hydrated silica has a surface area b e t of between 50 70 sq m gm a typical analysis of this amorphous precipitated hydrated silica is as follows ______________________________________sio sub 2 as shipped dry basis 94 0 nacl 1 7 cao 0 80 r sub 2 o sub 3 fe al 0 63 ph in 5 water suspension 7 0loss at 105 u00b0 c as shipped 5 3 cu and mn combined total 0 003 surface area b e t 60 sq m gm ultimate particle size 0 04 micronsrefractive index 1 46dbp absorption 182 ml 100 gm retained on 325 mesh 0 3 pellets 0 07 powder ______________________________________ it is believed that the high surface area of the amorphous precipitated silica contributes substantially to forming an intimate admixture with the alumina and further provides a substantial area that may be coated with b 2 o 3 flux any suitable alumina of colloidal size may be used as a constituent for the borosilicate composition a preferred alumina is a pseudo boehmite or alpha monohydrate of colloidal size the boehmite is prepared by the thermal dehydration of a gibbsite bayerite an amorphous aluminum hydroxide the material has a range of surface areas from 15 m 2 g to 400 m 2 g the preferred alumina sold by kaiser chemicals baton rouge louisiana under the trade name substrate alumina sa has a b e t of between.