Commercial underglazes, formulated to fire at about 1800 F, applied to the pottery as soon as the pots are finished being trimmed on the wheel in the raw state; these have been taken to 2232 F (cone 6) under my clear glaze. They darken or fade from their original colors a little bit. But the original colors are still vibrant. These small plates are to hang on the studio wall for reference when customers order dinnerware!
I already have 7 non-underglaze glazes that have been in regular rotation for a few years. Now, out of a round of new glaze tests, I found these to add (below). They have tonal variation within them that is rich and interesting when they are layered over the gray-white Cornwall Stone glaze. I got mottled pink-red (red chrome satin) and spearmint effects! Hoo ha!