          34.5 33.4 5.5  25.4 26.1 5.6  25.8 26.1 6.1  26.8PbO          --   --   --   --   --   --   --   --   --   --ZnO          --   --   --   --   --   --   --   --   --   --CdO          --   --        --   --   --   --   10.4 --   --La.sub.2 O.sub.3        27.5 28.7 28.9 30.5 31.1 30.5 23.9 20.8 20.8 30.4Y.sub.2 O.sub.3        --   --   --   --   --   --   --   --   --   --Al.sub.2 O.sub.3        --   --   --   0.5  --   --   --   --   --   --ZrO.sub.2    6.0  --   --   0.5  --   --   --   --   --   --TiO.sub.2    --   3.3  --   0.5  --   2.0  --   --   --   --ThO.sub.2    2.3  4.6  6.6  14.8 11.9 12.7 14.9 15.0 15.0 0.2Nb.sub.2 O.sub.5        --   --   --   --   4.2  --   7.9  --   10.14                                                     --Ta.sub.2 O.sub.5        --   --   --   --   --   --   --   --   --   5.5WO.sub.3     --   --   --   0.5  --        --   --   --   --F.sub.2 -O   3.2  3.3  3.2  3.6  3.7  3.7  2.2  3.7  3.7  5.6total        103.2             103.3                  103.2                       103.6                            103.7                                 103.7                                      102.2                                           103.7                                                103.7                                                     05.6As.sub.2 O.sub.3 (refining agent)        0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3  0.3highly refractive weaklydispersing substances        70.3 66.7 69.6 71.2 73.3 68.8 72.5 61.9 72.3 72.9strongly disp. substances        0    3.3  0    1.5  0    4.0  0    10.4 0    0n.sub.d      1.67621             1.68671                  1.66573                       1.69690                            1.71202                                 1.70668                                      1.73292                                           1.70976                                                1.74243                                                     1.68816v.sub.d      53.92             50.72                  56.42                       53.61                            50.60                                 50.05                                      46.71                                           51.66                                                44.58                                                     53.76n.sub.g -n.sub.F        0.5513             0.5607                  0.5501                       0.5530                            0.5571                                 0.559                                      0.5620                                           0.5546                                                0.5656                                                     0.5531n.sub.g -n.sub.en.sub.F -n.sub.C        1.0120             1.0238                  1.0102                       1.0139                            1.0192                                 1.0227                                      1.0255                                           1.0160                                                1.0302                                                     1.0142__________________________________________________________________________ 
    
    
    
     Reference is also made to the attached figures. FIG. 1 shows the optical positional range of the glasses in accordance with the invention. FIG. 2 shows the dependence of (n g  - n f )/(n F  - n c ) = P g ,F on (n d  -1)/(n F  -n C ) = v d  of known glasses and of the glasses according to the invention. In FIG. 2, both glasses of this invention as well as glasses of the prior art have the same optical position, but the glasses of this invention are shown to exhibit higher partial dispersion P g ,F than the prior art glasses. In these figures, the crosses indicate the glasses according to the inventon. The other markings correspond to commercial LaC and LaF glasses, which are to be found in the 1966 Schott catalogue and 1968 Min catalogue with the indicated numberings. 
    
    
     ILLUSTRATIVE EXAMPLES 
     The following examples are given by way of illustration of this invention only, and should not be construed as being in any way limiting upon the scope thereof. All percentages are by weight, unless specified to the contrary. 
     In the production of glass according to each of these examples, the following procedure was used. 
     The initial materials are well-mixed. The batch is melted at 1460° to 1480° C. in a platinum crucible and refined for two hours at 1520° C. Thereafter the temperature is lowered and stirring takes place from 1420° to 1030° C. The puring takes place at about 980° C. into a preheated casting mold. The reproducibility of the optical values is guaranteed by a melting assembly which is closed as tightly as possible, particularly so that the vaporization of the fluorine can be kept under control and influence the properties of the final melt product. 
     EXAMPLE 1 
     The following initial materials were used: 
     
         ______________________________________          Weighed quantity for 4.6          kg. melt in grams______________________________________SiO.sub.2        921.1H.sub.3 BO.sub.3 549.6BaF.sub.2        1329.1La.sub.2 O.sub.3 1395.7ThO.sub.2        668.3As.sub.2 O.sub.3 13.8______________________________________ 
    
     thus producing a charge for a 4.6 kg. melt having the following proportions: 
     
         ______________________________________        Percent by Weight______________________________________SiO.sub.2       20.75B.sub.2 O.sub.3           7.00BaO             26.08La.sub.2 O.sub.3           31.12ThO.sub.2       15.05Total Oxides:  100.00 percentF.sub.2 -O      3.74 (increase in Weight due to          oxygen replaced by fluorine)As.sub.2 O.sub.3 (refining agent)           0.30______________________________________ 
    
     EXAMPLE 2 
     The following initial materials were used: 
     
         ______________________________________          Weighed quantity for 4.6          kg. melt in grams______________________________________SiO.sub.2        1133.6H.sub.3 BO.sub.3 449.4BaCO.sub.3       308.4Ba(NO.sub.3).sub.2            394.5BaF.sub.2        1228.2La.sub.2 O.sub.3 1338.2TiO.sub.2        244.8ThO.sub.2        119.8As.sub.2 O.sub.3 13.8______________________________________ 
    
     thus producing a charge for a 4.6 kg. melt having the following proportions: 
     
         ______________________________________          Percent by Weight______________________________________SiO.sub.2        24.6B.sub.2 O.sub.3  5.3BaO              33.5La.sub.2 O.sub.3 28.8TiO.sub.2        5.2ThO.sub.2        2.6Total Oxides:    100.0F.sub.2 -O       3.3Overall Total:   103.3As.sub.2 O.sub.3 (refining agent):            0.3______________________________________ 
    
     Having now discussed in considerable detail illustrative and preferred embodiments of the invention, it should be apparent that the objects set forth at the outset of this specification have been satisfied.