This kind of devitrification is apparently due to Volatilization of the alkalies from the glass in the flame, and it is said that it can be partly remedied or prevented by holding a swab of cotton saturated with a strong solution of common salt in the flame from time to time as the glass is heated.
"Laboratory Manual of Glass-Blowing"
Francis C. Frary
The fire having been kindled and the blowing apparatus, already described, in operation, thick clouds of white, yellow, and orange-yellow smoke were evolved from the partial Volatilization of the sulphur, arsenic, and antimony, for the space of an hour; but as soon as only sulphurous acid was formed, and the heat by this procedure had attained its highest degree, the blowing was discontinued and the product taken out.
"The Former Philippines thru Foreign Eyes"
Tomás de Comyn Fedor Jagor Rudolf Ludwig Carl Virchow Charles Wilkes
Small amounts of nitrotoluene are lost by Volatilization, but this loss is not serious, as can be seen from the yield of product obtained.
"Organic-Syntheses"
Conant, James Bryant