Determination and Use of the Sag Point as a Reference Point in the Heating of Glasses

Glasses, whe n heated , undergo a gradu a l co nt inuous change from so li ds to iiq uids over a rather Wide te mperature range. As a co nseq uence, it has bee n fo und necessar y to d efi ne certall1 te mperature r eference poin ts in t his t ra nsit io n range so t hat different glasses m ay be co mpared w:ith each other. These poin ts a re chose n to co rrespond to establi shed stages In t he a.n neali ng or formin g process. A ne w refere nce point, designated as t he sag POll1 t,. IS descrIbed .. The ex perimental determ inat ion of t hi s poin t is ra pid a nd sim ple. A lso, It gives usefullnformatlOl1 concerning t he a nnealin g temperatures a nd ease of fo rmation o f glasses of w idely vary ing composit ion s . The sag point is defin ed as t he te mperature at whi ch a "lass fi ber 0. 5 to 0.8 milli mete r in di a mete r, horizo ntall y sup ported at .H-inchi ntervals, will sag under its own w eight in 25 ± 5 minu tes.