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How to Make Porcelain Casting Slip: Article or Project

E.P.K. (KAOLIN) 25%
TENN. #10 (BALL CLAY) 15%
SILICA (200 MESH) 20%
100% (total dry ingredients)
Distilled water 35% (dry ingredients)
Darvan #811 00.36% (dry ingredients)

More Details

How to mix a 10 lb. batch (total dry ingredients):
(A 10 lb. batch will yield about one gallon of slip)

1. Weigh distilled water to equal 35% of the dry weight of the clay (3.5 lbs)

2. Weigh .36 Darvan #811 (in this case 16.3 grams) Add to the water and mix.

3. (1 lb = 454 grams so----10 lbs. = 4540 grams so-----.36% of 4540 = 16.34 grams or 16.3 g)

4. Add the clays to the water one at a time and power mix in between additions. Try not to whip air into the mixture.

5. Add the non-clays and power mix between additions.

6. Chances are that the slip is too thick, which means more deflocculant/water is needed. You may want to check the specific gravity of the slip with a hydrometer. Good casting slip falls within the range of 1.65 (thin) to 1.8 (thick). Have a 50/50 solution of Darvan #811 and water on hand and add the mixture a drop or two at a time into the slip. Mix between additions and check viscosity levels and specific gravity. Add solution until the slip loosens up to your liking. Remember, adding too much deflocculant will actually flocculate (thicken) the slip.

7. At this point the slip may seem loose enough but no doubt there will be some apparent lumps in the mix (which equates to bits of clay not thoroughly wet). Leave the slip alone overnight so all the clay particles can “get wet” and mix again in the morning. Add more 50/50 solution if necessary. At this point some individuals put the slip through a 30M screen. I don’t.

Please remember that the way we are using the casting slip is regarded as non-traditional. Our slip formula has an extra amount of clay components to yield more plasticity for manipulating slabs. Higher amounts of deflocculant usually are needed (up to .70%). Specific gravity may be as high as 1.8.


You may want to experiment with colored casting clip by adding mason stains or mineral oxides and carbonates. The first thing you need to figure out is the actual clay to water ratio for the slip you are using. Refer to our clay formula and think in terms of grams. To a 100 gram volume of dry clay we are adding 35 grams of water, resulting in 135 grams of slip. Divide each of the two components by 135 to realize the true percentage amounts:

Water: 35 grams = 26%

Clay: 100 grams = 74%

Now that you know the true percentage ratio of your casting slip it will be easy to figure the actual amount of clay that is present in any volume of wet casting slip. Let’s say that you filled a 3 gram Styrofoam cup with slip, weighed it and it came up with 267 grams. Deduct 3 grams to account for the cup and proceed with the math:

264 x 74% = 195 grams of clay present.

Now it is easy to accurately add any percentage of colorant and be able to repeat the results the next time you need the same colored slip. To add 4% mazerine blue mason stain to the above example, do this:

195 (grams of clay) x 4% (stain) = 7.8 grams of stain to add to the slip

Just dump the stain in the cup and make sure you add a few drops of water to wet the stain addition. Then mix thoroughly. If you don’t wet the stain you will end up with lumps, which then have to be completely dispersed before you can use the slip.


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How to Make Porcelain Casting Slip

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