I'm learning how to paint plaques for a friend who builds stained glass windows. I'm using the Reusche paints (tracing black, enamels, etc). Things have gone pretty well, but I have a question I'm hoping someone can answer... When I fire the plaques, some of the painting on them has a matte appearance and some has a semi-gloss appearance. The only possible factor that I have found that may be causing the difference is which shelf they are placed on when firing. I didn't pay attention in the first several firings as to the position, but in the last few, it seems the ones on the top shelf have the semi-gloss finish (which is what I want) and the ones on the lower shelves have the matte finish.  Any ideas or suggestions? I have the space to fire at least 6 plaques at a time - but don't want to end up with 2 glossy and 4 matte..

Please help

I didn't find the right solution from the internet.

Creative production company

Thank you