Method of composing a mixture for lead glass
The invention relates to a method of composing a glass mixture comprising quartz sand, felspar and other glass-forming components intended for the manufacture of lead glass. 
     Lead glasses include those glasses within the range of compositions limited as follows in % by weight: 
     
         SiO.sub.2                                                                 
       20-65          ZnO          0-12                                   
B.sub.2 O.sub.3                                                           
       0-25           PbO          12-60                                  
Na.sub.2 O                                                                
       O-16 combined  Al.sub.2 O.sub.3                                    
                                   0.5-5                                  
K.sub.2 O                                                                 
       O-16 &gt;1.5      As.sub.2 O.sub.3 and/or                             
                                   0-1                                    
CaO    0-4            Sb.sub.2 O.sub.3                                    
BaO    0-8            MnO          0-1                                    
 
    
     The mixture used for preparing these types of glasses comprises quartz sand, optionally corrected for SiO 2  present in other materials, felspar (NaAlSi 3  O 8  - KAlSi 3  O 8 ), alkali carbonates and/or alkali hydroxides for suppletion of alkali oxide, minium or lead silicate and optionally calcite, dolomite, barium carbonate, zinc white and/or boric acid or borax. 
     In the glass obtained from this mixture difficulties occur due to gas inclusions. When drawing a tube from this glass, these inclusions become manifest as striae consisting, inter alia, for superficially open furrows in the drawing direction. 
     It was found from investigations prior to the present invention that the choice of the grain size of quartz sand and felspar is of essential importance for the melting behavior of the mixture in which these materials are present. 
     According to the U.K. Pat. No. 1,357,063 a mixture for a borosilicate glass consisting of a pulverulent mixture of quartz powder, pulverulent felspar and other glass-forming components is prepared while using quartz powder having a particle size at least 90% of which is between 30 and 120 μm, and felspar having a particle size at least 90% of which is between 30 and 120 μm. In this manner a very large reduction of the melting time and the refining time is achieved. The time required to prepare the mixture may be up to 50% shorter than the time for preparing a mixture in which the materials are present with a non-selected particle size, i.e. with a considerable portion of larger grains and also with grains having a particle size of less than 30 μm. Particularly materials having a smaller particle size were found to have a considerable negative influence. 
     It was found that for preparing a mixture to TV glass there was a quite different requirement to be satisfied by quartz sand and felspar in the mixture (U.S. Pat. application Ser. No. 400,402). A considerable acceleration of the melting process is likewise obtained, considering the freedom from inclusions in the glass melt and the dissolution of crystalline constituents, if in the mixture sodium and/or potassium felspar powder is used, the specific area of which is larger than 1 sq.m/gram in addition to quartz sand the specific area of which is smaller than 0.1 sq.m/gram. 
     According to the invention a possibility to accelerate the melting process was also found for the type of glass denoted by the name of lead glass and defined above, by selection of the particle size of quartz sand present in the mixture and, optionally, felspar. Surprisingly, the requirements were found to be quite different from those for the above-described TV glasses and borate glasses. 
     According to the invention the method of preparing a mixture for lead glass is characterized in that quartz sand present therein has a particle size of not more than 100 μm and felspar has a particle size of not more than 100 μm. 
    
    
     The effect of the invention will be illustrated in the following Tables. The periods of time within which the melt is free from inclusions are stated as criteria. 
     In this case a lead glass is concerned which is melted at a temperature of 1430°C in platinum crucibles of 100 cubic cm. in an electric furnace and which has the following composition in % by weight: 
     
         SiO.sub.2 60.2       PbO          21.3                                    
Al.sub.2 O.sub.3                                                          
          1.3        Sb.sub.2 O.sub.3                                     
                                  0.3                                     
Na.sub.2 O                                                                
          10.0                                                            
K.sub.2 O 5.4                                                             
 
    
     The following mixture is required per 100 g of glass: 
     
         quartz sand           56.8 kg                                             
sodium carbonate      15.3 kg                                             
potassium carbonate    8.0 kg                                             
lead silicate         25.0 kg                                             
aluminium oxide        1.2 kg                                             
sodium nitrate         3.1 kg                                             
antimony oxide         0.3 kg                                             
 
    
     The mixture is melted at a temperature of approximately 1300°C. 
     Sand of 3 different origins and felspar of 4 different qualities are used in the mixture, with the distribution of the grain size as mentioned below. The felspar was (1) flotation felspar from Norway, (2) Habera felspar from Germany, (3) sieved Habera felspar and (4) felspar from Norway, supplied by the firm Bjorum. The sand was supplied by the firm Duinger from Germany (1), quartz powder from the firm Sibelco (2) of Belgium and (3) from Dowson and Dobson (South Africa). 
     
         __________________________________________________________________________
Distribution of grain size                                                
felspar                sand                                               
 1        2   3   4    1    2   3                                         
__________________________________________________________________________
&gt;315 μm                                                                
       5% 1.0%         0.3%                                               
&gt;200 μm                                                                
      34% 8.5%         6 %                                                
&gt;160 μm                                                                
      62% 25 %         49 %                                               
&gt;100 μm                                                                
      82% 39 %                                                            
              8.3%                                                        
                   1.0%                                                   
                       95 %      2%                                       
&gt; 63 μm                                                                
      98% 56 %                                                            
              6 % 11.0%                                                   
                       99.7%    80%                                       
&gt; 45 μm                                                                
          61 %                                                            
              25 %                                                        
                  24.2%     15%                                           
&gt; 32 μm                                                                
          62 %                                                            
              70 %                                                        
                  71.8%     35%                                           
&gt; 10 μm                  70%                                           
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     As the following Table shows, there is also some difference in composition of the four types of felspar used. This is of course to be taken into account when preparing the mixture. 
     
         ______________________________________                                    
Al.sub.2 O.sub.3                                                          
           Na.sub.2 O                                                     
                   K.sub.2 O                                              
                            CaO   SiO.sub.2                               
                                        Fe.sub.2 O.sub.3                  
______________________________________                                    
1    17.8      3.4     11.2   0.5   66.6  --                              
2    23.0      0.6     10.0   0.15  66.3  0.12                            
3    20.3      0.7     10.7   0.05  68.2  0.13                            
4    13.8      4.6      3.0   1.0   77.0  --                              
______________________________________                                    
 
    
     The results are summarized in the following Table 
     
         melt no.                                                                  
        sand       felspar    free from inclusions                        
                              within (min.)                               
______________________________________                                    
1       &gt;100(1)    ˜200(1)                                          
                              160                                         
2       &gt;100(1)    &lt;100(4)    135                                         
3       &lt;100(2)    &lt;100(4)    55                                          
4       &gt;100(1)    &lt;100(2)    135                                         
5       &gt;100(1)    &lt;100(3)    135                                         
6       &lt;100(2)    &lt;100(2)    75                                          
7       &lt;100(2)    &lt;100(3)    55                                          
8       &lt;100(3)    &lt;100(3)    85                                          
9       &lt;100(3)    &lt;100(2)    90                                          
______________________________________                                    
 
    
     Also as regards the occurrence of &#34;knobs&#34;, i.e. regions having a high SiO 2  content in the glass a considerable improvement is achieved according to the invention. The above-mentioned melts 3, 6, 7, 8 and 9 melted at 1300°C are free from knobs after approximately half an hour. When a mixture not according to the invention (1) is melted, this is the case only after 21/2 hours under these circumstances.