Fish can be cast with plaster of Paris.
You can make a replica of your prize fish by using plaster of Paris to capture its details. With a clay mold, the image of your fish will be incised for plaster casting. Plaster of Paris has been used for thousands of years. When water is added it undergoes a chemical reaction and becomes hard and capable of picking up details from a mold. Fiberglass and polyester resins may also be cast into a clay fish mold.
Instructions
Making a Mold
1. Roll the clay into a 3-inch slab with a rolling pin; make it long and wide enough to accommodate your fish shape plus two inches of border all the way around the fish. Roll the clay on top of an old sheet; this will help get the clay off the table easily as you can simply peel the fabric off the clay.
2. Place the slab onto the plywood. The wood will hold the clay securely and keep it from bending.
3. Roll over the top of the clay surface to make a smooth top.
Forming the Fish Shape
4. Press the fish firmly into the clay.
5. Push the fish down into the clay until half of it is embedded in the clay, lengthwise. Pay close attention to the fins and make certain they have enough pressure to leave an impression in the clay.
6. Remove the fish carefully from the clay. You will now see the transferred image of the fish in the clay, and you have a working mold.
Casting
7. Mix the plaster according to the manufacturer's instructions. Always add the plaster to the water. Sprinkle the plaster evenly and slowly into the water until a small conical island appears in the center of the water.
8. Allow the plaster to sit for five minutes.
9. Mix slowly with a paint stirrer until the mixture is the consistency of heavy cream.
10. Pour the plaster into the mold slowly until the fish indentation is completely filled.
11. Tap the mold gently to release any possible air bubbles.
12. Allow the plaster to dry overnight. It will feel hot soon after pouring as a chemical reaction is occurring. It will seem dry to the touch in 30 minutes.
13. Remove the plaster fish replica from the mold.
Fiberglass Casting
14. Create the clay mold as in Sections 1 and 2.
15. Layer polyester resin with fiberglass to fill the fish form.
16. Allow to dry overnight.
17.Lift and remove carefully from the mold. Be especially careful around the fins.
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