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Of the 50+ pots I'm getting ready to bisque fire I only one has a small off center crack on the base.

 

The crack is very narrow and about an inch long.

The clay is stoneware and will eventually be fired to ^6

 

I'm not overly attached to the pot but I figure it would be a good learning experience learning how it fix it.

 

Any pointers?

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I have used Peggy Heer's recipe for a clay mend and that has worked well so far, even on bisqued items, though that is a bit more iffy than with greenware. Might need a little sanding after final fire. You can find the formula via Search...it's very simple, just a few ingredients.

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Found it! Interesting mixture

 

Peggy Herr was an early clay art participant. She developed this recipe. She died in 2006.I add paper pulp to this.about 20% to the clay portion.
This is her recipe.
Here is the recipe for Peggy's spooze if anyone would like to try it. It's
wonderful.

1/3 parts dry clay powdered....any body...preferably the one you are using with no grog
1/3 parts heavy karo syrup
1/3 parts white distilled vinegar

A drop of hydrogen peroxide to keep it from spoiling...it will bubble for a
few minutes and then settle down. if you don't add that the sugar goes bad
and smells awful in about a day.




Marcia

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When salvaging cracked pots, including S-cracks, my objective is to at least turn the crack into an esthetically pleasing decorative feature. 
 
I fire to cone 10; and when plugging cracks in bisque ware I pack the crack with a mixture of  powdered bisque and a very stiff glaze (usually Shino).  If packing the crack with glaze is not feasible, I may add a shard to cover the hole and 'glue' the shard in place with glaze.  Sometimes a runny glaze will work better than a stiff glaze; it all depends on ... .
On one pot I placed a red glaze drip salvaged from a kiln shelf on top of the shard to turn the patch into a focal point. 
 
When firing 'salvaged' pots I want the pot oriented in the kiln so that gravity is working for me, and this often requires placing the pot on wads or sea shells.

I will add Marcia's version of Peggy Herr's elixir to my recipe book.
 
LT

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Throw it out and make another one. S cracks are a part of pottery life, and should be expected from time to time. It cracked for a reason. There was something done wrong during the making process, and the crack is the result. Figure out what it was, throw out the pot, and make a new one without a crack. It's better to learn from the mistake and improve your skills than to waste your time trying to hide it.

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I really don't like throwing a downer into the parade, but why worry about fixing a cracked pot. Any sort of crack in the pot is just a weakness waiting for something to happen. Could you sell it?  Could you give it away to a friend or relative? So you might use it yourself? If it is not cracked all the way through, sure use it at home, enjoy it until the next one come along. In the long run, best to not get into the habit of fixing cracks for any reason. The time you spend repairing it could be better used with a little more time making a replacement. At the same time, maybe lesson learned.  Hope I don't sound to harsh, but that is my take on cracks.

 

best,

Pres

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Hey,

I lean toward to either throw it away or punch a hole in it and make a planter or frog house! The reasons are... Trying to fix anything gives a sense of false hope. If you glaze it and the crack widens, the glaze will seep thru onto the kiln. If by chance it comes out seemingly perfect, and try using it yourself, you might forget and give it away to a friend years from now! There might be some other reasons I can't think of ... But I think those are enough for you to use the time to make a good one.

 

Alabama

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If it was mine, I'd probably just remake it.  If it was something large or elaborate, that was just decorative, I might keep it.  Otherwise, try, try again.

 

I will attempt to mend student projects, with nearly any defect.  Not only does it make me better at repairing wares, but students don't have the time, nor skill level to remake a project.  And to them, they have so much time, pride and effort into making that one piece, in the beginning, telling them to remake it is pretty devastating.  

For any type of crack, I do tell them, that they are not functional.  Eventually, the crack will get worse and fail.

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So I guess the lesson I learned here is, "Life is to short to fix a crack pot" :)

 

Thanks for your honest replies.

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but RonSa should it be about fixing or just about looking at it in another light. esp. greenware.

 

of course take this with a grain of salt since its coming from a student perspective when time is not money yet.

 

i love, love, love blemishes on greenware. it gives me the chance to break rules. it gives me the chance to experiment.  i've turned a trimming hole on the side of a vase as part of design (making the fixing look intentional) and sometimes its become the focus point. in fact i've learnt more about trimming when i was rewetting a bone dry pot which i was told should not be done. 

 

for me its essential to play with such failed pots because it forces me to think differently. even my proff. has said why waste your time. for me its personal growth. a different perspective. so when i am faced with an unfamiliar situation i dont hide from it or try to destroy it ... but stare at it right in the face and push myself to 'now what?!!'

 

luckily my S crack bowls were the perfect shape to turn into a succulent planter. 

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LOL, yes planters and flower pots are good, that's exactly what I decided to do with this one.

 

I too look at mistakes and blemishes as design opportunities. I'm still trying to come up with an idea on what to do with those crescent moon shapes left by finger nails.

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It's good to have a few flower pots, but if you save every cracked pot your house will soon be overrun with planters and your friends will start to refer to you as a hoarder. Part of being a potter is throwing out bad pots. I toss a few into the dumpster every time I unload the kiln. Every pot cannot be a keeper, it's the nature of the process.

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You are right Neil, there is a limit.

 

I grow a lot of herbs and always give a lot of live plants away each summer. I'm always scrounging for something to put them when friends or family stop by. I'll keep a few cracked pots around for that.

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I have a theory which seems to have some base in physics as of the origin of too many S cracks, however; I shall not disclose this thought as it defies "the way". Some folks can get pretty testy when you defy their beliefs and knowledge base, especially when they paid a lot of money for it.

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For most beginners, and any ware meant for regular use I think learning the skills, and how to avoid the s-crack in the first place should probably be the focus. But there are points where repair becomes less time consuming than starting anew. I think if you're going to repair something, it needs to be 1) less work than starting again, and 2) really enhance the piece, and be well incorporated into it. I'm thinking of Mariko Paterson, who fires most of her pieces 5 times between inlay, underglaze, glaze, decals, lustre, and some China paint. If she has a piece crack, she will do a kintsugi-style repair with grey automotive epoxy, which she will then finish with craft glitter.

It works for her, but it's also a big part of her aesthetic and subject matter.

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Once upon a time, I embarked on a project to deliberately produce s-cracks in the center of mandalas.  The strategy was to not do the steps that traditionally are emphasized as 'must do' steps in throwing.  My success rate was very poor! 
 
However I had great success when I opened a fresh bag of commercial clay and just cut half inch thick slices from the square bag with a wiggle-wire tool and bisqued the slices.
 
Yes, I do compress the bottoms of my bowls, and yes s-cracks seldom occur in my pots, and yes I am still skeptical about the why they form.  When they occur I usually just use the pot for a difference purpose, which does include adding to the amount of clay to be reclaimed. 
 
  LT

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It just so happens that we were having a similar discussion over on Facebook about an hour ago. This is a cut'n'paste of my post.

 

{begin snip}

IMO, we have several different issues coming together and then bandied about in conversation under the heading of "compression." There is particle alignment, water content, and particle adhesion through water surface tension.

As noted above, clay par
ticles are flat platelets. When they are jumbled up all higle-di-pigle-di in the moist clay, they are not very strong, because of the limited contact area between the random particles aligned edge of one to the face of the next particle. The clay will be much stronger if you can stack the particles face to face. The adhesive force between the particles comes from surface tension of the water in the moist clay, and the more adjoining surface area you can promote, the greater the adhesive strength between the particles will be.

Furthermore, when the particles are jumbled, they take up more physical volume for the same number of particles, i.e., lower density; or from the opposite frame of reference, fewer particles of clay in the same cubic unit of volume. You can realign the particles to the stronger and more dense face-to-face position in 3 ways - by simple physical pressure applied to it to crush the particles into alignment (such as a slab roller), by rubbing it to push the edgewise particles flat, or just let it dry and the particles will flatten themselves together as the water departs.

I don't have a scanning electron microscope in my studio, but I have seen SEM pictures of clay as it was being centered. The rubbing of your hands on the clay lump as it rotates will push down and align the particles to about 1/2 - 1 inch deep below the surface, but the effect dissipates deeper in the clay mass. That is the benefit of coning up your clay as you center. A greater proportion of the clay mass is within an inch of the surface of the tall narrow cone, so all the particles in the higher regions of the cone will align, but not so many down near the base of the cone. The coning process, if done properly, squirts the randomized clay in the middle of the clay mass upward into the cone where it is aligned, and then a proper run-down of the cone deposits the aligned clay evenly around the centering mass. Another coning squirts more up from the middle, and this is aligned. And so forth until it has all been "processed" through the cone. However, there is still a pocket of unaligned clay remaining in the center of the mass because it is stuck to the wheelhead and was never squirted up into the cone. This soft pocket will not be exposed until you open. Then you must work the opened base to align it by compression. This is the purpose of all the rubbing back and forth of the newly opened base. I also believe that the force used to open the base pulls the particles outwards under some elastic tension, which must be relieved by some inward stroking of the base. This has been mentioned during the discussion.

However, we still have a problem. We may think of clay as a plastic solid, but actually it is a liquid because of the water content. It flows (albeit slowly) and you can make waves in it with your finger as the wheelhead moves it. When you are compressing the base with your finger(s), you compress the area directly beneath your fingertip and some will compress and stay put but some of it will flow out from underneath your fingertip and come back up behind that spot. You can work this out with many passes back and forth to even out the average compression across the whole base, or you can beat it with brute force. If you use the straight edge of a strong rib that just exactly fits the radius of the opening (i.e., covers the entire base from the center point out to the edge), you can apply uniform pressure over the entire rotating base as if one big wide finger; now there is nowhere the wave in the clay can pop back up.

And now for the next however... we may still have a problem. If you allow the moist clay to become oversaturated with throwing water, the excess water will be absorbed into the clay turning it into mush. I think this is where Tim was leading. Once it is mush, there is no way to unmush it . Compress all you want, but the water is still there and the faces of the clay particles will not adhere to each other but rather just float around in a mud puddle. Where this particular issue becomes problematic is during the pulling up of the wall. Lots of water is used to lubricate your fingers for the pulls, and that which was applied to the inside of the wall runs down into a pool at the bottom, filling up the beautifully compressed base. And floating all those nicely aligned particles up into a jumbled mess. Back to Tim's point - you can't compress the excess water out of the mud. As a result, you now have differential water content between the walls and the base. The walls are strong, nice and tight, while the base is weak, loose and sloppy. As the piece dries, there is more water that evaporates from the loose base compared to the tight wall, and the base shrinks more than the walls. The tension of that extra shrinkage relieves itself right down the middle - the infamous S-crack. The practical solution is quickly sop up all that extra water from the inside the bottom as soon as you finish every pull. Make it habitual - pull up, instinctively compress and strengthen the rim with light finger pressure while your fingers are still there, and instinctively immediately dab your sponge into the base to get the extra water out.

We can save for another time compression of the cylinder walls with a flexible rib after each pull.

{endsnip}

 

As for the slabs sliced off a new cube of clay, that is because the clay blocks were produced by a monster pugmill with a square outlet. The screw auger pushes the clay out the square outlet in a fat square lug, and then the machine chops it off into neat tidy cubes (or rectangular blocks depending on the brand) and into the bag. However, the auger is creating a spiral pressure ring inside the clay as it comes out. You probably won't notice it when you cut off a chunk for your work, and the wedging process will alleviate it anyway. But if you just slice off a side, those compression rings will do crazy things as the clay dries and fires. If you want to see this more clearly, let a new bag of clay sit outside for the winter through a few freeze-thaw cycles, and then bring it inside to warm up. Slice off some slabs the same is you did before, and watch the slabs come apart like onion rings. The freeze-thaws moved the water content according to the compression rings.

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"But if you just slice off a side, those compression rings will do crazy things as the clay dries and fires."

 

I do this for a lot of my concepts and just love the serendipity of the surprises. Honor the cracks, she sez & does LOL  I especially like what happens to boxes I make from cutting off the pugged block.  If I get an S crack (or line cracks in porcelain), I might just fill it in with micro-dust (glitter) after firing--depending of course on the nature and purpose of the piece. 

 

Never saw such a detailed explanation for the particle process re: centering/compression/pugging--cool!  

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When we get down to the nitty gritty, an S crack maybe decorative when force for purpose in non functional decorative piece of pottery, but for those things functional not so much. Furthermore, in those things used for food, the potential for bacteria, even in a "repaired" pot could be downright unhealthy>

 

 

 

 

best,

Pres

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Dick White: "As for the slabs sliced off a new cube of clay, that is because the clay blocks were produced by a monster pugmill with a square outlet. The screw auger pushes the clay out the square outlet in a fat square lug, and then the machine chops it off into neat tidy cubes (or rectangular blocks depending on the brand) and into the bag. However, the auger is creating a spiral pressure ring inside the clay as it comes out. You probably won't notice it when you cut off a chunk for your work, and the wedging process will alleviate it anyway. But if you just slice off a side, those compression rings will do crazy things as the clay dries and fires. If you want to see this more clearly, let a new bag of clay sit outside for the winter through a few freeze-thaw cycles, and then bring it inside to warm up. Slice off some slabs the same is you did before, and watch the slabs come apart like onion rings. The freeze-thaws moved the water content according to the compression rings."

 

I've had exactly that onion ring layering happen to about 20 boxes of clay, really does delaminate into slabs doesn't it?

 

When I got my pugger I cut off a dozen or so 1/2" - 2" slices straight from the pugger and just laid them out to dry to see how many got S cracks.  I think it was about 1/4 of them did. Maybe some studio size puggers don't have a problem with unwedged or coned clay being thrown straight from the pugger with the pug soup can style on end but doesn't work for me. (Bailey de-airing) Commercial pugged and boxed clay definitely does the same thing but just on a larger scaled size and on a different plane, depending on how pugs are cut.

 

I think the pics in this article illustrate beautifully  what you are talking about in your fourth paragraph.

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thank you for the explanation.  having experienced much of what you mentioned as problems, i have tried to throw dry as possible, cone the clay on the wheel and because my hands are getting a little difficult at age 76, i now WHACK the ball of clay down onto the center of the bat i throw on. a couple of whacks with a hefty wooden paddle before i start using my hands.  not sure exactly what is happening to the clay platelets after that but i rarely have any s cracks and then i kind of know what silly thing i did with that particular pot.  rarely being one or so a year.

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If you throw left-handed, do you get z-cracks? That is, does the crack curl the opposite direction?

 

Just curious.

Yes, the rotational stresses generate the characteristic S, so throwing clockwise would create a Z. But the Z would be an S when viewed from the top. Or maybe the usual S is actually a Z from the top. And now we are all topsy turvy :lol:

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