I put the plate in for about two minutes. I thought it would be enough, and the crayons did melt, but they didn't get that vibrant color that I have seen in other pictures. Next time I try it, I definitely will leave it in for longer.
However, it didn't really matter to the my little boy. He had a lot of fun.
And it did wash right off with soap and water. My plate looks as good as new!