I have begun making custom coasters using 4"x4" travertine tile and stamping images on them using Stazon ink pads. One of my customers reported that the design on her coasters has begun to run. I do not know what caused this. They have not used any cleaners on them nor have they run them in the dishwasher.
I chose stazon because it was supposed to be water-fast. I do not seal the coasters so that they will be more absorbant nor do I bake them.
How do I prevent this from happening in the future?