I followed instructions very carefully.  It sounded reasonable to warm in oven before using and I made sure temperature was set as specified. (I've had this oven for 9 years and it tests accurately.)  When I took the inserts out of the oven 1 insert was slightly smaller than the other and the black material on top was all curled at the edges.  I should have returned them and now it is too late.  Not a big investment but certainly disappointing after reading all the glowing recommendations..<br /><br />Cora