Sometimes if the glass holds to long at high temps, it can get that gritty look and can cause deventrification. Make sure your ramp schedule allows for this. For example, when I am doing small pieces only, I don't need a long ramp time: 1500 to 1500 hold for 10 minutes; asap/full to 900, hold for 30 minutes; 350 to 700, no hold; end. Just a suggestion.