I was so excited to see this on the Martha Stewart show, especially since I have a whole carton of plaster paris sitting around waiting to be used. It very easy all you need is:
plaster of paris
molds (I used water bottle ice cube trays from Ikea)
Silicon molds release really well, but if your ice cube tray is well used and looks like mine its not a very good shape so heres what I did:
I cut 1/2 inch strip of cardboard and stuffed it in between on the backside
not perfect… but much better
mix 2 parts plaster with one part water in a plastic cup. I like to add water a little bit at a time until I get the right consistency. I should be like thick pancake batter, like the original bisquick recipe before you go ahead and add more milk than it calls for.
If you want white chalk you can go ahead and pour now.
if you want colored chalk add enough pain to get the desired color. Blues, greens, and purples will dry slightly lighter, whereas yellows and oranges are true to color.
fill molds and allow to dry 1 hour to overnight. If you are using silicon molds you can leave them in as long as you like, other molds have a hard time releasing after the plaster is dried. The more paint you use the longer it will take to dry. My white dried in 30 minutes, my blue green and red in 1 hour, my yellow and purple took overnight. Carefully unmold when dry, they should be cold and still damp.
due to shrinkage while drying, the edges will be probably be slightly raised and sharp. break off as much as you can with your fingers then take and knife and carve the edges flat.
Set them out to “cure” for 2-3 days so they will dry out and become chalky. This is when the lightening occurs as all of the water is evaporating out of the plaster. Then they are ready to use.