1. Formula. Formula ⑴: 100 parts of plaster of paris, 5 to 20 parts of metal oxides, 0.4 parts of polyvinyl alcohol, 0.2 parts of polypropylene glycol, and 80 to 150 parts of water. Formula (2): 100 parts of plaster of paris, 100 parts of calcium carbonate, 0.9 parts of polyvinyl alcohol, appropriate amounts of glycol and metal oxides, 150 to 220 parts of water. Recipe ⑶: 500 parts of plaster of paris, 2 parts of glycerin, 500 parts of water. Recipe ⑷: 250 parts of plaster of paris, 250 parts of light calcium carbonate, 2 parts of polyglycol, 500 parts of water. 2. Production. Step 1: Dissolve the additives in the formula (glycerin, glycol, polyvinyl alcohol, etc.) with water, heat appropriately if necessary, stir well and set aside. Step 2: Use a brush to apply the release agent inside the mold. The mold should be placed flat with the large holes facing upward. The release agent can be made by dissolving 3 parts stearic acid in 100 parts diesel or kerosene. Step 3: After mixing all the powders in the formula, add the solution prepared in step 1, stir it into a paste, and then pour it into the mold. Because plaster of paris sets quickly when exposed to water, you need to work quickly. The mold should be vibrated at the same time when pouring, and excess slurry should be scraped off after pouring. It can be removed from the mold in about 4 to 5 minutes. After demoulding, turn the mold over with the big hole facing downwards. Use a nailed wooden board to align the small head of the chalk. Tap a few times to release the formed chalk. Dry it in the sun or at 80°C.