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Cracks In Slip


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#1 Shuli

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Posted 02 January 2014 - 12:58 PM

Hi all, would love some advice with the following problem.

 

I have recently been handbuilding mugs and I've tried using a thin slip to even out the interiors.  The slip is made from the same body as my clay, just strained through a mesh to take out grog and lumps.  It's the consistency of cream.  I pour it in when the mug is leather-hard, swirl it around, pour it back out and let the mug dry upside down for a while.

 

The problem is that the slip cracks around the edge of the base where it joins the sides.  The question is, what am I doing wrong?

 

Is the consistency of the slip too thick or too thin?

Do I need to add a deflocculant?

Am I applying the slip when the clay is already too dry?

 

I didn't have this problem when I was applying the slip with a brush, but I really like the smoothness of the poured slip layer and would like to figure out a way to use this technique without cracking!



#2 High Bridge Pottery

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Posted 02 January 2014 - 01:41 PM

This might be a stupid question but are you wrapping it up after applying the slip?


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#3 jrgpots

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Posted 02 January 2014 - 03:16 PM

1. Try applying the slip before leather hard state. Try placing the mugs in a sweat box for at least 3-5 days before starting to dry them. This would allow the clay and slip to reach equal hydration levels. Then Dry slowly.
2. If the base of the mug is too thick, it will dry more slowly than the walls, causing cracks in the slip.
3. Don't leave the mug on plaster. It would cause uneven drying and possible cracking.

Jed

#4 Shuli

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Posted 02 January 2014 - 05:35 PM

It's not a stupid question.  I do wrap the cup up in plastic immediately after applying the slip.

 

I think the bases of the mugs are a little thicker than the walls--I hadn't thought of that.  I'll try to make them more consistent.

 

I'll try the sweat box idea too.  Thanks so much!



#5 Nancy S.

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Posted 02 January 2014 - 05:42 PM

Sweat box? Is that like a damp box?



#6 Shuli

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Posted 02 January 2014 - 07:24 PM

I'm assuming that's what jrgpots meant--like a lidded container with plaster in the bottom.



#7 jrgpots

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Posted 02 January 2014 - 10:26 PM

Sorry for missed nominclature. Yes, I meant damp box.

#8 DarrellVanDrooly

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Posted 03 January 2014 - 02:02 AM

I'd recommend using the damp box as well. also, you might want to thin the slip slightly and try adding a little CMC gum solution to the slip. Since my studio is in colorado, everything dries extremely quickly, so cracking my biggest enemy. CMC is my solution to everything, and ive noticed that it sometimes prevents cracking and pinholing in slips and engobes.

 

Hope that helps!

 

Darrel


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#9 ayjay

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Posted 03 January 2014 - 05:35 AM

2. If the base of the mug is too thick, it will dry more slowly than the walls, causing cracks in the slip.
 

If the mugs are already drier than they should be when the slip is applied wouldn't  the thicker base be pulling more water out of the slip quicker?  

 

It's still going to crack so it doesn't really matter, but that's just the way I see it working.

 

If I'm using slip, (not often) I like to get it on when the clay is as damp as possible.






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