I attempted to make sidewalk chalk from eggshells, following the advice linked to from the Eggs and Sustainability page here on CocktailGreen. 

The basic procedure is:

1. Collect eggshells, boil them to clean them.
2. Mix with flour, water, and food coloring.
3. Put into a tube and allow to dry. 

I did the above, and spoiler: it turned out bad. In the first place, I buy mostly brown eggs (to encourage biodiversity) and thus the eggshells started with a beige base rather than white. 

The second and far worse thing was: they smelled awful! They took a long time to dry (I left them for more than two weeks on top of my baseboard heater) and were stinky after they started to get dry. I burned a scented candle every night.

Even when they were all the way dry they were stinky, very very stinky. Unforgivably stinky. 

In the end I unwrapped them to see the finished product (results: not great!) but I didn't want to handle them any longer than necessary. So much for that.