Thermally high pre-stressable glass with high hot stressing factors
This is a continuation of application Ser. No. 971,036 filed Dec. 19, 1978, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to glass compositions having a high heat stress factor. More particularly, the present invention concerns thermally high pre-stressable strength and consequently high mechanical strength-glasses (high σ B  B -values), simultaneously featuring low heat expansion coefficients due to high temperature shock resistance or, in other words featuring high hot stressing factors. 
     When expressing the serviceability of partly heated glass objects, the heat stress factor as outlined below ##EQU1## (please see &#34;Die Keramik&#34;, by Salmang-Scholze, Springer Verlag, Berlin, 1968, p. 334 et seq.), is more informative than the separate evaluation of σ B  and α. The greater is the value for R, the higher the resistance against partial heating. 
     Within this formula, the symbols have the following meaning: 
     σ B  =bending tensile strength (kp/cm 2 ) with undamaged surface or surface broached with wet 220 grain emery. In the latter case, the values are approximately 400 kp/cm 2  lower than the values obtained on undamaged test-piece surfaces. 
     μ=transverse contraction index--approximately 0.20 for silicate glasses 
     α=mean linear thermal expansion coefficient in the temperature range of 20°-300° C. 
     E=elasticity modulus (kp/cm 2 ): between approximately 6.5 and 9.0×10 5  kp/cm 2  for silicate glasses. 
     Of these four values determining the heat stress factor R, σ B  and α are of particular importance. Under approximately constant μ-values and limited differences in E-modulus for the glasses according to the invention, R is determined practically alone by those values. It follows from this that temperature shock-resistant glasses with high R-values are only obtained where α is low and σ B  is high. Practice shows that this aim has so far only been achieved with low heat expansion glasses even where, as in the case of borosilicate glass with an α/300 value of approximately 33×10 -7  /°C., the glasses allow a less pronounced thermal pre-stress (pressure stress produced in the glass surface) than glasses with a high thermal expansion, such as window glass (float glass) with an α/300-value of approximately 90×10 -7  /°C. 
     It must be recalled here that glasses with a high thermal expansion do not lend themselves easily to high pressure pre-stressing. The limit results from the fact that the equivalent tensile stress, automatically created within the glass during pre-stressing, must not exceed the basic strength of the glass as predetermined by internal defects such as bubbles, waviness and the like, since a fracture will otherwise occur. (Whenever glass breaks, this is invariably because of excessive tensile stresses, since the compression strength of glass is some 10 times greater than the tensile strength). 
     The specific pre-stressability (hardenability) is also dependent upon the linear thermal expansion coefficient of the glass; the surface compression stress achieved is approx. 400 kp/cm 2  for the above borosilicate glass and approx. 1200 kp/cm 2  for window glass, as determined upon 50×20×5 mm laboratory samples. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide improved thermally high pre-stressable glass compositions. 
     Another object of the present invention is to provide a method for the production of such glass compositions which possess high heat stress factors. 
     Upon study of the specification and appended claims, further objects, features and advantages of the present invention will become more fully apparent to those skilled in the art to which this invention pertains. 
     SUMMARY OF THE INVENTION 
     Briefly, the above and other objects, features and advantages of the present invention are attained in one aspect thereof by providing glass compositions characterized as follows in percentages by weight: 
     
         ______________________________________                                    
SiO.sub.2               61.60-79.50                                       
B.sub.2 O.sub.3         1.00-10.50                                        
Al.sub.2 O.sub.3        2.50-4.00                                         
Na.sub.2 O.sub.3        1.50-6.00                                         
CaO                     0-4.20                                            
MgO                     0-3.10                                            
BaO                     0-9.60                                            
ZnO                     0-12.00                                           
total CaO + MgO + BaO + ZnO                                               
                        3.20-17.90                                        
ZrO.sub.2               0-1.50                                            
As.sub.2 O.sub.3        0-0.50                                            
NaCl                    0-0.75                                            
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     and besides good chemical resistance and high devitrification strength, they have the following properties: 
     
         ______________________________________                                    
α × 10.sup.7 (20-300° C.)/°C.                   
                     = 33.90-53.20                                        
α&#39; × 10.sup.7 /°C. above Tg                            
                     = 149.80-367.90                                      
α&#39;/α     = 4.10-9.40                                          
Tg (°C.); approx. 10.sup.13.5p                                     
                     = 566-660.                                           
Ew (°C.); = 10.sup.7.6p                                            
                     = 821- 945.                                          
V.sub.A (°C.); = 10.sup.4p                                         
                     = 1211-1370.                                         
Ew - Tg (°C.) = 232-298.                                           
density              = 2.30-3.02                                          
R, thermal stress factor                                                  
                     &gt;300.                                                
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     DETAILED DESCRIPTION 
     The mechanical strength of thermally pre-stressed glass (as obtained by various procedures, for instance compressed air blowing of the hot glass component or quenching in an oilbath) results from basic strength (usually 700-800 kp/cm 2  for most silicate glasses with an undamaged surface and in the stress-relieved state) and from the compression prestressing (increased strength due to this specific prestressability/ability to strengthening). In the case of compression prestressed borosilicate and window glasses as shown above, tests show σ B  -values on the borosilicate glasses of 700-800+400=1100-1200 kg/cm 2  whereas the window glasses show 700-800+1200=1900-2000 kp/cm 2 . Taking into account the α, μ and E values for the samples, the R-values for the borosilicate glass are 429-468 and those for the window glass are 216-227. 
     The tensile strength σ B  in the unstressed and prestressed condition for technical products manufactured from these glasses (e.g. flat glass) exceeds the above values. This is of no importance in the present case, since it is only intended to show the relationship between the various glasses as determined from the values for laboratory glass melts. 
     All values determining the thermal stress factor R will thus finally control the resistance to partial heating, and the above figures clearly show that borosilicate glass is superior to window-glass from this viewpoint, even though the specific prestressability of the glass does not suffice to achieve crumbling on fracture (low α-values induce a high temperature shock resistance and high σ B  values induce a high mechanical strength, in conjunction with an increasing tendency towards crumbling under forced fracture). 
     Most surprisingly, glass compositions have been found which, with a low linear thermal expansion coefficient α of 33.9-53.2×10 -7  /°C. at 20°-300° C. will pre-stress to a much higher extent under similar prestressing conditions than known glasses of the same thermal expansion, and which consequently result in crumbling under forced fracture. 
     The thermal stress factors R for the new glasses are within the order of magnitude of the R-value as recorded for the above-named borosilicate glass. Consequently, in conjunction with the higher prestressability, they are superior to the borosilicate and the window glasses. For many applications it is particularly useful for safety purposes to achieve numerous small fragments (crumbling) as a result of a fracture, instead of a few large fragments. The difference in relation to the state of the art is also clearly emphasized by four very important properties, which are achieved simultaneously and exclusively by the glasses in according to the invention, as follows: 
     1. α&#39;/α≧4.0; i.e., to achieve a high compression prestressing (high σ B ) the ratio of thermal expansion coefficent α&#39; above transformation temperature (Tg) to the thermal expansion coefficient below Tg is decisive. α-values at 20°-300° C. (viz. below Tg) do not suffice alone to obtaind an optimum compression prestressing . 2. Softening point of glasses Ew (ρ=10 7 .6 poise)&gt;820°. 
     3. Thermal stress factor R is greater than 300 for undamaged glass surfaces and under a quench blowing pressure of 1 bar. 
     4. Ew-Tg=232°-298° C. 
     Without further elaboration, it is believed that one skilled in the art can, using the proceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. All temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all pressures are ambient and all parts and percentages are by weight. The following tables show the superiority of the range of compositions according to the invention as compared with the previously cited borosilicate and window glasses. 
     Prefered compositions illustrated in the following examples are those having a high SiO 2  content together with a low Al 2  O 3  content, i.e. thermally high prestressable glasses with high temperative shock resistance and thermal expansions coefficients in the temperature range 20°-300° C. of from 37.8 to 53.1×10 -7  /°C., characterized in that the transformation temperatures (Tg) are from 571° to 608° C., the softening temperatures (Ew) are from 821° to 897° C., the working temperatures (V A ) are from 1211° to 1355° C., the temperature differential Ew-Tg is from 232° to 294° C., the thermal stress factor R is greater than 300 for undamaged glass surfaces under a quench blowing pressure of one bar and the ratio α&#39;/α of the thermal expansion coefficient above Tg (α&#39;)to expansion below Tg (α) is from 4.6 to 9.4 whereby said glass exhibits a high compression prestessing and crumbles into small pieces upon forced fracture, wherein the composition of said glasses, in oxides percent by weight, consists essentially of: 
     
         ______________________________________                                    
SiO.sub.2             73.40-79.50                                         
B.sub.2 O.sub.3       1.00-10.50                                          
Al.sub.2 O.sub.3      2.50-9.00                                           
Na.sub.2 O            4.00-6.00                                           
CaO                   0-3.00                                              
MgO                   0-2.10                                              
BaO                   0-9.60                                              
ZnO                   0-5.10                                              
total CaO + MgO + BaO + ZnO                                               
                      3.20-13.50                                          
ZrO.sub.2             0-1.50                                              
As.sub.2 O.sub.3      0-0.50    and                                       
NaCl                  0-0.75.                                             
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     A second group of preferred glass compositions in accordance with the present invention are those having both a high Al 2  O 3  content of about 14 percent and an As 2  O 3  content of about 0.30 percent, i.e. thermally high prestressable glasses with high temperative shock resistance and thermal expansion coefficients in the temperature range 20°-300° C. of from 33.9 to 42.3×10 -7  /°C., characterized in that the transformation temperatures (Ig) are from 623° to 660° C., the softening temperatures (Ew) are from 909° to 945° C., the working temperatures (V A ) are from 1272° to 1370° C., the temperature differential Ew-Tg is from 249° to 297° C., the thermal stress factor R is greater than 300 for undamaged glass surfaces under a quench blowing pressure of one bar and the ratio α&#39;/α of the thermal expansion coefficient above Tg (α&#39;) to expansion below Tg (α) is from 4.1 to 6.8 whereby said glass exhibits a high compression prestressing and crumbles into small pieces upon forced fracture, wherein the composition of said glasses, in oxides percent by weight, consists essentially of: 
     
         ______________________________________                                    
SiO.sub.2             61.60-65.00                                         
B.sub.2 O.sub.3       6.20-10.00                                          
Al.sub.2 O.sub.3      14.00                                               
Na.sub.2 O            1.50-4.00                                           
CaO                   2.50-4.00                                           
MgO                   1.70-1.80                                           
BaO                   2.00-4.50                                           
ZnO                   0-6.50                                              
total CaO + MgO + BaO + ZnO                                               
                      7.00-16.70                                          
                                and                                       
As.sub.2 O.sub.3      0.30.                                               
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     A third group of preferred glass compositions in accordance with the present invention has an Al 2  O 3  content of about 10 percent and an As 2  O 3  content of about 0.30 percent, i.e. thermally high prestressable glasses with high temperative shock resistance and thermal expansion coefficient in the temperature range 20°-300° C. of from 46.1 to 53.2×10 -7  /°C., characterized in that the transformation temperatures (Tg) are from 596° to 621° C., the softening temperatures (Ew) are from 859° to 885° C., the working temperatures (V A ) are from 1288° to 1355° C., the temperature differential Ew-Tg is from 248° to 271° C., the thermal stress factor R is greater than 300 for undamaged glass surfaces under a quench blowing pressure of one bar and the ratio α&#39;/α of the thermal expansion coefficient above Tg (α&#39;) to expansion below Tg (α) is from 4.3 to 5.8 whereby said glass exhibits a high compression prestressing and crumbles into small pieces upon forced fracture, wherein the composition of said glasses, in oxides percent by weight, consists essentially of: 
     
         ______________________________________                                    
SiO.sub.2             66.70-70.70                                         
B.sub.2 O.sub.3       4.50-10.30                                          
Al.sub.2 O.sub.3      10.00                                               
Na.sub.2 O            4.50-5.50                                           
CaO                   2.20-4.20                                           
MgO                   1.70-3.10                                           
BaO                   1.50-3.50                                           
ZnO                   0-3.00                                              
total CaO + MgO + BaO + ZnO                                               
                      8.50-10.30                                          
                                and                                       
As.sub.2 O.sub.3      0.30.                                               
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     A fourth group of preferred glass compositions in accordance with the present invention as an Al 2  O 3  content of 6.00 percent and an As 2  O 3  content of about 0.30 to 0.50 percent and a relatively high ZnO content of about 9.00 to 12.00 percent, i.e thermally high prestressable glasses with high temperative shock resistance and thermal expansion coefficients in the temperature range 20°-300° C. of from 44.9 to 46.2×10 -7  /°C., characterized in that the transformation temperaturess (Tg) are from 566° to 575° C., the softening temperatures (Ew) are from 844° to 864° C., the working temperatures (V A ) are from 1239° to 1287° C., the temperatures differential Ew-Tg is from 274° to 298° C., the thermal stress factor R is greater than 300 for undamaged glass surfaces under a quench blowing pressure of one bar and the ratio α&#39;/α  of the thermal expansion coefficient above Tg (α&#39;) to expansion below Tg (α) is from 5.0 to 5.7 whereby said glass exhibits a high compression prestressing and crumbles into small pieces upon forced fracture, wherein the composition of said glasses, in oxides percent by weight, consists essentially of: 
     
         ______________________________________                                    
SiO.sub.2           65.70-68.70                                           
B.sub.2 O.sub.3     6.00-6.30                                             
Al.sub.2 O.sub.3    6.00                                                  
Na.sub.2 O          4.00-5.00                                             
BaO                 5.00-8.9                                              
ZnO                 9.00-12.00                                            
total BaO + ZnO     14.00-17.90                                           
                              and                                         
As.sub.2 O.sub.3    0.30-0.50.                                            
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                       TABLE 1                                                     
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Composition of Borosilicate-Glass and                                     
Window-Glass in % by Weight and Properties                                
                 Borosilicate                                             
                            Window Glass                                  
Composition/Properties                                                    
                 Glass      (Float Glass)                                 
______________________________________                                    
SiO.sub.2        80.5       73.0                                          
B.sub.2 O.sub.3  12.8       --                                            
Al.sub.2 O.sub.3 2.4        0.8                                           
Na.sub.2 O       3.6        14.0                                          
K.sub.2 O        0.7        --                                            
CaO              --         8.7                                           
MgO              --         3.5                                           
Total %          100.0      100.0                                         
α × 10.sup.7 (20-300° C.)/°C.                   
                 33         88                                            
α&#39; × 10.sup.7 /°C. (above Tg)                          
                 126.4      229.7                                         
α&#39;/α 3.9        2.6                                           
Tg (°C.)  530        528                                           
Ew (°C.)  815        724                                           
V.sub.A (°C.)                                                      
                 1265       1033                                          
Ew - Tg (°C.)                                                      
                 285        196                                           
Density (g/cm.sup.3)                                                      
                 2.23       2.49                                          
μ             0.19       0.25                                          
.sup.←.sub.B of 50 × 20 × 5mm                            
                 1100-1200  1900-2000                                     
test-pieces air-hardened                                                  
at 1 bar, undamaged                                                       
surface and polished                                                      
(in kp/cm.sup.2)                                                          
E × 10.sup.-5 (kp/cm.sup.2)                                         
                 6.3        7.5                                           
R (thermal stressing                                                      
                 429-468    216-227                                       
coefficient)                                                              
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                       TABLE 2                                                     
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Composition Examples Nos. 1-4 in % by                                     
Weight and Properties                                                     
Composition/Properties                                                    
               No. 1   No. 2    No. 3 No. 4                               
______________________________________                                    
SiO.sub.2      78.50   78.50    78.50 78.50                               
B.sub.2 O.sub.3                                                           
               10.50   8.50     5.40  8.00                                
Al.sub.2 O.sub.3                                                          
               3.50    4.00     3.50  3.50                                
ZrO.sub.2      --      --       --    --                                  
Na.sub.2 O     4.30    4.00     5.00  5.50                                
CaO            0.80    1.80     --    1.50                                
MgO            0.50    1.20     --    1.00                                
BaO            1.90    2.00     2.50  2.00                                
ZnO            --      --       5.10  --                                  
As.sub.2 O.sub.3                                                          
               0.30    0.50     0.15  0.50                                
NaCl           0.70    0.50     0.70  --                                  
Total %        101.00  101.00   100.85                                    
                                      100.50                              
α × 10.sup.7                                                  
               37.8    38.5     41.9  43.6                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               254.3   338.4    224.0 323.3                               
α&#39;/α                                                          
               6.7     8.8      5.3   7.4                                 
Tg (°C.); ca . 10.sup.13.5                                         
               576     593      571   578                                 
Ew (°C.; = 10.sup.7.6 p)                                           
               837     878      854   822                                 
V.sub.A (°C.); = 10.sup.4 p                                        
               1297    1345     1327  1245                                
Ew - Tg (° C.)                                                     
               261     285      283   244                                 
Density (g/cm.sup.3)                                                      
               2.30    2.33     2.40  2.36                                
μ           0.19    --       --    --                                  
E × 10.sup.-5 (kp/cm.sup.2)                                         
               7.04    --       --    --                                  
σ.sub.B of                                                          
               1548    --       --    --                                  
50 × 30 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
R (heat stress 470     --       --    --                                  
factor)                                                                   
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Composition Examples Nos. 5-8 in % by                                     
Weight and Properties                                                     
Composition/Properties                                                    
               No. 5   No. 6    No. 7 No. 8                               
______________________________________                                    
SiO.sub.2      79.50   78.50    78.50 78.50                               
B.sub.2 O.sub.3                                                           
               8.00    6.10     5.20  6.50                                
Al.sub.2 O.sub.3                                                          
               2.50    3.50     3.50  3.50                                
ZrO.sub.2      --      --       --    --                                  
Na.sub.2 O     5.50    5.00     5.30  5.30                                
CaO            1.50    2.40     3.00  2.40                                
MgO            1.00    1.70     2.10  1.60                                
BaO            2.00    2.80     --    2.00                                
ZnO            --      --       2.50  --                                  
As.sub.2 O.sub.3                                                          
               0.15    0.25                                               
NaCl           0.60    0.75     0.50  0.50                                
Total %        100.75  101.00   100.50                                    
                                      100.50                              
α × 10.sup.7                                                  
               44.3    44.9     45.4  45.8                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               291.9   333.0    328   361.4                               
α&#39;/α                                                          
               6.6     7.2      7.2   7.9                                 
Tg (°C.); ca . 10.sup.13.5                                         
               574     582      582   581                                 
Ew (°C.; = 10.sup.7.6 p)                                           
               823     839      848   831                                 
V.sub.A (°C.); = 10.sup.4 p                                        
               1243    1290     1292  1270                                
Ew - Tg (°C.)                                                      
               249     257      266   250                                 
Density (g/cm.sup.3)                                                      
               2.35    2.38     2.37  2.37                                
μ           --      0.19     --    0.19                                
E × 10.sup.-5 (kp/cm.sup.2)                                         
               --      7.46     --    7.48                                
σ.sub.B of                                                          
               --      1787     --    1766                                
50 × 30 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
R (heat stress --      433      --    419                                 
factor)                                                                   
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Composition Examples Nos. 9-12 in % by                                    
Weight and Properties                                                     
Composition/Properties                                                    
               No. 9   No. 10   No. 11                                    
                                      No. 12                              
______________________________________                                    
SiO.sub.2      78.50   78.50    77.50 78.50                               
B.sub.2 O.sub.3                                                           
               1.00    5.10     5.10  5.10                                
Al.sub.2 O.sub.3                                                          
               2.50    3.50     3.50  3.50                                
ZrO.sub.2      --      --       1.50  --                                  
Na.sub.2 O     4.50    5.30     5.30  5.30                                
CaO            2.70    --       3.00  3.00                                
MgO            1.90    --       2.10  2.10                                
BaO            4.90    7.60     2.00  2.50                                
ZnO            4.00    --       --    --                                  
As.sub.2 O.sub.3                                                          
               0.25    0.25     --    0.30                                
NaCl           0.60    0.60     0.50  0.50                                
Total %        100.85  100.85   100.50                                    
                                      100.80                              
α × 10.sup.7                                                  
               45.9    46.6     46.7  47.1                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               212.3   438.7    345.2 369                                 
α&#39;/α                                                          
               4.6     9.4      7.4   7.8                                 
Tg (°C.); ca . 10.sup.13.5                                         
               591     591      594   584                                 
Ew (°C.; = 10.sup.7.6 p)                                           
               885     831      857   854                                 
V.sub.A (°C.); = 10.sup.4 p                                        
               1355    1254     1290  1271                                
Ew - Tg (°C.)                                                      
               294     240      263   270                                 
Density (g/cm.sup.3)                                                      
               2.47    2.45     2.40  2.38                                
μ           --      --       --    0.19                                
E × 10.sup.-5 (kp/cm.sup.2)                                         
               --      --       --    7.50                                
σ.sub.B of                                                          
               --      --       --    --                                  
50 × 30 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
R (heat stress --      --       --    --                                  
factor)                                                                   
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Composition Examples Nos. 13-16 in % by                                   
Weight and Properties                                                     
Composition/Properties                                                    
               No. 13  No. 14   No. 15                                    
                                      No. 16                              
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SiO.sub.2      78.50   77.00    78.50 75.00                               
B.sub.2 O.sub.3                                                           
               4.10    5.10     2.50  5.10                                
Al.sub.2 O.sub.3                                                          
               3.50    5.00     3.50  9.00                                
ZrO.sub.2      --      --       --    --                                  
Na.sub.2 O     5.30    5.30     5.30  6.00                                
CaO            2.40    3.00     2.40  2.90                                
MgO            1.70    2.10     1.70  2.00                                
BaO            4.50    2.50     6.10                                      
ZnO            --      --       --    --                                  
As.sub.2 O.sub.3                                                          
               0.25             0.30  0.10                                
NaCl           0.60    0.50     0.50  0.50                                
Total %        100.85  100.55   100.80                                    
                                      100.60                              
α × 10.sup.7                                                  
               47.5    48.2     48.8  49.8                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               347.2   311      308   265                                 
α&#39;/ α                                                         
               7.3     6.4      6.3   5.3                                 
Tg (°C.); ca . 10.sup.13.5                                         
               588     593      586   608                                 
We (°C.; = 10.sup.7.6 p)                                           
               854     848      858   897                                 
V.sub.A (°C.); = 10.sup.4 p                                        
               1287    1281     1314  1344                                
Ew - Tg (°C.)                                                      
               266     255      272   289                                 
Density (g/cm.sup.3)                                                      
               2.41    2.39     2.44  2.35                                
μ           0.19    0.19     --    --                                  
E × 10.sup.-5 (kp/cm.sup.2)                                         
               7.52    7.36     --    --                                  
σ.sub.B of                                                          
               1871    1781     --    --                                  
50 × 30 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
R (heat stress 425     407      --    --                                  
factor)                                                                   
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Composition Examples Nos. 17-20 in % by                                   
Weight and Properties                                                     
Composition/Properties                                                    
               No. 17  No. 18   No. 19                                    
                                      No. 29                              
______________________________________                                    
SiO.sub.2      78.50   77.00    75.50 73.40                               
B.sub.2 O.sub.3                                                           
               1.00    4.10     5.10  5.10                                
Al.sub.2 O.sub.3                                                          
               3.50    3.50     3.50  3.50                                
ZrO.sub.2      --      --       --    --                                  
Na.sub.2 O     5.30    5.80     5.30  5.30                                
CaO            2.40    --       3.00  3.00                                
MgO            1.70    --       2.10  2.10                                
BaO            7.60    9.60     5.50  7.60                                
ZnO            --      --       --    --                                  
As.sub.2 O.sub.3                                                          
               0.25    0.25     0.25  0.25                                
NaCl           0.60    0.60     0.60  0.60                                
Total %        100.85  100.85   100.85                                    
                                      100.85                              
α × 10.sup.7                                                  
               50.0    50.3     50.9  53.1                                
(20- 300° C.)/°C.                                           
α&#39; (above Tg)                                                       
               258     368      372   322.7                               
α&#39;/ α                                                         
               5.2     7.3      7.3   6.1                                 
Tg (°C.); ca . 10.sup.13.5                                         
               593     589      588   589                                 
Ew (°C.; = 10.sup.7.6 p)                                           
               876     851      826   821                                 
V.sub.A (°C.); = 10.sup.4 p                                        
               1322    1257     1238  1211                                
Ew - Tg (°C.)                                                      
               283     263      238   232                                 
Density (g/cm.sup.3)                                                      
               2.46    2.47     2.45  2.49                                
μ           --      --       --    --                                  
E × 10.sup.-5 (kp/cm.sup.2)                                         
               --      --       --    --                                  
σ.sub.B of                                                          
               --      --       --    --                                  
50 × 30 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
R (heat stress --      --       --    --                                  
factor)                                                                   
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Composition Examples Nos. 21-24 in % by                                   
Weight and Properties                                                     
Composition/Properties                                                    
               No. 21  No. 22   No. 23                                    
                                      No. 24                              
______________________________________                                    
SiO.sub.2      65.0    61.60    65.00 65.00                               
B.sub.2 O.sub.3                                                           
               10.00   6.20     10.00 10.00                               
Al.sub.2 O.sub.3                                                          
               14.00   14.00    14.00 14.00                               
Na.sub.2 O     2.00    1.50     2.50  4.00                                
CaO            2.50    4.00     2.50  2.50                                
MgO            1.80    1.70     1.80  1.80                                
BaO            2.00    4.50     4.20  2.70                                
ZnO            2.70    6.50     --    --                                  
As.sub.2 O.sub.3                                                          
               0.30    0.30     0.30  0.30                                
Total %        100.30  100.30   100.30                                    
                                      100.30                              
α × 10.sup.7                                                  
               33.9    36.6     38.8  42.3                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               164     149.8    206.4 286.8                               
α&#39;/α                                                          
               4.9     4.1      5.3   6.8                                 
Tg (°C.)                                                           
               652     660      654   623                                 
Ew (°C.)                                                           
               945     909      935   920                                 
V.sub.A (°C.)                                                      
               1335    1272     1370  1340                                
Ew - Tg (°C.)                                                      
               293     249      291   297                                 
Density (g/cm.sup.3)                                                      
               2.40    2.54     2.40  2.37                                
μ           --      0.20     --    --                                  
σ.sub.B of .sub.B ×  20 × 5mm                           
               --      1910     --    --                                  
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
E × 10.sup.-5 (kp/cm.sup.2)                                         
               --      8.5      --    --                                  
R (heat stress --      491      --    --                                  
factor)                                                                   
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
Composition Examples Nos. 25-29 in % by                                   
Weight and Properties                                                     
Composition                                                               
Properties   No. 25  No. 26  No. 27                                       
                                   No. 28                                 
                                         No. 29                           
______________________________________                                    
SiO.sub.2    66.70   70.70   70.70 69.70 69.70                            
B.sub.2 O.sub.3                                                           
             10.30   5.80    5.80  4.80  4.50                             
Al.sub.2 O.sub.3                                                          
             10.00   10.00   10.00 10.00 10.00                            
Na.sub.2 O   4.50    5.00    5.00  5.50  5.50                             
CaO          2.30    2.20    3.10  3.70  4.20                             
MgO          1.70    1.80    2.40  2.80  3.10                             
BaO          2.00    1.50    3.00  3.50  3.00                             
ZnO          2.50    3.00                                                 
As.sub.2 O.sub.3                                                          
             0.30    0.30    0.30  0.30  0.30                             
Total %      100.30  100.30  100.30                                       
                                   100.30                                 
                                         100.30                           
α × 10.sup.7                                                  
             46.1    46.4    49.6  52.4  53.2                             
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
             266.6   199.1   232.4 265.2 246.7                            
α&#39;/α                                                          
             5.8     4.3     4.7   5.1   4.6                              
Tg (°C.)                                                           
             596     614     618   621   621                              
Ew (°C.)                                                           
             859     885     873   869   873                              
V.sub.A (°C.)                                                      
             1291    1355    1347  1303  1288                             
Ew - Tg (°C.)                                                      
             263     271     255   248   252                              
Density (g/cm.sup.3)                                                      
             2.39    2.40    2.41  2.45  2.44                             
μ         --      --      --    0.205 --                               
σ.sub.B of                                                          
             --      --      --    1910  --                               
50 × 20 × 5 mm                                                
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface                                                         
and polished                                                              
(kp/cm.sup.2)                                                             
E × 10.sup.-5 (kp/cm.sup.2)-                                        
                     --      --    7.52  --                               
R (heat stress                                                            
             --      --      --    385   --                               
factor)                                                                   
______________________________________                                    
 
    
     
                       TABLE 5                                                     
______________________________________                                    
Composition Examples Nos. 30-33 in % by                                   
Weight and Properties                                                     
Composition/Properties                                                    
               No. 30  No. 31   No. 32                                    
                                      No. 33                              
______________________________________                                    
SiO.sub.2      65.70   67.00    66.00 68.70                               
B.sub.2 O.sub.3                                                           
               6.10    6.00     6.10  6.30                                
Al.sub.2 O.sub.3                                                          
               6.00    6.00     6.00  6.00                                
Na.sub.2 O     4.50    4.20     4.00  5.00                                
BaO            5.70    7.80     8.90  5.00                                
ZnO            12.00   9.00     9.00  9.00                                
As.sub.2 O.sub.3                                                          
               0.50    0.50     0.50  0.30                                
Total %        100.50  100.30   100.50                                    
                                      100.30                              
α × 10.sup.7                                                  
               44.9    45.2     45.6  46.2                                
(20-300° C.)/°C.                                            
α&#39; (above Tg)                                                       
               229.6   258.4    260.1 232.6                               
α&#34;/α                                                          
               5.1     5.7      5.7   5.0                                 
Tg (°C.)                                                           
               566     574      575   568                                 
Ew (°C.)                                                           
               864     848      855   844                                 
V.sub.A (°C.)                                                      
               1239    1287     1270  1287                                
Ew - Tg (°C.)                                                      
               298     274      280   276                                 
Density (g/cm.sup.3)                                                      
               3.02    2.57     3.02  2.51                                
μ           --      0.206    --    --                                  
σ.sub.B of                                                          
               --      1465     --    --                                  
50 × 20 × 5mm                                                 
test-pieces air-                                                          
hardened at 1 bar,                                                        
undamaged surface and                                                     
polished (kp/cm.sup.2)                                                    
E × 10.sup.-5 (kp/cm.sup.2)                                         
               --      7.23     --    --                                  
R (heat stress --      356      --    --                                  
factor)                                                                   
______________________________________                                    
 
    
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those specifically used in the examples. From the foregoing description, one skilled in the art to which this invention pertains can easily ascertain the essential characteristics thereof and, without departing from the spirit and scope of the present invention, can make various changes and modifications to adapt it to various usages and conditions.