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Terra Sig To Smooth Out Groggy Surface


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This is the novice question of the day (read: dumb) -- I understand that slip is made from the same clay as the pot, but I keep seeing recipes for terra sig, and that confuses me. Can you just use any terra sig recipe and slap it on any unfired clay pot? Is there not a problem with shrinkage differences? Is there some magic ingredient in terra sig that makes it "one size fits all"? I've been wondering this for awhile, but it has suddenly become relevant. I've just made some coil pots from a clay that turned out to be groggier than I realized. After burnishing the pots with a river rock as I normally do with the local hand-dug clay, it looks like the surfaces are not smooth enough to be simply bisqued and pit fired. There's just too much grog showing on the surface. Can I "save" these pots by making a batch of terra sig from a recipe out of a book and coat the pot with the terra sig, then -- what? burnish it with a river rock or rub it with a cloth? And while I'm dreaming up ways to save the pots, what would happen if I made a slip out of the hand-dug clay and painted it on the groggy pot, then bisqued and pit fired it?

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If the pots are still damp, keep burnishing. The only way I know to smooth a groggy surface is to push the grog into the body with a hard rib or as you are doing with the rock. If that hasn't worked, adding a wet slip could wash away the clay around the grog and make it worse.

If you can get the clay soft again, just keep pushing the grog into the surface.

Good luck.

Marcia

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About terra sig ... You can make it out of any clay body you have.

 

As mysterious as people like to make it sound, it is a very straightforward thing to do that just requires measurements and time. It is basically the finest particles of clay that are left when you add chemicals to settle out the big particles.

 

People tend to use dry clays like OM4 for white and Redart for red because the results are lovely but I have made it from other clays. So go ahead and make a batch ... Google terra sig recipes and go for it!

 

Then follow Marcia's advice and smooth in as much as you can ... but you can still use terra sig after.

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About terra sig ... You can make it out of any clay body you have.

 

As mysterious as people like to make it sound, it is a very straightforward thing to do that just requires measurements and time. It is basically the finest particles of clay that are left when you add chemicals to settle out the big particles.

 

People tend to use dry clays like OM4 for white and Redart for red because the results are lovely but I have made it from other clays. So go ahead and make a batch ... Google terra sig recipes and go for it!

 

Then follow Marcia's advice and smooth in as much as you can ... but you can still use terra sig after.

 

 

 

So let me get this straight: ANY clay that is not groggy can be used to make terra sigillata? The recipe given in Lark's "Alternative Kilns and Firing Techniques" reads "1.5 cups ball clay, 1.5 cups EPK clay, 10 cups water, 2 tbl sodium silicate". Why those TWO clays? Do they each bring something to the equation that is necessary? If so, does that mean that I couldn't mix 3 cups of a smooth clay into 10 cups water and 2 tbl sodium silicate and get an equally useful terra sig? I'm asking that in particular because I'm using a hand dug clay and I don't want the color of that clay lost to another clay's color by mixing up "ball clay" and "EPK" and coating the work...

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If II were you, I would roll out and dry a sheet of your clay. You want to end up with more weight than the sig recipe calls for since you will not be using the grog. Then I would pulverize the dry clay and pass it through a fine sieve to get rid of the chunks. Then I would weigh and measure the clay powder and make the sig from that.

 

Remember, terra sig is just a suspension of the finest clay particles. You might get a quart or you might get less but it will work.

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Lots of good advice, wish I'd known before. I don't even know if what I use could be categorized as terra sig, but that's what I call it. I take the slop from throwing my regular clay body (most of the groggier stuff is in the pots) and collect it in two one-gallon buckets. Add water if necessary, and set them aside. After the water has separated on top, I pour or scoop the extra clear liquid off. I then pour the next section of slop (very fine and semi-fine particles) into another container, add water and leet sit again. The gritty stuff at the bottom goes on a plaster bat to dry and then into the garbage. Gradually the two buckets become one as the amount of slop gets smaller and so do the particles. I keep repeating this process until I end up with about two quarts of stuff--no grit--that I call terra sig (sounds so much better than slop).

 

I use this on pots where I want a smoother finish (for underglazing) and always on the bottom of pots so there's no errant grit to scratch furniture. I know I should probably be adding sodium silicate or have a hydrometer to make sure there is balance between water and clay, but this has worked for me, and I'm of the school if it ain't broke, don't try to fix it. Whatever it should be named--slip, terra sig--it does what I want it to.

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Yea, I suspect this is exactly how terra sig was invented!

Then someone made it expensive and complicated.

 

 

Thanks for the practical "how to" advice on making terra sig from my clay. Now, because I've gotten curious about this stuff, I really want to know:

Why two clays in the recipe?

What does the sodium silicate do?

How is it that you can make terra sig from a clay totally different from the one you're using for your pot, and there is no compatibility problem?

Does terra sig sometimes crack off in the firing, and why?

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Lots of good advice, wish I'd known before. I don't even know if what I use could be categorized as terra sig, but that's what I call it. I take the slop from throwing my regular clay body (most of the groggier stuff is in the pots) and collect it in two one-gallon buckets. Add water if necessary, and set them aside. After the water has separated on top, I pour or scoop the extra clear liquid off. I then pour the next section of slop (very fine and semi-fine particles) into another container, add water and leet sit again. The gritty stuff at the bottom goes on a plaster bat to dry and then into the garbage. Gradually the two buckets become one as the amount of slop gets smaller and so do the particles. I keep repeating this process until I end up with about two quarts of stuff--no grit--that I call terra sig (sounds so much better than slop).

 

I use this on pots where I want a smoother finish (for underglazing) and always on the bottom of pots so there's no errant grit to scratch furniture. I know I should probably be adding sodium silicate or have a hydrometer to make sure there is balance between water and clay, but this has worked for me, and I'm of the school if it ain't broke, don't try to fix it. Whatever it should be named--slip, terra sig--it does what I want it to.

 

 

I love this post! I do the same as you, Idaho Potter. Depending on the desired effect I may use different clay bodies (i.e. different colored slop from various buckets...) As long as I allow the process enough time, I've found that even clay bodies with different grog contents all mesh together into a concoction that smooths all of my pieces without fail.

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Yea, I suspect this is exactly how terra sig was invented!

Then someone made it expensive and complicated.

 

 

Thanks for the practical "how to" advice on making terra sig from my clay. Now, because I've gotten curious about this stuff, I really want to know:

Why two clays in the recipe?

What does the sodium silicate do?

How is it that you can make terra sig from a clay totally different from the one you're using for your pot, and there is no compatibility problem?

Does terra sig sometimes crack off in the firing, and why?

 

 

 

Coincidentally, I was thumbing through an old issue of Ceramics Monthly (September 2000) and found most of my questions answered in the "Questions" section. Thanks for the information, everyone.

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Yea, I suspect this is exactly how terra sig was invented!

Then someone made it expensive and complicated.

 

Thanks for the practical "how to" advice on making terra sig from my clay. Now, because I've gotten curious about this stuff, I really want to know:

Why two clays in the recipe?

What does the sodium silicate do?

How is it that you can make terra sig from a clay totally different from the one you're using for your pot, and there is no compatibility problem?

Does terra sig sometimes crack off in the firing, and why?

 

 

Coincidentally, I was thumbing through an old issue of Ceramics Monthly (September 2000) and found most of my questions answered in the "Questions" section. Thanks for the information, everyone.

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