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GiselleNo5

Question: Mixing Porcelain And Stoneware For Slip Casting

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I have been trying to find an answer to this online with no result.

 

If I were to mix two kinds of premixed casting slip with different shrinkage rates, what would happen? Would the shrinkage average out or would a mushroom cloud obliterate my dad's kiln shed? ;)

 

We want to try mixing Laguna Oriental Pearl (shrinkage 14%) with Lagina White Stoneware (shrinkage 10%). I know that stoneware/porcelain blends exist as a clay form, but I'm not sure how it works with slip.

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Thank you both for the answers! :) 
 

It will be a new shrinkage rate which to be know would have to be tested. Its just like wedging two clays together for anew better mix.

Mark

 

I realize that I made it sound like the question was about shrinkage, and it isn't, although of course that would be interesting to see the difference. I'm not as concerned with what the shrinkage rate would be, as with whether the two kinds of slip would even be compatible. I've been wondering about this for awhile: For example, if I wedged together a cone 05 and a cone 5 clay  (or a 10% and a 14% clay) what would happen? Would it need to be fired somewhere in the middle? Or a Cone 5 and a Cone 10 clay? Or should you stick to mixing only clay that is the same heat range/shrinkage? (These two slips I'm talking about are both cone 5/6). 

 

Sorry for the confusion.

 

I would mix small batch. Make small 2 plates and 2 cups. Put slip on cups and plates. Glaze one cup and one plate. Don't glaze the other cup and plate. Look at results closely. 

The slip I have isn't topical slip, it's special casting slip. This is a really liquid slip that you pour into a plaster mold; it then hardens to make a shell and when it's thick enough you pour off the excess then remove the item from the mold.  So the whole item is made from hardened slip. It has ingredients in it that make it set up really quickly and nicely specifically for molding (not sure what they are because I haven't made my own casting slip yet, I've been using premixed). I have an album of some of my slip cast dishes made from a set of vintage molds, and if you're curious about it I believe I have work-in-progress photos too. I've been pouring some molds yesterday and today and I have to say I vastly prefer wheel throwing. :) 

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Hahaha! Yes! You pour the porcelain into the stoneware ... suddenly ... a puff of smoke, a clap of thunder, and a portal into another dimension opens, releasing dread creatures with glowing eyes to wreak havoc on your studio and leave nothing but broken pottery and mangled equipment in their wake.

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Hahaha! Yes! You pour the porcelain into the stoneware ... suddenly ... a puff of smoke, a clap of thunder, and a portal into another dimension opens, releasing dread creatures with glowing eyes to wreak havoc on your studio and leave nothing but broken pottery and mangled equipment in their wake.

 

Oh, you mean that doesn't happen for everyone else, only me ?

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Joseph: that's okay! :) I love using contrasting slips too. I've been experimenting with sgrafitto and mishima using different kinds of clay and I love the results. I also will use white slip on portions so the color will show true on those parts. Do you stain your clay or use a naturally black clay? I'm looking for a good black clay, the darkest I have is dark brown and it's really brittle, prone to cracking.

 

Chilly: Oh, it happens to everyone at one time or another.

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The little cup I posted in my "pre heat help" post on the Studio forum is Cassius Basaltic-Aardvark Clay cone 6 from  Seattle Pottery Supply "Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5. Very black, ebony like finish. Needs lots of oxygen during bisque firing." I used a heavy clear glaze on the interior, so it looks dark brown (but darker in real life than appears in the photo) and the exterior is a deep true black-gorgeous!  

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I used black raven for a while, I gave up on it. It is a mess to clean up in the studio, and it was very difficult to get rid of the bloating. I finally got it manageable, but every now and then I would have bloating on a nice piece, and in the end It was a lot of work and I like throwing porcelain better. They sell the black raven at Stone Mountain Clay if your interested.

 

If I go back to black clay again i will try the Cassius Basaltic-Aardvark Clay.

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I'm wondering, perhaps I can use slip made from my dark brown over my white stoneware to get the look without the brittleness, less cleanup too. I use Hawaiian Red sometimes which is lovely to use, practically throws itself ... but I have to be in the mood for a LOT of cleaning, and in a position to have stained hands and fingernails for a week.

 

I've never used Aardvark but I've heard it's excellent clay.

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Cassius has a tendency to bloat when fired over cone 5. Outgassing can also be hard on the elements.

 

A friend slaked Cassius down to a slip and applied it on stoneware and fired it to Cone 10 (natural gas/reduction) with no issues. Going with a dark clay as a slip is a good alternative; plus you get more choices on colorful line glazes on a white interior than with black clay.

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That's what I did with the piece in my profile picture. Speckled Buff clay, white slip on the interior and flowers so the colors would show nicely while still able to contrast with the natural clay. Some tiny speckles showed through the slip and glaze but I like it anyway.

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If you mix two clay bodies from the same firing range, then they will be fine. Many clay manufacturers sell half-and-half bodies, a porcelain/stoneware blend. They are generally very nice. Start mixing ranges and you'll have to do some testing. As for mixing slips, the same rules would apply, however I'm thinking that the viscosity of the slip could possibly be affected when they are mixed together. Just a guess. It's worth a try.

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The little cup I posted in my "pre heat help" post on the Studio forum is Cassius Basaltic-Aardvark Clay cone 6 from  Seattle Pottery Supply "Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5. Very black, ebony like finish. Needs lots of oxygen during bisque firing." I used a heavy clear glaze on the interior, so it looks dark brown (but darker in real life than appears in the photo) and the exterior is a deep true black-gorgeous!

 

I wondered if they still called it Cassius! It's a lovely black, even unglazed, though I don't usually work with ^6, I made an exception for this. I didn't try throwing (couldn't face the cleanup or having separate everything) but I made some jewelry. I was delighted to find that it has a nice milk chocolate color at ^06, so I made some fake-chocolate pins and earrings, too.

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Rae: I throw with multiple colors of clay regularly. What I do is, I have little lidded buckets for throwing water and scraps, one for each color. It's not bad! If it gets mixed in the splash pan I just throw it out.

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