10 Slip Casting Problems and How to Solve Them Troubleshooting Plaster Mold and Slip Casting Problems Jonathan Kaplan

PS. For a full slip-casting how-to, pick up a copy of The Mold-Making Manual in the CAN Shop!

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Problems with plaster ceramic casting molds and casting slip can be interesting for sure, but they can usually be rectified. Here are a few of the major slip casting problems and suggestions for correcting them:

1. My mold leaks when I pour slip into it. Your mold parts don't match up correctly and/or your parting line has some serious issues. Your natches/keys could be prohibiting correct mold part registration. It could also be issues with the model itself, so I would advise looking at the model from another point of view to see if it needs to be fixed. Another possibility is that the mold parts have warped. This also happens when the mold parts are not strapped together when drying. 

2. The casting surface has a very rough or textured surface, and the cast piece's surface mimics this. Chances are that your plaster was mixed incorrectly. Make sure that your soak time is correct and that all lumps are broken up as you mix. Try pouring the plaster part through a handheld sieve to catch any unmixed plaster lumps and use a pour spout. Check your parting compound application.

3. There is too much clay to fettle off the cast greenware. Your parting line is not tight enough or your mold parts do not register perfectly. Revisit your technique of finding the parting line and making the plane of separation. Check your pouring techniques when you pour a second, third, or fourth mold part.

4. I can't move my mold. It is way too heavy. The walls of the mold are too thick. The weight can be reduced by making the clay walls proportional to the shape. You can remove extra weight by cutting the corners of the mold at 45° angles. Alternatively, an excellent solution is to make a bottom draining mold.

6. My plaster slip casting mold is soft and kind of crumbles in places. This is a plaster issue—incorrect plaster to water ratio, old plaster, plaster has become damp or wet in the bag, insufficient soaking time, or improper mixing technique.

7. The cast greenware does not want to release at certain places in the mold. You may have poured the plaster to make that mold part so that its stream hits just one part of whatever it is being poured onto forming a hot spot. It could also be that if it is in a corner, there is a hang up due to draft issues. To help your cast clay object release from these troublesome areas, fill an old sock with corn starch and knot the open end. In the locations where there are release issues, pounce that sock so that some corn starch appears on the mold surface. You don't need to cover that area with a large quantity of material. Just lightly pounce so that you can barely see the powder, and that should help the cast release in that area. Any corn starch left on the greenware simply burns off in the bisque firing. It could also be an undercut, a return, or other mold design issues. 

8. Cast object retains water after excess slip is poured from the mold. Casting rate is too long. This is a casting slip issue and is due to incorrect specific gravity or viscosity. It can also be due to too much ball clay in the mix.

9. Some parts of my mold do not come apart. This is most likely a parting compound/mold soap issue. Not enough has been applied to the mold surfaces, or, whoops, you just spaced on applying it before you poured the next mold part onto it. It's been known to happen. Always ask yourself, "Did I put parting compound on this mold part?"