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A Years-Long Battle w/ Teeny S Cracks on 70% of my Pots! Halp!


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Hi! I have been making pots for about 7 years and in the past 4 have struggled with small s-cracks appearing on the bottom of about 75% of my pots. The s-cracks are usually around a half inch long, thin, and only on the underside of the pot - never go all the way through and therefore the pots are all fully functional, just annoyingly cracked. In the photo attached, you'll see 3 cracked pots, and 2 planters that did not crack. My theory is that something is amiss in the center of my pots that causes stress & cracking during drying - because typically, when I make planters that have the centers removed, cracks do not appear.

The cracks don't show up to my knowledge til after the bisque fire (though because my studio fires my pottery, there are usually a few days that I'm not seeing the piece before it's loaded in the kiln.)

I have tried just about every remedy I can get my hands on and still can't manage to kick these cracks to the curb. My current regimen is:

1. Wedge each piece of clay (ram's head) 40x each. 

2. Align clay on wheel so the spiral-y part is horizontal per this video: https://www.youtube.com/watch?v=f7oIZiXmXFU 

3. Tap clay to center and use a finger to create a seal between clay and bat.

4. Add water, press down on the clay and get into flat hockey puck-ish shape. 

5.  Cone up and down three times.  I make sure my cone is very cylindrical, with the bottom being almost as narrow as the top, which I read was important for preventing cracking. I make the top of the cone teeny-tiny and focus on getting air bubbles out of that portion.

6. During coning down process, I bring the piece back into a hockey puck shape and run my finger across the top towards the center, occasionally finding air bubbles there.

7. Bring out the walls and compress bottom religiously with my thumb, a kidney rib and my sponge. About 10 times, compressing from the outside of the base to the center. Occasionally, I'll feel little air bubbles during this step and will continue compressing until I've popped them.

8. Continue to compress and flatten bottom as I shape the piece. I don't let water sit in the piece. 

9. Let the piece dry slowly and evenly on absorbent surface like canvas or foam (covered after a few hours). Dry with bottoms up once sturdy enough to stand.

Something I've noticed: Often when I use the wire to take pieces off the bat, there is a small air pocket right in the center of the clay that appears both on the remaining clay on the bat (picture attached), and on the piece itself. The indentation in the bottom center of the piece is never more than 1/16-1/8 of an inch deep I'd say. It's something I trim off later in the process, but seems to correlate with the pots that end up cracking.

A hypothesis: Could I be creating these air pockets during the coning up process? I.e. as I press the clay in & up the bottom air pocket forms? Maybe making my cone so cylindrical is part of the problem?

Another potential hypothesis: My wedging is bad and introduces too many air bubbles. Would I be better off using clay straight from the bag? (Or, sigh, trying to get better at wedging?)

Last note: I prefer a flat bottom to a footed bottom. It's possible I get less cracks when I trim feet, but I haven't really investigated.

Any advice you have on ending this years-long struggle would be so, so appreciated!

S-Crack Photo

Bat with Air Bubble

 

 

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6 hours ago, Rachel Hawkins said:

Hi! I have been making pots for about 7 years and in the past 4 have struggled with small s-cracks appearing on the bottom of about 75% of my pots. The s-cracks are usually around a half inch long, thin, and only on the underside of the pot - never go all the way through and therefore the pots are all fully functional, just annoyingly cracked. In the photo attached, you'll see 3 cracked pots, and 2 planters that did not crack. My theory is that something is amiss in the center of my pots that causes stress & cracking during drying - because typically, when I make planters that have the centers removed, cracks do not appear.

The cracks don't show up to my knowledge til after the bisque fire (though because my studio fires my pottery, there are usually a few days that I'm not seeing the piece before it's loaded in the kiln.)

I have tried just about every remedy I can get my hands on and still can't manage to kick these cracks to the curb. My current regimen is:

1. Wedge each piece of clay (ram's head) 40x each. 

2. Align clay on wheel so the spiral-y part is horizontal per this video: https://www.youtube.com/watch?v=f7oIZiXmXFU 

3. Tap clay to center and use a finger to create a seal between clay and bat.

4. Add water, press down on the clay and get into flat hockey puck-ish shape. 

5.  Cone up and down three times.  I make sure my cone is very cylindrical, with the bottom being almost as narrow as the top, which I read was important for preventing cracking. I make the top of the cone teeny-tiny and focus on getting air bubbles out of that portion.

6. During coning down process, I bring the piece back into a hockey puck shape and run my finger across the top towards the center, occasionally finding air bubbles there.

7. Bring out the walls and compress bottom religiously with my thumb, a kidney rib and my sponge. About 10 times, compressing from the outside of the base to the center. Occasionally, I'll feel little air bubbles during this step and will continue compressing until I've popped them.

8. Continue to compress and flatten bottom as I shape the piece. I don't let water sit in the piece. 

9. Let the piece dry slowly and evenly on absorbent surface like canvas or foam (covered after a few hours). Dry with bottoms up once sturdy enough to stand.

Something I've noticed: Often when I use the wire to take pieces off the bat, there is a small air pocket right in the center of the clay that appears both on the remaining clay on the bat (picture attached), and on the piece itself. The indentation in the bottom center of the piece is never more than 1/16-1/8 of an inch deep I'd say. It's something I trim off later in the process, but seems to correlate with the pots that end up cracking.

A hypothesis: Could I be creating these air pockets during the coning up process? I.e. as I press the clay in & up the bottom air pocket forms? Maybe making my cone so cylindrical is part of the problem?

Another potential hypothesis: My wedging is bad and introduces too many air bubbles. Would I be better off using clay straight from the bag? (Or, sigh, trying to get better at wedging?)

Last note: I prefer a flat bottom to a footed bottom. It's possible I get less cracks when I trim feet, but I haven't really investigated.

Any advice you have on ending this years-long struggle would be so, so appreciated!

S-Crack Photo

Bat with Air Bubble

 

 

This has happened on occasion to several experienced potters in the studio, so don't feel bad. That you have air bubbles is not a good thing, so careful maybe as you assemble the clay into a ball after wedging. Often we find folks trapping air at this point. Air under your clay has to be eliminated, this can be from placement and sealing to the batt or on rare occasion I have seen folks center and cone with enough uplifting force to actually pull the bottom center up, and create enough vacuum so the moisture under the clay evaporates slightly and voila an air buble right under the center of the clay! once this is there it will be near impossible to compress evenly as this little buble will chase everywhere and be difficult to remove.

Other than that, proper compression from out to in,  which you seem to have mastered and for folks who make sharp interior corners, apply reasonable pressure opposite to the clay on the outside so all your compression work doesn't simply get squeezed out. Your pictures, to me have signs of these issues.

Hope that helps, stay with it you will figure it out.

You tube compression videos below, we usually use to illustrate the in to out compression. Notice in the video as I compress it well, how dry it actually appears. One advantage to compressing things is the water in between clay particles gets displaced and your sponge will remove it while the clay becomes denser and much stronger and resistant to shrinkage and the eventual S crack pulling itself apart while drying. If there is an air buble under it, there is no way to compress it all the same density.

If you wedge your clay and finish in a cone, make sure the bottom of your cone is not flat. It should be convex so when you put it down on the wheel there is little chance of trapping air in the center.

 

Edited by Bill Kielb
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Hi Rachel,

Hope you find s-cracks diminishing to near zero very soon!

I'm in my third year, have seen two s-cracks only - one early on, in a classic Frankenstein beginner pot, one last year.

Thoughts:

   Interesting idea, the gentleman in the video. I throw clockwise, hence turning the ram's head (I'll spiral wedge, yes, but the last several turns of a weighed clay ball  I'm going with straight ram's head) on its right  side, as I believe in going with the swirl that's established in wedging. A counter clockwise thrower would turn the ram's head up on the left side, eh?

  Coning continues the process of swirling the clay (folk say "aligning the particles" - ok); this is important to me. The part that doesn't get worked is stuck to the bat (or wheelhead), particularly in the center, next to the wheelhead. Coning is important to me - because well swirled clay throws better.

  Opening and throwing the base - as the s-crack is in the middle of the base, my ideas on how the vertical portion is handled, and in particular the transition, are not germane - I'm including a move that pushes clay toward the middle, forming a small solid cylinder that I shear away with my fingers and toss to reclaim. From there, working (folk say "compressing" - ok) the base, I'm moving clay in and back out again, such that a hump forms in the middle, and is then flattened, several times. In short, I'm looking to work that base! Said work drags the clay around. Dragging the clay around is good.

  Water left in there, hmm, I'm not seeing where a thin slick of slip left in the bottom of the pot is a problem, meh.

  Drying, hmm, I'm only turning the pot upside down during drying when looking to gravity train a handle - otherwise, the base dries out last. That said, I often set the trimmed pot on a plaster bat, which does speed things up a bit, however, the base is still the last thing to dry out, likely due to the extra mass the ring portion adds, and that there's less air circulation.

  Trimming - I turn a foot in just about everything, hence the portion of the work that is least conditioned (I call it "swirled") - what was against the wheelhead -  is cut away; furthermore, I'm ribbing the bottom smooth after trimming, then burnishing with the backside of a loop tool, which I believe strengthens. Burnishing does help seal the clay, any road. It might be important that the base thickness is about the same as the rest of the pot; it's what I shoot for. Are your bases thicker than the rest of the pot?

I'm curious to see what others will offer, and please do post back reports on your progress.

What clays are you using?

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Rachel, just to muddy the waters a bit more,  have you always used the same clay??  Just something to think about.  For whatever reason, I seemed to get more s cracks with bmix than any other clay I have used.  Also, if I trim when pots are a bit too dry and with flat bottomed pieces, I also tend to get an s crack or two.  I like trimming feet or at the very least, simply shaping my pots, and I feel I get fewer s cracks when I trim.  But that could simply be a coincidence.  Clay is humbling, that is for certain.

Roberta

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Those are classic compression cracks-more pressure on bottom after its opened -use less water and get the water out of pot bottom with sponge as soon as you see some. 

After trimming pot bottom while on wheel spinning use a Smoth wooden tool (rounded)to compress bottom from bottom side.

If this helps at all it means you are not comprssing the bottoms enough  in forming stages.

I feel air bubbles are a red herring

One last note from a professional is rub with some pressure in are area that cracks with a rounded tool (heck a spoon will work) before any crack appears in the semi dry state (just past trimming wetness  state). I do this will all lids thrown off the hump as they will S crack with out doing it.You can also thumb that area with around end of any tool as well-you are just comprising the particles before they crack. It works with all off the hump pots and lids.

Please let us know if this cures your issue?

Sorry you waited 4 years for answers.Hopefully this will now end for you

Ps what is the clay body you are using stoneware or Porcelain ??

 

Edited by Mark C.
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Rachel, after wedging with the spiral wedging at least 100 rounds,  I take about 20 rounds to smooth out the cone, then  I slap the flatter end of the cone into a low dome so that when I throw the cone on to the wheel the center of the dome spreads first to assure no air is trapped. Then I slap center the clay part way with the wheel moving slowly. Then center in a normal manner. 

 

best,

Pres

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13 hours ago, Rachel Hawkins said:

Another potential hypothesis: My wedging is bad and introduces too many air bubbles. Would I be better off using clay straight from the bag? (Or, sigh, trying to get better at wedging?)

I would stop table wedging completely. I have not wedge clay in years. Straight from the bag to the wheel and cone wedge 3 times like you do.

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dhPotter, 

"I would stop table wedging completely. I have not wedge clay in years. Straight from the bag to the wheel and cone wedge 3 times like you do." Great if you live in the Southern clime, where nothing freezes or not for long. However, my clay sits outside under the kayak rack with tarp over. Freezes in December usually and does not thaw til Spring. Try that trick with that clay! Really though most folks have said here they do not do much more than slam the bag down on the sides, remove and throw. Mine I can't, and so recycling is not a problem as all the clay I throw is wedged.

 

best,

Pres

 
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One more thing to consider, Rachel. The only time I experience s-cracks is when there is an uneven thickness situation involved. i.e., the floor is thicker than the walls, or the floor is thinner than the walls. S-cracks happen the most often for me on pots where the floor is flat and the walls are vertical and there is a right angle in between (just like the ones in your photo). This is because it’s easy to leave behind extra thickness in that right angle area.  Uneven thickness causes uneven drying, which means part of your pot is done shrinking while another part is still trying to shrink. 

Take some of your cracked pots and break them in half so you can see if you have thickness variations anywhere. If so, this is easy to overcome. Start using a needle tool to measure the thickness of the wall, floor, and corner before you trim. You may need to trim the inside of the corners, if trying to maintain the form from the outside, The act of trimming will seal up any holes you make with the needle. 

Edit to add: I found an old comment of mine with a photo of an s-cracked pot that I broke in half. The floor is thicker than the walls, and there is lots of extra thickness in the corners.

 

Edited by GEP
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26 minutes ago, dhPotter said:

I would stop table wedging completely. I have not wedge clay in years. Straight from the bag to the wheel and cone wedge 3 times like you do.

It depends on the clay body. Stoneware bodies generally work well this way, but I find that porcelain and some fine-grained white stoneware bodies need to be wedged a little bit.

@Rachel Hawkins Don't worry about aligning your wedging direction with the rotation of the wheel. Just make your clay into a ball and smack it onto the wheel. The coning process will align everything as needed. Also, make sure you have a ball or cone shape when you smack the clay onto the wheel, nothing remotely flat, or you'll trap an air bubble in the middle.

I don't think poor wedging or air bubbles are the cause of your S cracks. Most likely they're from uneven thickness, and exacerbated by using a smooth white stoneware body. 

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18 hours ago, Bill Kielb said:

This has happened on occasion to several experienced potters in the studio, so don't feel bad. That you have air bubbles is not a good thing, so careful maybe as you assemble the clay into a ball after wedging. Often we find folks trapping air at this point. Air under your clay has to be eliminated, this can be from placement and sealing to the batt or on rare occasion I have seen folks center and cone with enough uplifting force to actually pull the bottom center up, and create enough vacuum so the moisture under the clay evaporates slightly and voila an air buble right under the center of the clay! once this is there it will be near impossible to compress evenly as this little buble will chase everywhere and be difficult to remove.

Other than that, proper compression from out to in,  which you seem to have mastered and for folks who make sharp interior corners, apply reasonable pressure opposite to the clay on the outside so all your compression work doesn't simply get squeezed out. Your pictures, to me have signs of these issues.

Hope that helps, stay with it you will figure it out.

You tube compression videos below, we usually use to illustrate the in to out compression. Notice in the video as I compress it well, how dry it actually appears. One advantage to compressing things is the water in between clay particles gets displaced and your sponge will remove it while the clay becomes denser and much stronger and resistant to shrinkage and the eventual S crack pulling itself apart while drying. If there is an air buble under it, there is no way to compress it all the same density.

If you wedge your clay and finish in a cone, make sure the bottom of your cone is not flat. It should be convex so when you put it down on the wheel there is little chance of trapping air in the center.

 

Thank you, all so helpful! I will try putting additional pressure on the outside the clay per your suggestion r.e. the sharp corners! I also hadn't tried making the ball of clay convex  - the bottom is always completely flat when I put them on the bat, so maybe adding that curve will help eliminate the air bubble / vacuum problem.

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18 hours ago, Hulk said:

Hi Rachel,

Hope you find s-cracks diminishing to near zero very soon!

I'm in my third year, have seen two s-cracks only - one early on, in a classic Frankenstein beginner pot, one last year.

Thoughts:

   Interesting idea, the gentleman in the video. I throw clockwise, hence turning the ram's head (I'll spiral wedge, yes, but the last several turns of a weighed clay ball  I'm going with straight ram's head) on its right  side, as I believe in going with the swirl that's established in wedging. A counter clockwise thrower would turn the ram's head up on the left side, eh?

  Coning continues the process of swirling the clay (folk say "aligning the particles" - ok); this is important to me. The part that doesn't get worked is stuck to the bat (or wheelhead), particularly in the center, next to the wheelhead. Coning is important to me - because well swirled clay throws better.

  Opening and throwing the base - as the s-crack is in the middle of the base, my ideas on how the vertical portion is handled, and in particular the transition, are not germane - I'm including a move that pushes clay toward the middle, forming a small solid cylinder that I shear away with my fingers and toss to reclaim. From there, working (folk say "compressing" - ok) the base, I'm moving clay in and back out again, such that a hump forms in the middle, and is then flattened, several times. In short, I'm looking to work that base! Said work drags the clay around. Dragging the clay around is good.

  Water left in there, hmm, I'm not seeing where a thin slick of slip left in the bottom of the pot is a problem, meh.

  Drying, hmm, I'm only turning the pot upside down during drying when looking to gravity train a handle - otherwise, the base dries out last. That said, I often set the trimmed pot on a plaster bat, which does speed things up a bit, however, the base is still the last thing to dry out, likely due to the extra mass the ring portion adds, and that there's less air circulation.

  Trimming - I turn a foot in just about everything, hence the portion of the work that is least conditioned (I call it "swirled") - what was against the wheelhead -  is cut away; furthermore, I'm ribbing the bottom smooth after trimming, then burnishing with the backside of a loop tool, which I believe strengthens. Burnishing does help seal the clay, any road. It might be important that the base thickness is about the same as the rest of the pot; it's what I shoot for. Are your bases thicker than the rest of the pot?

I'm curious to see what others will offer, and please do post back reports on your progress.

What clays are you using?

Hmm, I think you may be onto something r.e. my base being thicker than the walls - I think they usually are. It's quite possible that the 25% of my pots that aren't cracking are the ones that are the lightest / with the least heavy base. I'll try trimming a bit more from the bottom and see if that makes a difference.

I've been using Laguna 45 Buff for the bulk of the s crack years... I recently used white clay for the first time (I think maybe 65 White Stoneware) and got no s cracks on those 10 or so pieces, which is interesting because it seems like most people have had more issues with s cracks with the white clays.

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18 hours ago, Roberta12 said:

Rachel, just to muddy the waters a bit more,  have you always used the same clay??  Just something to think about.  For whatever reason, I seemed to get more s cracks with bmix than any other clay I have used.  Also, if I trim when pots are a bit too dry and with flat bottomed pieces, I also tend to get an s crack or two.  I like trimming feet or at the very least, simply shaping my pots, and I feel I get fewer s cracks when I trim.  But that could simply be a coincidence.  Clay is humbling, that is for certain.

Roberta

I've been using Laguna Buff 45 for the past several years but recently switched to a white stoneware - I think 65 - and got no s cracks on those pieces. So now I'm using more white clay and seeing if that makes a difference. 

And r.e. shaping - I'm wondering if the cup shape I go for - basically a wide cylinder with 90 degree wall to base, very straight / sharp cornered piece - tends to lead to more stress and cracking? Maybe more rounded shapes hold up better during the drying process? 

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18 hours ago, Mark C. said:

Those are classic compression cracks-more pressure on bottom after its opened -use less water and get the water out of pot bottom with sponge as soon as you see some. 

After trimming pot bottom while on wheel spinning use a Smoth wooden tool (rounded)to compress bottom from bottom side.

If this helps at all it means you are not comprssing the bottoms enough  in forming stages.

I feel air bubbles are a red herring

One last note from a professional is rub with some pressure in are area that cracks with a rounded tool (heck a spoon will work) before any crack appears in the semi dry state (just past trimming wetness  state). I do this will all lids thrown off the hump as they will S crack with out doing it.You can also thumb that area with around end of any tool as well-you are just comprising the particles before they crack. It works with all off the hump pots and lids.

Please let us know if this cures your issue?

Sorry you waited 4 years for answers.Hopefully this will now end for you

Ps what is the clay body you are using stoneware or Porcelain ??

 

Thank you Mark! I will try to compress more during the forming stages and am excited to try the spoon trick too. Do you compress from the inside bottom of the pot or on the outside bottom? 

I'm using Laguna Buff 45 stoneware, I recently tried white stoneware and got no cracks in a batch of 10 pieces (unheard of for me with the buff) but am not yet sure if that was a fluke or if perhaps the Buff is part of my problem.

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8 hours ago, Pres said:

Rachel, after wedging with the spiral wedging at least 100 rounds,  I take about 20 rounds to smooth out the cone, then  I slap the flatter end of the cone into a low dome so that when I throw the cone on to the wheel the center of the dome spreads first to assure no air is trapped. Then I slap center the clay part way with the wheel moving slowly. Then center in a normal manner. 

 

best,

Pres

Thank you Pres! I think this might be a big part of my problem. I've been throwing the clay onto the bat in a flat-bottom cone shape and I think making it convex will *hopefully* really help!

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6 hours ago, GEP said:

One more thing to consider, Rachel. The only time I experience s-cracks is when there is an uneven thickness situation involved. i.e., the floor is thicker than the walls, or the floor is thinner than the walls. S-cracks happen the most often for me on pots where the floor is flat and the walls are vertical and there is a right angle in between (just like the ones in your photo). This is because it’s easy to leave behind extra thickness in that right angle area.  Uneven thickness causes uneven drying, which means part of your pot is done shrinking while another part is still trying to shrink. 

Take some of your cracked pots and break them in half so you can see if you have thickness variations anywhere. If so, this is easy to overcome. Start using a needle tool to measure the thickness of the wall, floor, and corner before you trim. You may need to trim the inside of the corners, if trying to maintain the form from the outside, The act of trimming will seal up any holes you make with the needle. 

Edit to add: I found an old comment of mine with a photo of an s-cracked pot that I broke in half. The floor is thicker than the walls, and there is lots of extra thickness in the corners.

 

Thank you!! I've looked at cross sections of my pots before and you're completely right, even if the base and walls are the same thickness the way I trim seems to leave a solid hunk on the outside corners. Hadn't ever considered trimming that part from the inside, will try it now!

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6 hours ago, neilestrick said:

It depends on the clay body. Stoneware bodies generally work well this way, but I find that porcelain and some fine-grained white stoneware bodies need to be wedged a little bit.

@Rachel Hawkins Don't worry about aligning your wedging direction with the rotation of the wheel. Just make your clay into a ball and smack it onto the wheel. The coning process will align everything as needed. Also, make sure you have a ball or cone shape when you smack the clay onto the wheel, nothing remotely flat, or you'll trap an air bubble in the middle.

I don't think poor wedging or air bubbles are the cause of your S cracks. Most likely they're from uneven thickness, and exacerbated by using a smooth white stoneware body. 

I usually use a buff stoneware - any excuse to wedge less, I'll take! I think I'm definitely trapping an air bubble in the middle because I've been putting the clay down on the wheel in a totally flat cone shape. Will try with a rounded edge and really hoping that helps! Plus working on trimming to ensure even thickness across base, walls and corners! Thanks so much for your help!

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9 minutes ago, Rachel Hawkins said:

Music to my ears! I will try without wedging and keep coning the same

Doesn't work with all clays.  Mine stays hard in spots and is generally not nice to throw with if it's not wedged.  It's a stiffer clay and I don't own a pug mill.  A lot of people who don't wedge forget to mention they pug their clay before throwing.

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2 hours ago, Rachel Hawkins said:

Music to my ears! I will try without wedging and keep coning the same

If you can get away with not wedging, by all means do so. It'll save some wear and tear on your body, especially your wrists. But see how your clay body handles it. If you still need to wedge, do so. Your wedging technique is not the cause of S cracks, nor are air bubbles. Bad wedging and air bubbles can cause a lot of other problems, though, so it is in your best interest to get good at it.

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(Thank you Mark! I will try to compress more during the forming stages and am excited to try the spoon trick too. Do you compress from the inside bottom of the pot or on the outside bottom? )

I only do this on off the hump pieces which are much more prone to s-cracks-but since you are having so many issues it cannot hurt. I use a rounded end of a wood tool but as I said a spoon will work. I do not work on the inside at all on any pot. I also use a wood tool and thump the middle on off the hump lids.

In my 45 years of production work I do not have s crack issues with my porcelain any more. Its somethink you work thru in the beginning (my beginning was making pottery for living ) as it is now so you get to the bottom of issues fast. I gave you all the tips-but really its your throwing and commpressing thats the issue combined with your clay body.

Once those are mastered this will fade away I'm sure.

Its like everyone said-flat bottoms straighter walls seem to be more prone . For me french butterdishes are that exact match but rarely in 100 do any ever s-crack. Pie plates are more likely in general . They are large flat bottoms and need extra compression while making-if not they tend to s crack-so I really work the bottoms while throwing.Maybe one in 50..

Work the bottoms more compression wise.

 

Edited by Mark C.
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Liam, on your 40# planter try the asian pounding method of opening up, while wheel is moving slowly, use your right fist swinging downward into the center of the mound rhythmicallyly. continue at an even pace until you have an opened up, but slightly ridgey inside. Use your hands and a sponge to smooth the interior with some not-pulls, then finish as you usually do.  This technique will leave a very compressed base, and the fist also drives clay into the outer wall compressing the walls there. Has worked for me for years, but takes a little patience in the beginning to master.

 

 

best,

Pres

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