“…H ence it is necessary to explain why , alo ng the BaO·Ti02-Si02 join, glasses are formed only in the llarrow composition range of 40 to 50 mole percent of SiOz, and in particular, wh y , as Si02 is increased slightly to 52.5 mole percent or more, a m ixture of glass and crystals is obtained. The phase diagram of the B aO· Ti02-i02 sys tem [14] s uggesLs a simple explanation . The portion of th e liquidus cur ve for the Si02-rich region of the system i extr em ely steep ; t herefore, it is dif-:acult to keep the primary phase, Si02, in solution as t he melt is cooled .…”