The hot spots appear due to many factors among which the major factors are listed below for reference:

1. The dampproof powder sticks to the glass when it is damped or there are sundries on the surface of the glass, which are not cleaned up before tempering;

2. Some of the thermal insulation layer falls off from the top of the heating furnace;

3. If the glass is not washed after edging, after a long time of storage the residual glass powder will become dry and hard. And the glass is not cleaned up before being tempered.

4. The ceramic roller in the heating furnace is not cleaned properly so that dirts accumulate;

5. Tiny glass cullet is remained on the roller because of the glass breakage in the furnace.