>Is it possible to put lowfire glazes over commercially made mugs? We
>got some school mugs and would like to personalize them.
HARD TO APPLY
Yes, it can be done. The application of ordinary glazes is challenge because the piece is not porous like bisqueware. Experiment. You may find a good way to do it.
Anytime we re-fire a pot we take a small risk that the pot will explode when the moisture in the body of the piece turns to steam. Take it slow (like with greenware) while it passes through the steam forming temps.
AVOID MOST OVERGLAZES
Pottery is often decorated using overglazes, china paints, and lusters. However, to make them easy to brush on, many of these materials are suspended in TOXIC aromatic hydrocarbons. I use lusters, but only when I can sit outside with the wind from my back to carry the vapors away. If you can smell it, your brain is being effected. It is not safe to use. Firing also releases the toxins.
I would not use materials suspended in aromatic hydrocarbons in a school setting.
MAYBE A SAFE ALTERNATIVE
There are water base overglazes as well.
I have not used them, and cannot vouch for the safety. Perhaps other teachers are familiar with them, and can vouch for them. You could call or email Amaco to verify that these are safe to use in a school setting.