I have some old glazed ceramic plates that I won't use any more. One of them developed a crack half way through and I noticed that, in the microwave, food on it

I have some old glazed ceramic plates that I won't use any more. One of them developed a crack half way through and I noticed that, in the microwave, food on it would stay frozen but the plate would be blazing hot. The glaze was conductive with presumably lead and was absorbing all the energy. The crack created a slot that blocked eddy currents.