i am an engineer with a picture tube manufacturing unit. since last two weeks we are facing the problem that some sort of white powder is coming on the top face of the picture tube glass, after furnance baking. We have changed the supply mixture of LPG 2~3 weeks back, but the same mixture is being supplied on other line in the furnance where we are not facing any problems. Can any one help. what should we look for and what can be the proble??