A two part slip cast mold made of plaster allows for the creation of more complex cast forms than are possible with a one piece mold.

Ideal objects for slip casting should be relatively simple and not have deep undercuts . More complex objects may need more than two mold parts or may need to be disassembled into multiple parts to be used with a two part plaster mold.

Process

1. Select your object.
2. Analyze the object and determine if there are any undercuts that are unavoidable. These can be filled prior to making to mold to remove the undercuts.
3. Draw a parting line on the object. The parting line represents where the two mold halves will separate. It should be tangent to the outermost features of the object.
4. Mold clay is used to build up a base around the object to be cast. This clay should not be mixed back in with clay to be used in kiln fired ceramics since plaster inclusions in the clay may cause the fired clay to explode.
5. The build up of mold should meet the half way point of the object, or the parting line. This is the infinitely thin plane that is perpendicular to the tangent point between the divide of undercuts.
6. Make sure that the parting line is level to the table or work surface. You can use a ruler to check completely around the form.
7. Continue to build up the mold clay around the form and up to the parting line.
8. Cottle boards are used to support the outer edges of the mold clay and to create a square boundary for the plaster to be poured into. They should be approximately 1.5" from the outermost part of the object on all sides.
9. Use C-clamps to attach the cottle boards to each other.
10. Construct a sprue out of mold clay for a place to pour the slip into the mold when making slip casts.
11. Use coils of mold clay to seal all the edges and the corners.
12. Apply mold soap to prevent the plaster from sticking to the cottle boards or the object to be cast.
13. Calculate the volume of the mold cavity. Mark a vertical line on the cottle boards 1.5" above the highest point of the object.
14. Mix the plaster according to the plaster mixing instructions .
15. Pour the plaster in gently and check for leaks.
16. Wait for the plaster to set up, then remove the cottle boards.
17. Reattach the cottle boards.
18. Rebuild the slip pouring sprue with mold clay.
19. Seal all seams with mold clay.
20. Apply mold soap to all surfaces.
21. Mix and pour more plaster.