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Knobs detach during bisque firing


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I am having problems with my knobs.  They detach during the bisque fire about 50% of the time.  I score both pieces and use a joining slip that works well on my mug handles (same clay as body, water, sodium silicate and sometimes some ground up tissue).  They appear to adhere well but when they come out of the firing they have come apart.  Picture below.  What are some of the things that might cause this?

knob.jpg

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I used to have this problem on small knobs of clay I would add to lids (at leather hard) and carefully throw.  The lid and knob are at too different stages of drying.  Another potter showed me a simple trick once that has worked every time.  I take the blunt end of a needle tool (1/4" round) and push up through the lid into the newly added handle.  I don't mind the small hole on the inside of the lid and have not had a single handle/knob detach since. 

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This is a common deal with clay not well attached. There are many ways to attach knobs. I throw mine on the lid. Your looks like two-three things going on not enough scoring and not enough slip and maybe two uneven moisture contents. I would score deeper-use real slip made from the clay body and cover lid to all clay gets to the same mositure content (at least one night)

 

Edited by Mark C.
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Hi Rick!

Doesn't look fully stuck to me. Did you see the recent thread where potter is attaching and joining slabs? The piece depicted a building/domicile, if I recall ...any road, similar appearance where the join had failed.

When joining, I'm wetting the ware - typically, the bigger piece is dryer, eh? -  where the join will be, just a damp finger, then I'm watching the shine fade as the moisture is absorbed. I might dampen that spot again, depending. Then I'll score it and dampen again. Whilst that shine is fading, I'll prep the to be joined bit - which is typically wetter, eh? - which I score in a radial pattern, then load with a dab of thick-ish slip. I save off some hand muck in a bowl whilst throwing just for the next day's joining. "Properly" dampened, the join doesn't need slip, my opinion/observation; the slip squeezes out of the join (ah, the radial scores), and from there I'm smoothing it with a wet finger and/or wet brush, leaving a small fillet. I find that easier and faster than wiping the edges together, and I prefer the look of a small fillet over leaving a smooth crease. If I wasn't aiming for the small fillet, there wouldn't be any need for the slip, as the clay on both sides was wetted.

Any road, both sides hafta be wet/damp enough to mush together; by "sides" I mean the actual clay, not whatever is applied on top. Some potters don't score, some don't use slip; I'm not seeing anyone joining dry bits.

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1 hour ago, fergusonjeff said:

I used to have this problem on small knobs of clay I would add to lids (at leather hard) and carefully throw.  The lid and knob are at too different stages of drying.  Another potter showed me a simple trick once that has worked every time.  I take the blunt end of a needle tool (1/4" round) and push up through the lid into the newly added handle.  I don't mind the small hole on the inside of the lid and have not had a single handle/knob detach since. 

Very interesting!  You push up thru the leather hard clay?  Do you do this right after attaching the knob?  Any idea why this works?

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52 minutes ago, Hulk said:

Hi Rick!

Doesn't look fully stuck to me. Did you see the recent thread where potter is attaching and joining slabs? The piece depicted a building/domicile, if I recall ...any road, similar appearance where the join had failed.

When joining, I'm wetting the ware - typically, the bigger piece is dryer, eh? -  where the join will be, just a damp finger, then I'm watching the shine fade as the moisture is absorbed. I might dampen that spot again, depending. Then I'll score it and dampen again. Whilst that shine is fading, I'll prep the to be joined bit - which is typically wetter, eh? - which I score in a radial pattern, then load with a dab of thick-ish slip. I save off some hand muck in a bowl whilst throwing just for the next day's joining. "Properly" dampened, the join doesn't need slip, my opinion/observation; the slip squeezes out of the join (ah, the radial scores), and from there I'm smoothing it with a wet finger and/or wet brush, leaving a small fillet. I find that easier and faster than wiping the edges together, and I prefer the look of a small fillet over leaving a smooth crease. If I wasn't aiming for the small fillet, there wouldn't be any need for the slip, as the clay on both sides was wetted.

Any road, both sides hafta be wet/damp enough to mush together; by "sides" I mean the actual clay, not whatever is applied on top. Some potters don't score, some don't use slip; I'm not seeing anyone joining dry bits.

I will put this to work right away.  Thanks for all your help and advice.

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1 hour ago, Mark C. said:

This is a common deal with clay not well attached. There are many ways to attach knobs. I throw mine on the lid. Your looks like two-three things going on not enough scoring and not enough slip and maybe two uneven moisture contents. I would score deeper-use real slip made from the clay body and cover lid to all clay gets to the same mositure content (at least one night)

 

Thanks

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You might set up a few tests - to destroy, err, check (both!) later on; try tearing apart when cheese (sharp cheddar) hard, almost dry, fully dry. Learn by doing - wrecking is part of doing; it's going to happen anyway, take control of the destruction, arrrr!

If you watch vids, you'll see other potters' joining methods do vary...

I typically make two lids for every opening (credit Bill Van Gilder) - the extras come in handy: one fits better than t'other; one gets oops'd; customer oops'd; lids for coffee cups - some folk like that; use as test tiles... and etc. This is where standard sizing can really help! I have a box of lids to nearest half inch; now I'm doing 60, 80, 100mm...

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I feel like it is physically impossible for dryness differences to cause this seperation in these instances. 

You are applying moisture to this very place of detachment, so moisture will be even there, moving more into the drier piece. Where it stops moving into the drier piece, there will be an expansion difference there, so the crack from 2 seperate drynesses will be somewhere further into the drier piece, or the piece that dried faster after attachment.

Handles that attach twice are the reason for this "myth" of 2 drynesses. Different shrinking beast.

That's with scratch and water though. Not sure what reactions your "soup" is creating, but I still wouldn't blame the soup. I would work simpler though, scratch and water droplets.

Are you pressing up against the bottom while attaching?

In the thread Hulk brought up, I blamed trapped air for not allowing a connection. Yours looks to be that bright white spot.

I like the push up through trick, but it sounds like a bandaid, and an extra step.

Sorce

 

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If a knob or a lid is attached properly, if it breaks off it’ll be on either side of the join. If it pops off like this and you can see where you scored, you didn’t apply enough pressure or friction (little wiggles). Whether you’re using water or slip, you need to gently push and wiggle the piece you’re attaching until you feel it grab.

No grab=not attached. 

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9 hours ago, Callie Beller Diesel said:

If a knob or a lid is attached properly, if it breaks off it’ll be on either side of the join. If it pops off like this and you can see where you scored, you didn’t apply enough pressure or friction (little wiggles). Whether you’re using water or slip, you need to gently push and wiggle the piece you’re attaching until you feel it grab.

No grab=not attached. 

Sounds like good advice.  Thanks Callie.

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Using paper clay  slip made from the same clay body can help with attachment when there is some degree of difference in moisture content in the pieces being joined.  Also, covering the piece tightly in plastic for the first day after attaching  will let the moisture equilibrate between the two pieces.

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