silica sand is fed to this furnace and allowed to melt... furnace temperature is around 1600-1700°C

LPG burner in glass annealing lehr (furnace)... after the bottle or jar or whatever is made... the glass product is very brittle due to its high cooling rate... the annealing process allow glass grains to re-grow... this increases its strength properties... as in, it allows you to drop the bottle or jar a few more times before it would break :)