First I have students choose a selection of natural objects, we may go on a walk to find these pieces. Shells, leaves, twigs etc. all work great for this project. I also let them choose from some plastic toy bugs, frogs or dinosaurs I have in the studio. All of these pieces are pushed into a slab of clay so that they leave a texture and fairly deep indent in the clay. This is a great opportunity to discuss negative and positive space in art.
Next students build a wall around the sides of their slab, attaching it on so that there are no holes or weak spots on the edges where plaster may leak out. Then I mix up the plaster, I don't generally measure the plaster to water ratio, but try to get a consistancy similar to pudding. If I have a large class, the plaster is mixed in batches (maybe 5 students at a time). I then pour the plaster into their mold filling it so that the plaster is about 2 inches deep. Students are asked to gently tap the sides of their mold and the top of the plaster to wiggle out any air bubbles that may have gotten trapped. Once the plaster has started to hold its shape but it still soft a wire loop can be placed in so that the fossil can be hung on the wall when finished. The plaster takes about 20 minutes to set but 24 hours to cure completely. Once the plaster has set, the clay can gently be removed from the sides and the "rock" of plaster can be flipped over to remove the clay from the bottom. This will reveal the positive shapes of all the nature items that were pushed into the clay. Write students names on their plaster and let cure until the following day.
Once the plaster is cured you can paint it will any type of paint, I like to use watercolors for this project. This is a great opportunity to pull out your handy-dandy color wheel and discuss warm and cool colors. I usually let students choose which they'd like to use and then challange them to mix up 10 colors that are either warm or cool to use on their project. Once painted, the plaster will dry rather quickly as it sucks the water out of the paint and then I like to use a bees wax furniture polish to seal the surface. It also gives the surface a semi-gloss sheen to it. If you've included a wire hanging loop on the back of your plaster, you fossil can be hung up for display!