Zircon streaks from furnace glass contact material
Inclusion of stagnant glass by increase in pull
Inclusion of stagnant glass after colour change
Volatilisation of alkalis and borates - borosilicate glasses
Volatislsation of lead oxides - lead glasses
Volatilisation of flourides - opal glasses
Colouring forehearths
Incorrect stirrer action
Incorrect stirrer section length
Incorrect stirrer configuration
Incorrect frit feed