the following corning glass compositions can also be used 7052 65 sio 2 7 al 2 o 3 18 b 2 o 3 1 li 2 o 2 na 2 o and 2 k 2 0 1720 62 sio 2 17 al 2 o 3 5 b 2 o 3 1 na 2 o 7 mg and 8 ca 1723 57 sio 2 15 al 2 o 3 5 b 2 o 3 7 mg 7 ca and 6 na 2 o 7740 81 sio 2 2 al 2 o 3 13 b 2 o 3 and 4 na 2 o and 7913 96 5 sio 2 0 5 al 2 o 3 and 3 b 2 o3 furthermore leaks could be drawn from other compositions of borrosilicate clear glass and silica it should be further noted that although kovar tubing is preferred other metals such as for example molybdenum tubing copper tubing stainless steel tubing and tungsten tubing may also be used however the proper glass must be used with each of the above metals to insure that their coefficient of expansions are substantially the same so that the seal between the metal tube and the glass does not break at high temperatures kovar tubing is preferred over the other metals because of its stable characteristics at high temperatures and.