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Stone Spiral

Clay Comparisons & Identifying Unmarked Clay

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Hey there,

 

I have several bags of clay that are not in their original boxes, and so I can't identify them as "M325" "H441G" etc etc. I can tell colour and consistency, obviously, but my eye is not good enough yet to know precisely what type of clay each one is. I compared to what I do have labelled and did my best educated guess, but there are a couple that I can't figure out.

 

Can I assume they are all okay to fire to Cone 6, and just not attempt any up to Cone 10?

 

What is your advice for using unknown clay bodies?

Also, a newbie question for you... is it okay to mix different kinds of clay bodies together if they are both able to fire at the same temperature? For example, I want to make a piece with medium fire grey clay and then put a relief on it with a different (still medium fire) red clay. In what instance would this NOT work, or what should I look out for?

 

Part 2 - What's the rule on mixed clays for reclaim? If my wheel slop pan has mixed types of clay together, can I reclaim that mixture (as long as it is all medium fire)? I realize mixing different colours of clays and slips could be used for effect, but could also be undesirable if done unintentionally/accidentally. I'm just wondering about how it would fire, if I chose to do it. For example, medium fire clays of different colours/textures mixed together into one ball.

 

I realized that was a lot of questions in one! haha. Thanks for taking the time to share your wisdom and experience... this community is such a valuable resource! You're all gems : )

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Unmarked bags ... A very common problem. (Not that I've ever done it myself ... Ha ha)

You should default to your lowest firing temperature unless you like to clean kiln shelves.

I would not mix clay bodies on functional wares but for decorative stuff I would try it.

When using two clay bodies on the same piece, try to match their shrinkage rates and their stage of wetness.

Reclaim of mixed clays ... Donate it to your yard.

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Default to the lowest firing temperature you use. Use the clay in a non-functional way. A cone 6 clay fired to cone 10 will bloat and blister but will not ruin a shelf. An earthenware clay fired to cone 10 may become a puddle. So... earthenware sculptures are the best use for unknown clay.

 

It is best to reclaim by firing temperature. Mixing same temperature bodies shouldn't cause problems. But I wouldn't use it for food/drinking vessels. I have never seen a problem caused by mixing clay bodies.

If some cone six clay ends up in the cone ten reclaim, it should be dilute enough to not be noticed. In general, I recommend the one way street of cone 10 -> cone 6 -> earthenware.

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I "inherited" a number of bags of random clay a few years ago.  I was mostly certain they were cone 6.  But not positive.  I agree with Min.  The time I would have put into identification would have been significant.  So, they returned to the earth. 

 

As for mixed clays....I started in a studio that put all the reclaim in the pugger.  So bmix, stoneware, red, white, heavy grog, porcelain all were mixed together.  At first it was good clay to practice with.  But then we all began to have issues with glazes, structural concerns, cracks.  I do not do that now, at my studio.  Even though I might have different clays, same cone,  I keep them separate.  just because. 

 

Roberta

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If I mixed a red cone 6 clay with a gray cone 6 I make sure the shrinkage is the same.  We mostly have Laguna clay in my area and they have that information on there web site.  If the info isn't available you can always to a shrinkage test.   Denice

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You didn't say if the bags were given to you, or if they were old stock of yours.  Since you seem to have no idea what they are, I would guess they weren't yours.  If you want to spend the time playing around with this, you could always fire small test pieces at the various cones making sure to place the pieces in catch vessels when high firing so you don't have shelves to clean up.  Seems like a lot of work, but if you're not pressed for time it could be interesting to play with.

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I agree with Mark. I once mistakenly fired some pitchers made with Cone 6 clay in a Cone 9 gas firing. Lost all the pitchers as they melted. Luckily, they were all fired on one shelf. Both clays of mine were white, so it was an easy mistake.

Cost me $60.00 to replace the shelf and keep the friendship.

Beware of unmarked materials. The gift bearer is not dong you any favours. They are cleaning out their studio.

TJR.

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For the unknown clay: are you 100% positive that there is no low fire in there? No way at all? If so, you could gamble and take it to cone six, but not cone 10. Plainsman goes from "mature" to "dunted" in a very narrow temperature range. If you really need it, you could make garden sculptures or planters or something, bisque them and seal it with some sort of concrete seal/marine varnish/other non-ceramic finish, and call it done. There really isn't any harm in turfing it for safety, though.

 

The relief question: make a small test and see. I can say that M370 makes a compatible slip over M390, as long as you keep he application under 1/4". I have noticed it tries to crack off if it's on heavier than that, though, and their shrinkage rates are pretty close to each other. Plansman has approximate wet-to-dry, dry-to-bisque and bisque-to-maturity shrinkage rates posted on their website, so you can judge more closely what will work before even making a purchase. Beyond that, you have to test for yourself.

 

If you're going to reclaim multiple types of clay together, you will see the best results from taking the whole mess down to slurry, mixing thoroughly and drying it out in whatever pillowcase/plaster slab/ Mark's pants/other method you have set up. You'll have a whole new clay body result from this process. As Roberta has noted, you get problems eventually if you're just putting it through the pug mill, because it isn't as thoroughly mixed. The glaze fit issue may still crop up, because the shrinkage rate of this new clay will likely change from the originals.

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Thank you all for your responses. The clay came with the studio I bought off of a retiring potter... so no, was not originally mine. Unfortunately she moved over 1000km away from me and it has been a challenge to receive clarity through phone calls and e-mail.

 

I think I will use this stuff as "practice clay," just to help get my throwing technique dialed in, but likely won't fire it after reading this advice. If I do fire it, I'll likely just do it as low (cone 4) sculptures. Otherwise, back to the earth. That makes the most sense now that I've read your posts - I can't be certain that there isn't low fire clay mixed in there. In fact, there probably is, as the studio also came with a raku kiln and she did fire that on occasion. I don't want to do preventable/avoidable damage to my electric kiln.

 

Thanks for all your advice ~ I will set up a process for reclaim that keeps the mixing of clays to a minimum. Since I am new to my own studio (one month in!) I feel like it's important to develop good practices/habits... for certain I will be labeling bags (and everything else)! haha

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Oh, it should be fine for raku firings too. As long as there is some coarseness it will survive.

 

And remember there is a Big difference between ∆4 and ∆04. The 0 is effectively a negative sign.

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Interesting that I'm having a similar problem, that I started working on 2weeks ago. I've decided to segregate my unknown clays into groups. I have about 4 types. There are earthen ware sand tempered, earthenware shell tempered, earthenware raku clay, cone 6 stoneware, cone 9 stoneware, and some unknown white clay. My plan is to mix the unknown clay into the sand tempered earthenware and make pit fired things with them, and throw the rest in the pug mill to make items.

 

As for you problem, I'd combine the throwing out in the yard advice with the make things for practice advice, and make flower pots to bisque only and set outside.

Use the clay to make sprig moulds and any other bisque tool you can think of.

Good luck,

Alabama

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