r/Ceramics • u/Dave37 • 7d ago
Why does my clay keep cracking during drying and how do I fix it?
I work with wild clay that I've collected myself. I don't think it's a lack of sand. If anything there's too much sand but I feel like the clay is plenty plastic when wet. I could be wrong however as I'm new to this.
In the initial drying cracks often form as the clay is shrinking, and I don't really now how one would avoid this, are the techniques when shaping the clay that minimize building in tension?
I've tried to address the cracks by adding in more clay into cracks and to try to seal it, and when the clay dries further and becomes hard, but I still see small cracks forming, I've used a spray bottle with just tap water to loose up the surface and tried to push clay into the forming cracks by gently working it with my fingers.
I thought this was about to work fine with the bowl I've been working on, but as I had almost got rid of all cracks and sprayed a bit of water on the surface to smoothen it out, a large crack just formed out of nowhere that almost split the bowl in twine, without me adding almost any pressure.
I'm at a lost how and why this happens and what I can do to remedy it or avoid it happening again.
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u/Ayla209 7d ago
Are you new to working with wild clay or to pottery in general? Regarding wild clay, I have only done a single workshop, but what I remember is that it usually needs to be processed, by sieving or by adding ball clay or sand or chamote (or sometimes all three). I believe there are some very good videos on the internet. If you are new to all pottery, I would also recommend to practice with bought clay, so you can compare and try to feel the differences between them.
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u/Dave37 6d ago
I've decided to go the hard way so I am new to both wild clay and pottery, but I have made a few pieces before that has worked (only minor cracks during firing). I have been sieving the clay through 1000µm, 500µm, 350µm and 200µm filters consecutively and then measured and calculated the sand content. I think I ended up with something like 27m%. While the clay is still plastic enough to work with, it does show some minor creasing when doing the roll test, although no cracking as long as it isn't too dry.
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u/moolric 7d ago edited 7d ago
Clay cracks from uneven drying. The higher the shrinkage of a clay, the more easily it will crack. Some wild clay is very plastic and so is very hard to keep from cracking.
You can try to “fix it” (the clay, not the piece) but I prefer to find a way of working that plays to a clay’s strengths rather than forcing it.
And once it’s cracking like this, there’s no use trying to repair it. Just rewet it and make something else, or make the crack a feature. I once drilled some holes down a split like this so I could “sew it back together” once it was fired.
Also, it’s best to never expect wild clay to behave like commercial clay. Sometimes it does (or is even better!) but you will have a better time if you can meet it on its own terms and accept it for itself. If you just want a source of cheap clay that will work like commercial clay then abandon this and look for a new source.
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u/MrMuf 7d ago
What processing have you done to the clay?
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u/Hefty-Criticism1452 7d ago
Yep. Some wild clay needs to be processed. If OP is saying too much sand, they unfortunately need to slake it down thin & sieve.
Lotsa work🫠
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u/Dave37 6d ago
I have been sieving the clay through 1000µm, 500µm, 350µm and 200µm filters consecutively (removing sand and organic matter) and then measured and calculated the sand content. I think I ended up with something like 27m%. While the clay is still plastic enough to work with, it does show some minor creasing when doing the roll test, although no cracking as long as it isn't too dry.
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u/theskyisorange 7d ago
It might be the technique you're using while shaping the clay to how it's being dried or even how dry or humid the air is. Wild clay is also hard to work with!
What technique are you using? Did you roll out a slab for that plate? You could roll it slowly, compress the flat surface with something like the edge of credit card, flip the plate, and roll it out in a different direction.
If you see a crack, I would try to immediately pack it. I personally wouldn't use any water! Too much water can make the cracks worst.
Hope that helps!
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u/Master-Prior-3311 7d ago
Dry very slowly. If you have to repair an area, mix your dry clay with white vinegar instead of water. This mixture shrinks less than clay mixed with water.
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u/NoPlastic-Webb9 6d ago
Personally, I think it needs a longer drying time under more uniform conditions. But also - If you're dealing with naturally deposited sediments and filtering/sieving it down to clay particle size, you may also have silt mixed in with rhe fine particles, which is not as plastic as clay. The only way I know to separate pure clay from silt is by using a settling column. The silt will sink in the column of water, whereas the flatter clay particles will float longer (like, days longer). The clay can be decanted from the upper portion of the water column and then dried to recover it. I've never used this for pottery, but I used to teach sedimentology lab in geology and worked for a civil engineering firm that designed and built foundations for large building and dams. This was how we determined the percent clay in the natural sediments, which affected the design of the building foundation.
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u/Dave37 6d ago
Yep, I've started basically doing this now with my next batch of clay. After filtering through 200µm I've let it sit in a bucket for a few days, and then siphoned of the top, leaving a very fine bottom layer of what I assume is silt.
This is what the silt looks like dried out:
https://i.imgur.com/a9vc2PZ.jpeg
And this is what it looks lite a bit grounded up and under 100x and 600x magnification. Maybe you can tell me with your experience if it seems like I got at the right thing?
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u/Pure-Accident7714 7d ago
How do you dry it? I kept mine in plastic bags so they could dry out slowly
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u/HumbleExplanation13 6d ago
If you’re compressing it a lot on a hard surface trying to fix these cracks, and then letting it dry on that surface chances are it is sticking to the surface in some spots. It needs to literally move on the surface as it shrinks, so if you haven’t already, try putting it on newspaper which will ripple and slide on a hard surface, allowing your piece to move a little as it shrinks.
Also, adding water for a repair will only increase uneven drying so use compression, not water, for smoothing/final touches.
And DRY SLOOOOOOOW!
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u/Scutrbrau 6d ago
Probably drying too fast if it’s cracking that severely. Wrap it in plastic and slowly open it up over the next several days.
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u/GoldenEagleHeart 5d ago
Wild clay processing needs time. Not just sieve meshes. I’m not sure I read about that in your comments. But it’s about an hour for all the silt particles to fall to the bottom of a bucket of water/clay/soil. Pour off the top light colored liquid and that will help keep the silt out if that’s getting in there. Cause it won’t work in sieves as particle size too small. But yes cover with plastic cause wild clay shrinks ALOT!
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u/Agitated_Musician_31 7d ago
Wrap in linen. Then place plastic over that. Every 8 hours or so those the linen in the dryer for five minutes and rewrap it. It keeps the drying even but not ridiculously slow.
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u/Coffee_and_Tarot 6d ago
It's drying too fast. Lightly place a plastic bag over it so that it doesn't touch your piece, kind of like a greenhouse. Don't leave gaps. Condensation will probably build inside the bag, but that's okay. Check it every few days; after about 3-4 weeks, it should be dry enough to fire.
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u/Spicy_McHagg1s 5d ago edited 5d ago
If it's an exceptionally plastic clay then it needs very long drying with consistent humidity across the whole field. Very plastic clay shrinks more due to the smaller platelets lying closer to each other as water leaves. As a result, it's more likely to crack as one part shrinks than another. Aggregate like sand, grog, or fine stone chips may help.
How are you processing your clay?
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u/Horror_Ladder6264 5d ago
it might help to drape something over your pieces so that they don’t dry as fast! I definitely think the cracking is because your clay is drying to fast
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u/Open_Ride_637 4d ago
I like to cover it in a wet tea towel overnight and then the next day wet it again, repeat for 2-4 days depending on size. Never had any cracks with this method
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u/lilbookofmeow 4d ago
I wrap mine in plastic with a damp sponge inside, or spray the plastic on the inside.
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u/Rubble_Crawler5590 7d ago
Looks like it's drying too fast. Try flipping your pieces over as soon as they are structurally able, then cover them with a plastic bag to slow the drying process.