I used, at the suggestion of another chipper, my wife's hot air hair drier.

I put the chip on an absolutely flat surface, a ceramic dish, I used another, perfectly flat chip to be certain the dish was flat. I HOT AIRED the chip, the clay became softer and I gently pressed on the chip with a large, flat, stainless steel utensil.

I turned the chip over several times as I did this and I must say the WARPS were literally reduced to 0. You could actually tell as you were pressing that the clay was becoming soft enough to UNWARP! I did NOT go beyond that point for fear of distorting the chip's dimensions in any way!