Vitrification or densification is due to the feldspar or fusible sand.
It is fusible at 122° Fahrenheit, and volatile at higher heats.
To be of value a fusible plug should be renewed or changed once a month.
You should act at all times precisely as if there were no fusible plug.
It occurs as a red, crystalline, fusible, deliquescent powder.
A fusible brownish-red, translucent ...