Make sure you use tiles that are lead free and made only made from clay, some non-food intended tiles have bonus ingredients.
Can I ask where you got this information from, in relation to unglazed tiles?
Additional -Genuine question by the way, as it could be interpreted in other ways and I wish to be very clear.
I'm saying that terracotta is fine, but there are tiles that are made to emulate the look of it that may have a matte glaze.
@lynx started
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Make sure you use tiles that are lead free and made only made from clay, some non-food intended tiles have bonus ingredients.