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We often get asked about painting on already glazed commercial ceramics such tiles, plates and vases. This is very possible to do, and makes a fun project. In fact several shopping malls near us have done murals where kids painted on glazed tiles, then the tiles were fired and mounted.
The biggest problem with this is that you don't know the firing temperature of the clay and glaze that was used on the commerical piece. What you don't want is to fire it too hot and have melted clay and/or glaze ruining your kiln. So the best solution is to fire a sample piece inside a bisque bowl, that will catch the clay or glaze if it does melt or run. Instead of a bowl you could use a flat piece of clay with the edges turned up. If you only have a single piece, such as a vase, you can't test a sample of it. So either get two and test it on one first, or you will have to just paint the whole thing, fire it, and cross your fingers.
But if you use low fire glazes you are pretty safe. Most ceramics can withstand low fire temperatures such as Cone 06.
Sometimes the color of the original piece will change when it is re-fired. I once did this with black tiles and they turned a gray-ish black. So just be aware that this could happen.
What glaze to use? Many low fire glazes will work, and it is hard to say without testing. In a class I attended a few years ago we used whatever glaze was lying around. Most of the glazes covered and fired fine, but a few pooled when fired (so there were large blobs of glaze with none in between). It actually gave an interesting effect, but might not be what you want. So if you have the ability to test it is very desireable!
There is one line of glaze that has been used a lot for this purpose, and it is the Duncan Concepts Underglazes. Yes, they actually are underglazes instead of glazes, so it sounds very strange to use them over glaze. But they have some frit which causes them to melt a little, like a glaze. And they are commonly used to paint on commercial ceramics. No clear glaze covering is required. (The Concepts have another feature that make them great to use. The colors come in Light, Bright or Dark. So if you use colors in the same family it is easier to make them match. You don't get a dull color next to a bright color.)
Slip trailing the glaze (squeezing it out of a bottle) also works great in this application. You can make line drawings and write words easily this way.
Finally, there is the option of using the acrylic paints. This option won't work for functional pieces because paint won't hold up to washing and silverware scraping, etc. But for decorative pieces that are going to sit on a shelf, it works pretty well. I recently did a test where I painted acrylic paint onto a very glossy glaze. The paint doesn't stick too well, the first coat streaks. So for solid coverage you need to let it dry then apply another coat. But I let my single coat dry for a couple weeks and found it was much tougher than I imagined. I can scrape it with my fingernail, but it takes effort. I don't believe that picking up up to look at it, dusting it, etc. would damage it. So that is another option if you don't have easy access to a kiln.