Copper diffusers throw out a lot of heat, and they do so quite unevenly.
Usually, the center could have a lot more heat than the outsides, and this type of uneven & high heat could cause your pot to crack.

As a result, we strongly recommend avoiding using a copper diffuser with your MEC pure-clay pots and pans. Diffusers made of cast iron, carbon steel, or aluminum should work fine.
If using on glass/ceramic cook tops, we recommend the Simmermat diffuser or the ILSA diffuser.