I completely ripped off this recycled crayon project from this site. However, I found that 350 degrees was too much for our bunch of broken crayons. Maybe my oven is off kilter? In any event, 250 degrees for 10 minutes was plenty. We made ours in muffin tins and the kids decided they look a little like mutant Reese's peanut butter cups. As such, you should probably monitor little ones carefully.
We first discovered these homemade rainbow crayons during a homeschool class at our local art museum last spring. The kids used these on black paper and the effect was incredible.
Go scrounge under the car seats and you will probably find enough broken crayon pieces to make your own batch. Or maybe that's just us...