because the classic ink is water soluable and would therefore come off the tumbled tile - you need a permanent type ink in order to do tiles. At least black Staz-on - you can color in using chalk, pencils (have to seal both) or sharpies or some other type of permanent marker...
However if you look at this post
Forums at Splitcoaststampers
this person did use classic ink on tumbled tiles and really sealed it...
Make sure you get tumbled tiles and not ceramic tiles