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Hello! I have run into a problem making casting slip. I have been using a tried and true recipe from an instructor several years ago for a casting slip that calls for grolleg kaolin. I mixed up a new batch and did not realize that when I said grolleg, the guy at the sales counter heard EPK. I thought when I saw the bag that perhaps it was from a different supplier so added it. Realizing that it was not at all the same in color and substance, I did some quick research and saw that it is essentially the same but will shrink a little more than the grolleg. Okay, cool. I can deal with that as I am experimenting with slip casting anyway, so I added it as I was in the middle of it all and did not want to put it all away again.  But then when I got home I looked in the Andrew Martin book he said it will not work at all because it holds on to the water and will cause the casting to have a thin wall and be brittle. 

So now: do I toss my batch and spend another $100 on supplies or would you attempt some sort of save? I did cast one small plate as a test and I didn't notice any extra brittleness, though it did take longer to cast. It will be going through bisque fire in the next day or so, which might help me decide. The only other thing that I noticed is that it immediately starts to settle and turn into oobleck on the bottom of the bucket. The specific gravity is 1.6, which is a lot lower than normal. I was trying to decide whether to let it evaporate for a while and then add some Darvan or SS (though I made it with a bit of soda ash which I have heard doesn't play well with Darvan). Or maybe I just start over and learn to look more carefully at what they are loading in my car.

Thanks in advance for any advice or knowledge you can impart.


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You can see the differeance at Digital fire.com-somewaht minor

I just looked them up

You will notice minor changes but if your test pieces work I would jusat use the material-they are both fine kaolins

The big story is ceramics is to pay attention-it happens to us all sooner or later. I just toosed out 1/3 of a kiln load with a glaze mistake-the pots and the glaze and started over.

In ceramics all the details matter-so always double check everything

I would avoid the soda ash as it can damage the plaster over time.You could also make another batch with the grolleg and dilute your other batch.I doubt you will notice much difference with the two batches.

Mixing  up two kinds of kaolins is no big deal really

How many gallons is your batch size??

work off a materials list next time you go to the store-hand them a list with it written out.

Instead of removing water add  a little more material to batch  to thicken

Getting the specific gravity right and using dry molds is the most important part.

Darvan is your best choice over soda ash 

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Thanks, Mark. I will continue to fiddle with it - I just got worried when A. Martin said it wouldn't work at all. I might do as you suggest and mix another batch with grolleg and then combine a bit of each to see what that looks like. I mixed 14 gallons because I dislike doing it and so just try to do as much as I can at once.  It's always a learning experience, which is one of the reasons I keep doing it!

The only real difficulty is the settling - it is almost instantaneous and it seems like the bottom of the cast piece is always heavier than the higher sides. Would adding Darvan help with the suspension?

I appreciate your thoughtful response.

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Since it is already mixed up would you advise diluting with water (or perhaps even a bit of the slip mixture) and adding it that way? I will continue to try to digest the super-dense digital fire article (thanks for posting) and do some more research on Darvan. So many variables to keep in mind. 


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 I have also read that in Andrew Martin's book and I have had great experience using Grolleg in casting slip. Another thing to consider is that Grolleg has some potassium (flux) and EPK does not. EPK will require more added flux to reach the same level of vitrification for an equal amount of Grolleg. 

Not sure if I read this on digitalfire or Martin's book but this way to mix casting slip works well for me: mix your slip, let it sit overnight, measure specific gravity, add water to reach target specific gravity, then add deflocculant to adjust the fluidity of the slip.


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I love the grolleg slip I have been using. This is kind of an interesting problem that is expanding my knowledge so I'm kind of enjoying it even as I am struggling with it. The slip is about 1 1/2 weeks old now. I think today I will measure the specific gravity again and perhaps add some grolleg to bring it up if it is low (two days after mixing it was 1.6) and then maybe add some Darvan if needed.  Thanks, tinbucket!

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