I use StazOn and Fabrico Markers on tumbled tiles. No need to ever heat set and the coasters remain totally absorbent. I happen to have two sets of markers for sale, if you're just getting into these. PM me if you'd be interested.
If you use craft, dye inks or chalks, you must seal the tiles with a clear acrylic to keep the colors from coming off.
Hope this helps you!
Diane
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