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What happens if I high-fire a low-fire underglaze?

I’m doing some sgraffito work on porcelain, but the underglaze is low-fire. I want to high-fire this piece (durability), but there’s only one more firing before the student show at my university. I know I can low fire the greenware piece, then high-fire it with a clear coat, but I’ve found that my pieces lose their definition with the high fire clear coat, even with a firing preceeding it to fix the underglaze, but it seems the high-fire clear takes the underglaze for a ride. The underglaze (grey, in this case) can be high fired if there’s a clear coat on top, but what happens if it’s high fired without that protection?? I really don’t want to screw up the coloring.

For the most part, depending if you are using a commercial or custom mixed glaze the higher the firing temp the more you will loose your color.

Most commercial glazes designed for low fire will go clear as you go higher.

Mason stains on the other hand will retain their color integrity even at porcelain temps.

Please do not take my word for it without knowing what kind of glaze you are using or the formula anything can happen.

I had another thought do you need to fire it high? It is true the best durability comes from firing the clay body to maturity but if you want it done in time for a show maybe you shouldn’t worry about that. Fire it for the best appearence and handle with care.

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