Jump to content
Mike.Kelly

Glaze cracking bottom of pots

Recommended Posts

OK so I have to ask the experts.  I cant figure out what I am doing wrong here.  I think it is the pot and not the glaze since it doesn't happen all the time.  I am compressing the bottoms and they don't crack when they are bisque fired to cone 06 but this is what happens after glazing and firing to cone 6.   Any ideas?  This is happening more than it is not. :-(

IMG_2680.jpg

IMG_2679.jpg

Share this post


Link to post
Share on other sites

No expert bit my guesss is the bottom has been trimmed to a too thin syate in comparison to walls and prob too thin. Crack one open and post a pic if this is not the case.

Keep an eye on amount pf clay left on bottom of pot on throwing.

It looks like itz's cracking around inside of footrim?

Making a distinct footring means you have to leave enough clay there pn throwing.

Imo

Share this post


Link to post
Share on other sites

Yeh I think it it a difference in thickness.

Your pot has a ring cut on underside but your trimming imo needs to happen on wall of pot also. The wall of your pot is rising from athicker clay area lleft at trimming. 

Maybe spmeone else can put it more eloquently.

Share this post


Link to post
Share on other sites

I think I am following.  So perhaps because the thickness of the base is 2X the thickness of the wall and trimmed out bottom it might not be expanding/contracting the same?  meaning the bottom is either shrinking quicker than the rest of it causing it to separate and crack?

Share this post


Link to post
Share on other sites

Could be glaze sqeeze also.

That's mo

Uniformity of thickness in bottom and walls rising.

The profile of outside of pot the same as inside of pot footring exception the re.

You can see where the inside profile is quite different. The inside base tises to a wall abruptly, outside curves up

Just saying

 

Share this post


Link to post
Share on other sites

Your first two photos look like they cracked along what are uneven thick/thin spots. Could also be exacerbated if you are leaving too much water in the bottoms after throwing; it will rot the floors of your pots, and make them much weaker. Use only enough water to comfortably throw, and remove any water/slop after throwing.

The blue bowl could be a thick to thin issue; not consistent thickness. It looks like from the bottom of your trimmed foot to the floor of the bowl, its maybe 1" thick, however your wall looks to be half of that or less.

However, Id like to see another photo or two of the area on the blue bowl showing the wall profile, and the area where the wall meets the floor of the pot. From the angle you posted; it looks like there is a definitive line which runs around the foot(in the cracked/unglazed area), and into the wall of the pot; looks to be the "center" of the crack/fault and where it meets the wall, it looks like there is a pyramidal/cone shaped chunk missing. Its possible that as you opened, you rolled the clay on top of the softer/wet mush that was sitting around your clay after you centered. If this wet/sloppy clay was then "trapped" inside of the wall, it could cause those cracks because that clay wasnt compressed well, and left wet. If this was the case; make sure to compress the clay firmly to the bat while centering, and while opening pull more from the bottom and not "roll" the top of the clay outwards. You can also compress the "doughnut" once you've opened; fingers in/out, pressure from top, force clay to bat/wheelhead, squish out any softer clay chunks.

I would agree that if you're only having these issues with some pots, and not others that have the same glazes on them, that its a forming issue, not a glaze one. That being said, you want to make equal comparisons; are pots that are fine (no cracks) the same thickness, same shapes, fired in same locations, etc, etc, as the ones which are cracking? Your glaze thickness doesnt look to be overly thick.

Looks like you compressed after you trimmed, but if your not, a finger/rib/etc run back over your trimmed areas removes the scars from trimming, and compresses the clay further; especially important on thick bottomed pots, which are difficult to get enough compression all the way through, while throwing.

Share this post


Link to post
Share on other sites

First two pics too thin on trimming floor of foot of foot.

Blue pot thickness left when trimming wall to foot ring.... you didn't post this pot initially so did crack running around foot occur in your smashing of the pot.

These cracks occurring with  2 different glazes but may be happening for same or different reasons. Dunting.

Hamer and Hamer  goes into cracking really well. See if you can borrow a copy.

Share this post


Link to post
Share on other sites

Try to trim like the first three examples  (badly drawn—my hands are a little cooked today), so that you’ve got the same thickness throughout the same pot.  The foot ring shouldn’t create any thick spots where it joins the rest of the pot.

The two X’d out show examples of pot bottoms that are likely to cause problems, either in drying, firing, or as weak points in use. They’re supposed to represent a bowl with an outer contour that doesn’t match the inner (bottom bowl) and a bowl with an extra thick ring around the base (middle right), which will cause a whole constellation of problems for all the reasons others have listed above.  Uneven drying stress, uneven firing and cooling stress (dunting), etc

Likely extraneous, but sometimes visual examples help learning.  When you trim, check your work by cutting a few in half.

572FEA95-E1A7-4058-B482-9EFF064BCBE4.jpeg.05b3df9241de7983318b1e7fb51d19a0.jpeg

Share this post


Link to post
Share on other sites
2 hours ago, Babs said:

Hamer and Hamer  goes into cracking really well. See if you can borrow a copy.

The text Babs mentioned is a terrific reference book, The Potter's Dictionary of Materials and Techniques by Frank Hamer and Janet Hamer. The information regarding cracks is available in it's entirety in the preview here.

Share this post


Link to post
Share on other sites

Definitely uneven thickness of wall vs bottom and somewhere in between.

Maybe try throwing the clay upside down after the clay is wedged into a cone. Usually solves S crack problems, but may fix yours, who knows.

 

Share this post


Link to post
Share on other sites

Thanks all for the suggestions.  I agree the thickness is a concern and what I will focus on.  Looking at all the pots that have no problems they are an even or almost even thickness.  To be honest this wasn't something I previously thought about as being an issue.   Such a shame the pots really came out nice except for the crack.  I also ordered the book so will read that once it arrives. 

Its a shame I really like this vase :-(

DSC_0109_small.jpg

DSC_0107_small.jpg

Share this post


Link to post
Share on other sites

Are the ones that broke also unglazed on the outside with sodium silicate decoration like that?  If so it could be uneven glaze coverage pulling the pot apart.  Would pull the glazed inside toward itself and partially imploding.  Have had that happen myself more than a few times.

Share this post


Link to post
Share on other sites

No some are unglazed and others are glazed.  The blue mixing bowl was glazed on the inside and out.  I thought it might be the white glaze I was using inside but I had the same results with an opal glaze.  I never have cracks anywhere except for the bottoms.  You can see the bottom is fine from the outside on this one.   I think I have picked up a bad habit somewhere along the way.  3 in 5 pots have this issue.

The foot is not an even thickness.  I don't think I am compressing enough or evenly.  My bowls dont crack but I spend a good deal of time making sure they are smooth and rounded.  I don't do the same with my vases and I think that is the start of the problem.

Share this post


Link to post
Share on other sites

Lsaw got it on the unglazed part putting stress on a thinkness issues as well . Either can cause this but both may be to much. Many good sugestions in above post as well.

I think more attention to wall thickness water left in bottom of pots and unglazed un even thickness all can be issues.And as you noted compression issues which will get you always in the end .

Edited by Mark C.

Share this post


Link to post
Share on other sites

Think you need to look at the design of your feet:-)))

You are turning a bit of clay out of centre and leaving a lot of clay all around it......maybe focus on how folk make feet...the aesthetic of how the outside interfaces with space around can be enhanced with a little thought and will prob. help your problem  Now folks show us your feet:-)))

Edited by Babs
Autoprompt so clever...

Share this post


Link to post
Share on other sites
11 minutes ago, Babs said:

Think you need to look at the design of your feet:-)))

You are turning a bit of clay out of centre and leaving a lot of clay all around it......maybe focus on how folk make feet...the aesthetic of how the outside interfaces with space around can be enhanced with a little thought and will prob. help your problem  Now folks show us your feet:-)))

Wow I just looked through my pictures and the feet are something I guess I haven't taken pictures of!  My feet are pretty gross though, not sure you want to see them

Oh well, took a picture anyway since I'm doing dishes haha

IMG_20190204_212425-1209x1612.jpg

Edited by liambesaw

Share this post


Link to post
Share on other sites

Nice vase Mike Kelly! ...how'd you get that pinkish tone?

Good foot Liam

https://www.youtube.com/watch?v=VgGwI12zMJg  (It's a theme - foot problems - them things at th'end o'm'legs, that is).

Ahem, could the lil' spots be related? Are they voids/gas pockets or solid-ish inclusion of some kind? Note also, appears the glaze is thicker on the outside than inside.

I'm also curious how big is that pot where we see a hand in the background?

fwiw, I'm burnishing m'pot feet - the big flat, bottom and outside of ring with flex metal rib (an' round the corners, aye), the inside and inside corner with tip of small curved ribbon tool - in belief that it enhances strength, reduces absorbency, an' looks nice. ...oh, Sam said that a'ready... 

 

what's thaat.JPG

Share this post


Link to post
Share on other sites

I think that they're too thick over all, and too uneven. When you have a flat bottom on the inside of the pot, really work at defining that corner where the bottom meets the wall. Get it crisp and tight using the corner of a wooden rib. A rounded, sloping transition can cause problems. Also really focus on getting things thinner and trimming them more evenly. Some clay bodies can handle it better than others, but smooth white bodies do not like these issues.

Share this post


Link to post
Share on other sites

I try to make my feet the same thickness as the rim, max 1.5 times the thickness of the rim.  And following the curve of that bowl you have a lot of excess you could have cut out still.  These are goals of mine too, something always I'm working on is even thickness

Share this post


Link to post
Share on other sites

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.

Edited by Babs

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use.