Landscape in a Coffee Cup

This piece, created by Yukihiro Kaneuchi , is a little landscape in a coffee cup that appears over time from the stain of the coffee inside the mug.

(Image credit and original article from designmilk.com : http://bit.ly/i7NoSb)

I love this idea!  It's cool to make art that changes over time, reflecting how often it's used (and making art that encourages being used).  It reminds me of Japanese tea ceremony cups described in the book Thousand Cranes, that sometimes were stained on the rim from the lipstick of generations of women.

My only concern is how the stain is able to stick to the mug... the only way I can think this would be possible would be for the bare clay to be exposed, which unfortunately isn't very sanitary.  I would be interested to know how it works!