These paints are opaque and become glossier with higher firing temperatures.
If you fire at higher temperatures, be aware that your glass may pick up the structure of whatever material it is laying on in the kiln.
We often fire these paints at 630°C (1166°F). At this temperature the paints may not be weather resistant but the result is excellent for interior use. You will also have less pigment loss, which is especially notable with the red tones.