Making fossils

To make fossils my partner and I used three different molds (10oz). Each mold had the same amount of water (4oz). The remaining 8 oz were plaster and sand. Mix one contained 75% sand and 25% plaster. This mixture was very easy to break apart and the impression from the shell wasn't very visible. The 2nd mold was 75% plaster and 25% sand. This mixture had the best fossil but was very hard to break apart. The best mixture to use in the classroom would be the 50% plaster and 50% sand.