Chemically stable optical glass
BACKGROUND OF THE INVENTION 
     This invention relates to an optical glass of the optical position nd (refraction index) = 1.650 ± 2 × 10.sup. -3  and vd (Abbe number) = 55.5 ± 1.0. 
     Glasses of this optical position are conventional; however heretofore, all of these glasses had a very low chemical stability. Consequently, there were restrictions on their manufacture and application. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an optical glass of the optical position nd = 1.650 ± 2 × 10 -   3 , vd = 55.5 ± 1.0 having a high chemical stability and a sufficiently low tendency toward crystallization that it can be manufactured by a continuous tank melting method into gobs, pressed articles, and block glass of any desired form as a mass-produced glass. 
     Other objects are to provide starting glass composition intermediates in the production of and/or process to produce said optical glasses. 
     Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the 
     To attain these objects, it has been discovered that the desired optical glasses can be produced on the basis of a glass having the following composition (in percent by weight):SiO 2            33.0 ± 0.5B 2  O 3      11.6 ± 0.5BaO                 36.6 ± 1.0La 2  O 3     14.0 ± 2.5ZrO 2             4.0 ± 2.5As 2  O 3                      0  - 0.5 as refining agentor Sb 2  O 3 preferably nd            =        1.650 ± 1 × 10.sup. -3 preferably vd            =       55.0  ± 0.5 
     The glass of the present invention has the following properties: 
     THE COEFFICIENT OF EXPANSION α × 10 7  /° C. between 20° and 300° C. is 74 ± 2; 
     THE TRANSFORMATION TEMPERATURE IS 683 ± 5° C.; 
     the softening temperature at 10 7 .65 poises is 810 ± 10° C; 
     the specific gravity is 3.78 ± 0.10. 
     The chemical resistance is 2-3 classes better than that of the glasses on the market which have this optical position, i.e., La K22 nd = 1.6511 vd = 55.89. The chemical resistance test was conducted in accordance with the acid resistance and spotting tendency tests customary with Schott, as described in the Schott pocket catalogue No. 3050 of 1972. 
     The crystallization test shows, after a tempering of 60 minutes in a gradient furnace, no devitrification in the glass and only on the surface in isolated instances a crystallization growth rate of &lt;0.3 μ/min. The test was conducted according to O. H. Grauer and E. H. Hamilton: &#34;Improved Apparatus for Determination of Liquidus Temperatures and Rates of Crystal Growth in Glasses,&#34; J. Research Natl. Bur. Standards, 44 (5) 495-502 (1950); RP 2096. 
    
    
     The following table shows examples of glasses according to the invention in percentages by weight: 
     
         Oxides        Example    Example    Example                               
                1          2          3                                   
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SiO.sub.2     33,4       32,8       33,0                                  
B.sub.2 O.sub.3                                                           
              11,0       11,6       11,6                                  
BaO           37,1       36,6       36,6                                  
La.sub.2 O.sub.3                                                          
              12,7       15,0       15,8                                  
ZrO.sub.2      4,8        3,8        3,0                                  
Sb.sub.2 O.sub.3                                                          
               0,2        0,2        0,2                                  
nd             1,651      1,651      1,651                                
vd            54,5       55,1       55,9                                  
Density s      3,75       3,76       3,73                                 
Cryst. growth  0,3        0,3        0,15                                 
Acid resistance                                                           
glass f        4          4          4                                    
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     Example 3 with an Abbe-number vdof 55,9 at a refraction index nd of 1,651 represents a particularly preferred embodiment of the invention. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiment is, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. In the following example, all temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight. 
     For a glass according to Example 3 a blended batch of 300 kg. is formulated as follows: 
     
                       EXAMPLE 3                                                   
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WEIGHED-IN QUANTITY                                                       
Raw Materials      kg.                                                    
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SiO.sub.2          99.15                                                  
H.sub.3 BO.sub.3   61.81                                                  
Ba(NO.sub.3).sub.2 33.96                                                  
BaCO.sub.3         116.03                                                 
La.sub.2 O.sub.3   47.64                                                  
ZrO.sub.2          9.08                                                   
Sb.sub.2 O.sub.3   0.60     as refining                                   
                            agent                                         
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 The batch is well mixed together. The thus-mixed blend is melted in a
 continuous tank at about 1350°C, refined in the refining section of
 the tank at 1380° - 1400°C and stirred in the discharge
 section up to 1160°C. The melt is discharged through a platinum
 pipe at between 1100° and 1020°C (viscosity 250-10.sup.3
 poise) and is worked into gobs, bars, pressed elements or blocks.
 
    
     The preceding example can be repeated with similar success by substituting the generically or specifically described invention for those used in the preceding example. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.