S cracks in tall footed pots

[Babs]

I have been getting cracks on base, sometimes not through to inside, but maybe, on tall footed beakers. I sometimes throw off the hump, sometimes not. Any and all advice welcome. Time of trimming or is that irrelevant. Too cautious re making funnels so leaving thick? Uneven drying.... anyway I know much has been written but I await thoughts. May need to fashion a tombo thingie but feel damage may be done before that.

[Bill Kielb]

In my experience - compression is a huge contributor. Off the hump a bit of a challenge for sure. Variations in thickness often make the issue worse as drying and shrinkage rate differences often have the earmark to not fracture all of the way through the material.

For hollow knobs (closed forms) I usually take the backside of a needle tool inserted into the hollow to compress the closure a bit when still green. It’s the easiest way to finally get some compression in that joint and seems to limit failures to nearly zero. If I don’t, probably 1:10 develop some tiny crack at that joint. So finding a way to compress can be adaptive to a form.

For drying without plaster I found setting the object on the cheapest of paper towels keeps it from sticking and grabbing as well as dries the bottom very evenly without flipping. No more out of round pieces from dragging, no flipping so the most surface area possible can remain exposed and everything drys very evenly so less cracks from thickness especially hump thrown (pop up) lids. The paper towel wicks the water out from under and probably works as good or better than plaster for nearly no effort and minimal cost. I throw as much as possible off the wheel head so less use of batts as well. We call it set it and forget it.

Edited June 13 by Bill Kielb

[Min]

3 hours ago, Babs said:

I have been getting cracks on base, sometimes not through to inside, but maybe, on tall footed beakers.

pictures?

[Mark C.]

compression most likely 

After triming thump the bottoms especially if off the hump. I use a wooded tool and beat the bottom in middle  in all hump work.

[Hulk]

Very fortunate to have only two or three "s cracks" over six plus years; on the other hand, I haven't learned much (by direct experience) about them...

The clay in contact with the bat (or wheel head) can't move about as much, and hence isn't "compressed" nearly as well, I believe.
I cut feet in just about everything, so the "uncompressed" bit is eliminated.

In my process, the upper portion of the form is dryer/firmer than the bottom portion when it's time to trim; the bottom portion is last to dry.
The section within the foot ring, if it is able to dry out - and hence, shrink - well ahead of the surrounding clay, well, something has to give!
Where the bottom isn't appreciably thinner than the surrounding clay, very likely that helps...
The arc* in that section - if any - can flatten as it dries; that seems to help.
Mark, does your thumping stretch the bottom, hence providing some slack for drying?

I'm burnishing that same section (within the foot ring) immediately after trimming; maaaybe that helps, a little.
 

*I'm trying for the base to have a slight arc.
On the final pass when the wares are thrown, I'm expecting the base to be lowest in the middle, by a smidge.
Then, when trimming, the outside follows the inside.
As the ware dries, the base flattens, a lil' bit...

Edited June 13 by Hulk
lower, not thinner

[davidh4976]

How are you drying them?

If you are letting them dry by having the base of the piece on wood, plastic, or non-absorbent surface, it can be uneven drying. It would be worth letting the bases dry on pieces of drywall or cement board which are both somewhat absorbent. This lets the full bottom of the piece dry at the same rate as the rest of the piece.

You could do an experiment and make some pieces that are exactly the same, with bottom compressed by the same amount, and then let them simultaneously dry on different surfaces. I've not heard of anyone doing a nice, somewhat controlled experiment like that, but would love to see someone give it a go.  My personal hypothesis is that once you take some care to keep excess water out of the bottom, s-cracks are more about uneven drying than anything else. My pieces initially dry on cement board or drywall and I don't get s-cracks. Whenever possible, I let the pieces dry upside down so all surfaces are evenly exposed to the air. My bottom compression is minimal and mostly to just even out the interior bottom.

[Callie Beller Diesel]

My impulse is also to think it’s uneven drying, especially since you mention a thick base to start with. If your wareboards aren’t the absorbent kind, flipping the pieces upside down as soon as they’ll support themselves and drying upside down after trimming may help. If your cross section is uneven, that will contribute, so your idea about the tombo for a foot ring could be the ticket as well. 

[Babs]

13 hours ago, Min said:

pictures?

Straight onto dry wall from wheelhead or hump

Edited June 14 by Babs

[Babs]

Came across the crack during trimming. Pots had been turned. 2 out pf a batch but annoying.

[Bill Kielb]

Compression in my experience, but these are too thick for me to compress (especially off the hump), and really hard to dry evenly as well. So I think I would throw these nearly flat and throw a matching foot ring right after throwing the mug to reduce failure rates. 

Edited June 14 by Bill Kielb

[Babs]

Thinking of adding a coil to bottom at time of trimming and throwing foot upwards, very uneven thickness here on finished foot but damage already done. See lots of folk trimming a tall foot but they may be doing it at a wetter stage.

[Min]

Babs, when you break one open how thin is the floor of the pot. Thinking it might be too thick. Have a look at around the 20 minute mark of Hsin's video below, similar pot. 

https://www.youtube.com/watch?v=5dp-cWbM6O4

Edited June 14 by Min
correction made

[Pres]

I often throw off of the hump with the chalices and other pots. My biggest take away over the years is to make a pancake when opening up, instead of starting with a square base on the inside. Then bring up the walls from the pancake . Continue pulling as you usually do until you have finished the form. Make your cut off and start the next pot. Reasoning: The alignment of particles is all erratic at the base of the form where the walls meet the base, by throwing a pancake and bringing up the sides from the base continues the alignment of particles from the base up to the wall.  The forms you are throwing are perfect for this sort of opening up, as the are slightly bowled at the base.

IMHO

 

best,

Pres

[PeterH]

An earlier posting by Pres.

 

[Babs]

9 hours ago, Min said:

Babs, when you break one open how thin is the floor of the pot. Thinking it might be too thick. Have a look at around the 20 minute mark of Hsin's video below, similar pot. 

https://www.youtube.com/watch?v=5dp-cWbM6O4

Thanks Min, I am coming across the crack whem trimming and at that point it may not show in the interior. I'll go for trimming sooner and being more observant of thickness.

Thanks for the link.

[Babs]

@Min, @Pres, et al!! 

Now compression.

Somehow drying very evenly, 

and trimming at an even drier state, measuring and tapping.........and

Even then, ut depends, eh?

[davidh4976]

If I understand the photos correctly, you ended up trimming into the foot almost 3/4 inches deep. That means the bottom was about an inch thick while it was drying before it was trimmed?

If that's the case, it seems to me that a bottom that thick will take a LOT longer to dry in the middle than on the edges. Even if drying on drywall, the interior of that bottom is going to be really damp while the exterior is leather hard.  My personal hypothesis is still that it's the uneven drying that causes S-cracks. With that sort of thickness, slowing down the drying with plastic, so that everything dries more evenly, might be the answer.

[Min]

Stress the clay and you'll get a crack so finding what works for the clay one is using is going to be trial and error. Several good things to try in this thread, adding one more thought is after wedging the clay how is it placed on the wheelhead or batt? Spiral wedged and the spiral going in the direction of a cinnamon bun can also cause a crack in the base whereas placing the cinnamon bun on it's side often helps. I agree with David about even drying, don't want to create a torque in the pot by the walls being significantly dryer than the base. 

[Babs]

Yes spiral wedged ending with a cone shape, the wide end on the cone thumped  then placwd on wheel head. Coned up and down a bit then thrown .

Off the spiral wedged hump  ditto for each  cup but will now add the saucer re @Pres advice thought the crack may be forming in the clay below that when I go to cut off.

The all through drying to same leather hard state would be ideal too.

[Hulk]

Nice shape, burly foot!

I'm curious how thick is the portion that's cracked, after trimming is complete.
The foot ring appears to taper down to the portion that touches the table - thicker where it intersects with the cracked part, the web across the bottom.
Is the cracked part appreciably thinner than that surrounding ring? If so, would it not dry, shrink, and be held taught by the still damp ring around it?

Yesterday I trimmed and handled two mugs.
At trimming, the bases are a bit damper than the upper portion. When set aside, there's a millimeter or so clearance between the middle of the base and where it would touch a table.
This morning, that arc has flattened; now there's two+ millimeters of clearance.

[Babs]

@Hulk lots of things going on for sure. Yes the foot chunks up towards the bowl of the pot. The crack was evident as I trimmed . I make the bottom of bowl humped , thrying to match the inner profile. In this case I saw the crack and didnt take the same time to finish properly.

I am now pretty convinced compression AND uneven drying  so not going to lift onto drywall board for these shapes and after lose moist sheen, turn onto rims  wrap the board in plastic and leave for a while. The link @Min submitted shows the clay in  a perfect state of uniform moisture content...

Thanks everyone.

[davidh4976]

12 hours ago, Babs said:

 "so not going to lift onto drywall board for these shapes and after lose moist sheen, turn onto rims  wrap the board in plastic and leave for a while."

Thanks everyone.

Did you mean to say "now" instead of "not"?

 

[Bill Kielb]

14 hours ago, Babs said:

I am now pretty convinced compression AND uneven drying 

+ 1 for compression however you get it. I have had real good results just removing from the wheel and setting on the cheapest paper towel. For one, way more surface area for the pot to dry while the towel constantly wicks water from underneath to dry the bottom from the inside and underneath surface fairly evenly. Turning the pot over traps air for a good deal of the pot, including the inside of the bottom. If drying too fast, cover with a bucket or plastic. At least the pot does not stick to anything while drying so it stays round and drys fairly evenly. Might be worth a try. Once I tried it, I never contemplated any other way. Too cheap and way too easy.

Edited Monday at 01:56 PM by Bill Kielb

[Babs]

11 hours ago, davidh4976 said:

Did you mean to say "now" instead of "not"?

 

I mean "not".

I feel the dry wall sucks the moisture out of  bottom of pot at the same time the atmosphere sucks out of rim and cup shape leaving  a moist thick yet to be trimmed foot, lots of stress going o n there...

Though you'd think that would be more stressful for exterior of pot???

What do you think of that logic?

Edited Monday at 11:04 PM by Babs

[davidh4976]

58 minutes ago, Babs said:

I mean "not".

I feel the dry wall sucks the moisture out of  bottom of pot at the same time the atmosphere sucks out of rim and cup shape leaving  a moist thick yet to be trimmed foot, lots of stress going o n there...

Though you'd think that would be more stressful for exterior of pot???

What do you think of that logic?

I think we are all talking a little bit hypothetically on this.  I think you should try a few on drywall and a few not on drywall and do a side-by-side comparison. 

However, I also think that the thickness of your foot may cause S-cracking no matter what you do...unless you can somehow get everything to dry evenly...maybe by drying very slowly under plastic.