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Handle cracking question


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Hello, I am a ceramics instructor and I have been making ceramics for over 30 years. I use Laguna clay. Recently I have a new issue with their bmix. I am having over 1/2 of my handles on my mugs crack at the join. This has never happened before. I dry extremely slowly. In magic boxes for over a week then in plastic wrap for over a week. I never used slip with vinegar before but tried it when this started happening and it still cracks. I also never waxed my handles but tried that as well. Is anyone else having issues? Or does anyone have any suggestions? Did they change their recipe?

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Liz, just want to know something about your slow drying.   if i have understood the situation with regard to handles and additions, the purpose of covering the final piece is to allow the water content in each part to equalize so the  whole cup can dry without cracking.

if that is true, why do you slow down the entire process by keeping finished work under cover for weeks?  surely a day or even two days is enough to equalize the moisture content.  what is the point of covering so long?

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     @Liz Comay     Is the handle actually cracking or cracks appearing at the joint?

Handles pulled or coils and flattened?

I ask because I suddenly had a batch of mugs where the handles cracked , not the attachment. I thought I was following same old same old. Rethinking, only difference was I placed the handles on hardieback instead of a wooden board with newspaper on it.

Went through to glaze fire before I was faced with the cracked handles awaited me when I cracked the kiln.

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  • 1 month later...

I'm having the same issue, pulled handles cracking as the mug/handle dry. I work in porcelain. I've tried slow drying, wax over the handle and joint where it attaches to the mug, thicker handles, thinner handles..... And I'm still having about 60% of my handles crack as they dry. Any thoughts or suggestions would be greatly appreciated.

Thanks!!!!

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B mix is notorious for uneven drying-hence cracking (B mix is white stoneware)

I produce tons of handled mugs in  C 10 porcelain (Daves-from Laguna ) with very little cracking-about 100 a week currently

I make my slip from my throwing slop off my hands and splash pan add a very small amount of vinegar now and then

I score the join with a serraeted tool and add the slip -join-then cover the 5o mugs with plastic for the night. Uncover them next day and dry.

if I have any small cracks I just rub it out when dry with a sharp wood tool.

now cone 5 clays are very diffent  I suspect and I have never used them. More fluxs are used and I'm unclear about that in the drying stages

I used to throw large B mix pitchers and always used daves porcelain for the handles with zero issues . The porcelain dries more evenly

B mix drys unevenly-the rim is dry and the bottom of pot is damp. I like porcelain as its all quick drying not uneven

I would call the Laguna clay tec and see whats going on-they may help and may not-no harm with more info

Edited by Mark C.
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Hi everyone :)

I use a cone 6+ porcelain from a local supplier and attach with a process and slip much like described by Mark C.. I've talked with the supplier but am still having problems. Mugs usually are covered about 30 minutes after I throw them and handles are attached the next day. The clay body is pretty soft and needs the overnight time to setup enough to be handled (no pun intended). The handle cracking seems to be caused by shrinking. 

Thanks!!!IMG_1999a.jpg.d573fbe98ab73de3ec632c0be92ae6cd.jpg

 

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From Hamer and Hamer, those type of cracks look like they are from one of two reasons. Either the clay is too weak, perhaps from overworking or the handle is too thin. Doesn't look like the latter. Are those handles cut with a handle ribbon tool or extruded or ? Are you using recycled clay for them? How dry or wet are they when you apply them? How dry are they when you shape them?

Snippet from Hamer and Hamer cracks here with a drawing of similar crack, type "T".

Edited by Min
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If you are allowing the handle to set up (dry) some before attaching to the leather hard mug, do you pre-shape it?
Setting the arc(s) earlier may help.

I pull handles and hang them off the edge of the splash pan, hence, part of the curve is learned from the start.

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On 8/3/2022 at 10:11 AM, Liz Comay said:

 Did they change their recipe?

In answer to your question about changing the recipe. . . April of 2022, Laguna says they had to change the talc used in Bmix. They state that the only change should be a slight change of color from grayish to more cream. Link is here:

https://www.lagunaclay.com/_files/ugd/e5330f_3caae3bfde7b4456a33ea0b74f0890fc.pdf

best,

Pres

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On 9/23/2022 at 12:51 PM, jrklark said:

Hi everyone :)

I use a cone 6+ porcelain from a local supplier and attach with a process and slip much like described by Mark C.. I've talked with the supplier but am still having problems. Mugs usually are covered about 30 minutes after I throw them and handles are attached the next day. The clay body is pretty soft and needs the overnight time to setup enough to be handled (no pun intended). The handle cracking seems to be caused by shrinking. 

Thanks!!!IMG_1999a.jpg.d573fbe98ab73de3ec632c0be92ae6cd.jpg

 

I think these are just snapping because the handle is being applied to dry. To me with the clean break it looks like it was stressed when bending the shape and then as it dried the stress caused it to crack. There is really no other reason a handle would snap at this location besides stress during application. Cracks from a handle from drying normally would be around the application point, where the body of the cup is pulling moisture from the handle.

I would try to apply your handle a bit sooner, no amount of slow drying is going to save a fractured piece when being bent.

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On 9/23/2022 at 11:51 AM, jrklark said:

The handle cracking seems to be caused by shrinking

Interesting the attachments are intact yet when the material shrunk it exceeded the strength of the clay rather than bend during shrinkage. Better clay, thicker form all possibilities but handles should also have a way to shrink and uniformly bend a little as they shrink. Simple C form handles are often great examples as the perfect C often will collapse a bit into an over bent C but the handle because of its shape takes a bit of the axial stress and bends along the spine of the handle. Without getting too nerdy, some handle shapes allow the distribution of axial stress more evenly. So stronger clay may be the answer but handle shape and pre shape are something to experiment with. As others have likely said, good practice throughout production and attachment are always important

some handle shapes have a bit of an over loop at the top which can distribute this stress along the clay better than having all the force converge into one spot. 

A little illustration of how we allow for this stress in pipe work below. Very similar to a C shaped handle.

 

 

C985C24D-FF84-4C94-8706-4F8E3027B407.jpeg

Edited by Bill Kielb
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Generally for typical domestic water type stuff loops, offsets, changes in direction allow the pipe to shrink and grow. Fastening specifications vary but generally allow uniform support and axial movement.

Here is one of many guides out there: https://www.flowguard.com/blog/how-to-account-for-pipe-expansion-in-a-plumbing-system

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Hi everyone, and thank you for all the good information!!

General responses to questions from people's postings:

I use both recycled clay (broken up dried clay bodies, slip from throwing added to rehydrate, mixed, dried and wedged on a concrete slab), and new clay.

Handles are pulled not extruded.

I let the pulled ribbon"hang" from my workbench just long enough to have it maintain shape once attached to the mug. I usually start attaching handles about 20 minutes or so after pulling the ribbon.

I do not "preshape" the handles. My process is: take a ball of clay and roll it in my hands to make a cylinder, using water pull into a ribbon, hang from the edge of the workbench for about 20 minutes, lay the ribbon on the bench and cut to proper length, score and apply slip to mug, score and apply slip to ribbon, attach ribbon to top of mug, bend ribbon and attach to bottom of mug. If ribbon is too soft I will use put strip of newspaper, dipped in water, over the outer curve of the handle and into the mug to support the handle while it dries.

Min: What do you mean by "overworking the clay"?

I have started experimenting with "C" shaped handles, but not in love with the shape :)

Bill: What do you mean by "a bit of over loop at the top"?

Again, thanks everyone for the thoughts and suggestions. This has been driving me a little crazy for some time now!

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6 minutes ago, jrklark said:

I use both recycled clay (broken up dried clay bodies, slip from throwing added to rehydrate, mixed, dried and wedged on a concrete slab), and new clay.

 

I would stick with just using new clay when making handles as clay scraps from throwing can be short on the fine particles that aid plasticity. Over exaggerate bend in the handle when making it,  clay memory will want to straighten out the curve as the handle dries. Wedge, make the handle blank, either like a carrot for a single handle or a tapered lump for pulling multiple handles, pull the handle then stick it by the thick end to a table edge (or wherever) then when it's lost it's stickiness and you can pick it up and handle it attach it to the pot. Don't leave it longer than necessary.  Your joins look good and secure. Cover the pots and leave them to dry slowly. Some people dry mugs right side up, others upside down, try both and see what happens.

12 minutes ago, jrklark said:

Min: What do you mean by "overworking the clay"?

If you use recycled clay plus spend forever pulling the handle you will be removing a lot of those plastic fines and the clay will loose it's flexibility.  When throwing overworking a clay can cause it to get overly wet and will become floppy and likely collapse. 

16 minutes ago, jrklark said:

I have started experimenting with "C" shaped handles, but not in love with the shape

Handle in your image above looks fine for a mug handle, shouldn't have to change the shape.

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Ok, now that you’ve explained your process more I’m confident that bending the handle long (20 minutes or so) after you pulled it is the culprit. Like Min said, over bending it before you attach may solve this or I think putting a curve in it right when you pull it will prevent that unusual crack. The ribbon is trying to go back to its straight state, the attachments are solid so it gives in the middle. It may take a few tries, but giving it a couple pulls to train the curve into it after you’ve attached the top and before you attach the bottom is worth trying. That “bend ribbon and attach to bottom” step should include a couple of pulls.

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