a better homemade sidewalk chalk

I did an experiment making homemade sidewalk chalk using: 1 cup plaster of Paris, 3/4 cup water, & 2-3 T powdered tempera paint. This recipe is ALL over the internet. While it works, the chalk is super hard.Unless used on VERY smooth pavement, it doesn't write very clear, crisp lines. So, I decided to try adding things without compromising the integrity of the chalk.  Through many small batches of adding things like rice flour, glycerine, and cornstarch (in varying amounts), I came up with this recipe: 3 part plaster of paris to 1 part cornstarch   (3/4 c pop to 1/4 c cornstarch) and about 3/4 c to 1 cup water. I added just enough water so it mixed easily. I discovered using a wooden or plastic stirrer worked best, using metal, for whatever reason, I had clumping problems, even though the end result turned out fine. I also replaced the powdered tempera paint with some food coloring, and it seemed to work just fine. and since the kids aren't eating it(for those of you who avoid artificial food dyes), it was cheap and easy. so there we go, simple, inexpensive, slightly better than the typical recipe. oh, and it seems to dry faster. just thought i'd share