After posting that we had tested our homemade sidewalk chalk and it was "kid-approved," several people asked me how we had made it, so I promised a blog post about the process, and here it is!
You will need:
*Plaster of Paris
*a mixing bucket/bowl and spoon
*a measuring cup (you obviously can't use these for food afterwards, so pick up some cheap ones!)
*Toilet paper tubes or foil shapes to use as molds
*cooking spray (to use as mold release)
*I also recommend wearing gloves.
And also, some water. It's a good idea to have a container of water nearby as you're doing this so you can have it handy if you need to add more. And you will probably have to add some more.
Get your molds ready first. Close of one end of the cardboard tubes with some duct tape, and then spray the molds with the cooking spray.
Use your measuring cup to measure out 1 part water and 2 parts plaster. Put the water in the bowl first, and then add the plaster. I used 1 cup of water, 2 cups of plaster, and that worked out well.
Mix it up until it's smooth(ish.) Squirt some of the Tempera Paint into the mix. You can tell that I use very precise measurements on this. *grin* ADDNEDUM: If you can find powdered Tempera, instead of the liquid stuff, the chalk sets up much slower and gives you more time - I recommend using it!
Now you have to work fast, because the paint makes the plaster start thickening quickly. Mix the paint into the plaster; you may have to add some more water at this point to keep it workable. Scoop the mixture into your molds. I had to re-shape the foil star and heart a little bit. I should have shoved the plaster into the cardboard tubes a bit more firmly as they didn't get completely filled and the chalk was a bit misshapen but, eh, they still work as chalk.
Let the chalk set up for about thirty minutes. Pull (rip, tear, cut shred, y'know - whatever works) the molds off of your chalk, put it on a rack and let it dry for about 24 hours. Tah Dah! Now you can make a masterpiece on the sidewalk!