Glaze cracking bottom of pots

[Mark C.]

Nice  feet are that reflect ones style are a skill that takes a long time to hone down. Think about a rounded edge to avoid chipping over time/use.

[Mike.Kelly]

2 hours ago, Babs said:

Yeh mike was aware my comment could be a bit offensive. Sorry about that.

I think the less unnecessary clay in a pot the better  and where feet join pots can be an area where clay makes an otherwise great pot clunky. Same with rims really. The illusion of thinness but infact a sturdy functional rim is an art in itself .

I still say check feet.  Run your fingers in inside of bowl/ mug and thumb on outside and draw your hand up the pot of any ztuf you see, you'll feel what I'm talking about.The two sensations should be very similar

You can tell by. sound made by tapping pot over area of trimming when turning pots if you are getting s uniformity of thickness.

Just saying because your pic shows a lovely pot that the foot on your pot Could  still carry less clay but look the same when standing on its base.....

Old lady here Mike...forgive me.

No need to apologize I wasn't offended in any way.  I do think it is funny though and I am kicking myself.  Clearly I can see the difference in workmanship between the first post and that bowl.   IT comes down to laziness on my part and I think we have honed in on the problem.  Well multiple to be honest.  I really appreciate all the feedback and it has helped ground me a bit and again go back to the basics.  Looking forward to reading that book.  

[Babs]

Yeh as Mark says feet are individual hallmark of potters.

One potter I know turnd the feet so the pot ends up sitting on the narrowest run of clay. Stable and functional.

I'll try to post pic. The bottom of his pot curves to match the inside curvature.

I'll take prob awful pics to show you.

[Mullins Pottery]

I've had issues in that past with cracks forming around the foot area. In every case it was the result of a foot that was too wide. It doesn't happen every time. But I did discover that by thinning my foot a little ended the issue.

[Bill Kielb]

From the pictures thickness seems a bit uneven or overly thick in some of the examples.

I always ask this question: how do you compress the bottom to your satisfaction?  Do you have a specific plan  or method that you always follow? I spent several months verifying different methods and am always interested if others have a specific method or not.

interesting that I am just reading this as one of our members used a test glaze with great result on a test pot except his clay was too thick and basically poorly constructed.

He was hoping the glazed caused the crack

 

[liambesaw]

I compress after opening.   I push clay from the wall toward the center and back with my middle and index.  If it's big I will use a rib to compress afterward. How you do it, @Bill Kielb

[Bill Kielb]

@liambesaw

I see you are the only taker

Actually did a bit of science on this trying to figure out why experienced potters occasionally  fell into the unknown crack zone.  The time tested remedies of not too much water, not thick, slow dry  ........ all were fine but there was always someone that threw with lots of water and nothing happened. There was always those that threw, stuck their pot in the sun and trimmed it a couple hours later .........what?

So after all the testing we tried to figure out if there was something that we could define better to improve the chances their work would be better. Compression was something that had a very jello like definition.  I won’t bore you with the tests  ( and often funny trying to assess the density of clay with home made penetrometers) but we realized it was important at some point to make sure after they opened up they simply did not continue to pull all their clay out from the center.

So the simple point was to stress always compress (at some point) from the (interior) perimeter wall into the center. They can actually feel the compression and see it as they travel from out to in sort of like making a pop up knob. So include this motion preferably when you are near finished with the bottom just to ensure the density of the clay is uniform throughout the bottom.

This actually worked so well that we started to have them do this with bowls with their ribs to form a no bench, no sump, bottom. Same principle work from top to bottom into the center, dead smooth bowl interiors are prettty easy with a little practice.

Obviously for plates this works extremely well and is cool to watch. It doesn’t really take additional time, just raises their awareness that pushing down on something often allows it to squirt in one direction or another rather than uniform compression and distribution of the clay particles which can make it weaker in the center destined to crack in the glaze firing for no obvious reason.

So the plan, compress evenly which often means move from out to in on some of your stokes or at least the last few strokes.

A bit pottery nerdy but it seems to have reduced the unexpected cracks a whole bunch.

[Dale pots]

Mike, everyone's got you going in the right direction with thickness & maybe too much water for sure,  but one more thought by looking at the first pic is needing more compression on the base. Watch a video of a platter being thrown, you don't have to be as excessive as a platter but it will give you a good look at good compression.     

Oops!  Bill got it, I posted before reading Bill's post.  

[neilestrick]

I think compression is very important, but it won't overcome poor construction, especially in a bowl with a rounded bottom. They are much more prone to cracking from uneven thickness than flat bottomed forms.

Water is only an issue if you leave it in the bottom when you're done. Whether you have a 1/16" thick layer of water or a 1/2" layer of water in the bottom of your pot during pulling doesn't matter- it's not going to soak in all of it either way. It's more about time than volume of water. The longer you work the clay, the more time it has to soak the water in and get saturated to the point that it causes problems. You can use all the water you want as long as you don't take too long. And too long is really a pretty long time. When I teach first-time students, I have them drench their pots with water during the pulling and shaping process. Most have a 1/2" deep puddle in the bottom of the pot. We take at least 15 minutes to get through it with all that water in there, and it never causes problems.

[oldlady]

someone asked for foots.   (thank you, hulk!  see the post below for the photo i tried to post. )   these are rounded.   the bowl is shaped with a disk shaped wooden rib.  the original pat of clay is shaped like a doorknob on the bat.  the first shape is like a flowerpot.  when tall enough, insert rib straight down just like a jigger and jolly pot is shaped.  that is the compression that forces the excess clay downward and outward. 

 finish bowl rim and flare if any.  THEN the good part.   a large circular ribbon tool is placed against the bat, slid into the clay at the base of the bowl and pivoted upward to remove the thick clay that was formed when the disk reached the bottom.  all that fat part is removed in one revolution.

trimming later is made very easy because there is no lump to fight with, just smoothing, ribbing and shaping a foot.

this is so discouraging.  cannot post picture.   it is under my profile in my album section titled  "recent work, fall of 2015" and shows  bowls upside down with a variety of feet.  

 

[Hulk]

Variety o'feet, indeed! From your (oldlady) aforementioned album:

...when I get up from the couch, will take some pics o'my feeet...

[Mark C.]

The power of commpression is easy to see-throw a small lid off the hump.

Fire it and it usually has a small S crack

Throw another and when it leather hard tap the middle bottom with a tool like the rounder end of any wood tool-smash the clay where the other one had that S crack. now fire it no more cracks. 

Hump throwing is very hard to compress the center as you are working with a soft clay mass underneath the center. Taping the centers after trimming the lids fixes the compression S cracks.

That bowl in  Bill s  rutile glaze photo is a classic compression issue.

 

[Min]

One point that hasn't been brought up is how coning can greatly reduce the chance of S cracks. Compressing the clay on the base of the pot, both inside and out, is going to align up the clay platelets on the surface but not through the thickness of the base. Coning the clay properly, ie rod shaped cone with a convex not concave top and not a wide bottom pyramid shape, is going to align much more of the clay into the proper orientation than compression alone.

[Hulk]

Some feets, clockwise from top left

  Bisqued Aardvark bmix mug, not yet sanded*

  Raw Aardvark SBF bowl, just after trimming this afternoon

  Bisqued Aardvark bmix bowl, not yet sanded

  Bisqued Aardvark bmix mug, not yet sanded

I try to leave about 5mm of clay within the ring; a peek at the inside contour (and remembering, haha) helps there.

Note the tool marks on and near the inside edge  of the foot ring - where the flexy metal rib doesn't fit very well.

...it's all evolving for me - might be different later; for now, I like the look and feel of a footring.

 

*sanding after bisque fire seems the best way to smooth the lil' burrs on the chattermarks.

 

*

[oldlady]

hulk, why not a damp sponge right now instead of sanding later?

[liambesaw]

@Hulkthese foam brushes work very good just after you chatter, brush them down with one of these foam brushes DRY, and it brushes the burrs right off without marking up the surface.

[Bill Kielb]

I often flip my wheel direction and use a nice dry quality soft paintbrush (house paint) to knock off remaining bits.  I can also lightly brush anything out of the chatter and  with all the bits gone from the grooves can lightly damp sponge if I want to soften the chatter to taste as a final touch.

[Hulk]

Thanks Bill, Liam, OL!

Will try't. In bisque state, the clay doesn't move - the cuts stay clean.

Good point on the coning Min. Seems that throwing is easier when clay has been thoroughly swirled! ...hadn't noticed the less s-cracks, haven't had one for quite some time now.

[Min]

I'm just adding a bit to what I said earlier about coning, @Hulk, glad you don't have this problem anymore! Couple photos from Jeff Zamek illustrating the importance of coning in regards to S cracks, first picture shows the clay on the wheel after throwing a pot with red and white clay that hadn't been coned prior to throwing, second picture shows the effect of properly coning the clay. The contrasting claybodies make it really obvious how effective coning is at aligning the clay platelets with the direction of the wheel. When the platelets are aligned like this there is far less chance of S cracks forming. It would be interesting to see how deeply compressing the clay does this versus coning.

[Bill Kielb]

Uh oh!

I am going to have to break out the super nerdy how well do you really mix your clay video. (Even more embarassing than the home made penetrometer one) nice pictures! A good point to make is it is truly amazing how much effort (and time) one would need  to get their clay mixed relatively evenly.

After testing it’s near impossible with any wheel method  to make this homogeneous but shows the validity of always attempting to do an adequate job without overworking the clay. It will make throwing much easier and consistent for sure.

[oldlady]

the best coning demo for making the clay behave is the one that neil did in the 12 inch contest.    watch it and weep.

[Hulk]

imo, fully mixed/homogenous and fully coned/centered/swirled are related but not same - both being important.

Snip is from Tim See's video on centering, where he adds a tootsie roll of dark clay to the side of his light clay. Per experience, notorious un-homogenous bits can hide just above where his index fingertip is, can I get an aye on that?

Bill, by overworking do you mean too dry (wedging table) or wet (wheel head)?

Neil does make it look easy! Looking baaack (haha! ...been throwing just over a year now), learning to cone is key step to learning to throw, not just because the clay 'haves* better - we're learning how to apply pressure to and move the clay where we want without tearing/shearing.

I won' fight the term "compression" (much, ahem); does the clay volume actually change? Pressure is applied, yes; what does the clay actually do, hmm? I believe it moves - away from the pressure and also dragged out (or "swirled").

 

* 'have == behave  (I love you Mom, I love you Grandma!) - it all got a lil' easier once the notion o' "memory" started to sink in; the clay remembers, which is prob'ly 'bout the same as "aligning platelets"...

Wedge/pug: preparing the clay to learn (imo, the clay is learning alla time, hence, as clockwise, I turn the lump right side down onna wheelhead, can I get 'nother aye on that?) evenly

Center - more that just creating a uniform shape to throw, also teaching the clay to be round

 

[Bill Kielb]

3 hours ago, Hulk said:

imo, fully mixed/homogenous and fully coned/centered/swirled are related but not same - both being important.

Snip is from Tim See's video on centering, where he adds a tootsie roll of dark clay to the side of his light clay. Per experience, notorious un-homogenous bits can hide just above where his index fingertip is, can I get an aye on that?

Bill, by overworking do you mean too dry (wedging table) or wet (wheel head)?

Neil does make it look easy! Looking baaack (haha! ...been throwing just over a year now), learning to cone is key step to learning to throw, not just because the clay 'haves* better - we're learning how to apply pressure to and move the clay where we want without tearing/shearing.

I won' fight the term "compression" (much, ahem); does the clay volume actually change? Pressure is applied, yes; what does the clay actually do, hmm? 

 

Much like Tim’s discovery, it certainly mixes the clay more uniformly and I believe we concluded as he did, moving clay up from the center and down the sides is probably most efficient to getting it wheel wedged, however you do that. 

Overworking is pretty subjective, there are some that say, cone no more than three, four, whatever the number. Obviously there is a point where the clay may become too plastic, too inelestic, too oriented for a particular potter ......maybe too much moisture introduced to answer your question.

with respect to compression getting a uniform density of clay materials distributed across the bottom seems to be a reasonable definition. As the density decreases so does the strength in that region. So how to get uniform density becomes the goal. When we truly compress things or compact them if you will, they become denser but still consume the same volume relative  to that which is not compressed. In general this improves the overall strength of the area compressed. (Less voids) I often relate it loosely to when we compress gravel for a roadbed or under a new sidewalk. Compression  often makes the water if present, pump upward. We displace the water with fines or other solids which improves the overall density and strength of the finished product.

Clay orientation to me is difficult unless we always want some circular spiral polarization. Uniform, even distribution is definitely important though, so, good coning skills = all good. I have seen directional  wedging theories that may or may not be valid as well as removing your centered clay and flipping it over. Maybe, maybe not - if it works for you, great!

If Tim was adding a clay of lesser strength in his demonstration we would conclude that he found a way to make uniform cracks likely to appear in his finished result along predetermined lines.  His distribution is pattern uniform but not necessarily homogeneous.

Coning - wheel wedging,  a good skill to develop, good practice, definitely helps but compression is pretty important as well so we sought to define it for new throwers because folks often do not provide any specific direction other than push down.

here is an interesting thought: as we pull and stretch something upward at some point should we compress downward to regain strength? Most folks intuitively have no issue with compressing the rim being helpful, but should we rib downward on a finished wall as well? Interesting question for me.

finally Neil’s three pull video is excellent no matter what the answers are.

[neilestrick]

18 hours ago, oldlady said:

the best coning demo for making the clay behave is the one that neil did in the 12 inch contest.    watch it and weep.

Thank you for the kind words! Poor coning technique is the root cause of many issues that show up later in the throwing process, especially once you get into larger pieces of clay, like 3 pounds and up. If you don't have a good foundation, everything else will be affected. With good coning, good compression, and even thickness, S-cracks should rarely occur.

[Babs]

Wrll i  rib downwards on inside and take it past the centre of bottom of bowl to get shape of medium  to large bowls. Get no cracking in rims and upper bowl body but this is prob due to notice given to clay available which determines the widyh of final diametrr of opening...