Just like old records, Sunlight or heat. Some years ago a collector had his chips in the trunk in a case in Las Vegas and they warped. Don't ask me how I know this one... Dishwasher.

Microwave between two perfectly flat objects or as mentioned, two pieces of glass. VERY CAREFULLY for seconds at a time, until you can see that it has adjusted itself. Let cool or it may just warp again.

If I had a heat gun, it's probably a better method.

Someone who's got an accurate oven thermometer might run some tests some day.