The back side of the coaster has flocking on it which is made out of nylon threads.
The color does not transfer of the flocking or the pottery. There could be flocking dust on the top of anything that these coasters are put on, which wears off after use of 2-3 days. It is not harmful.
Please note that the coasters need to be cleant with a wet cloth from the surface.
If you have any further questions please do feel free to revert.
