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How Strong Is Unfired Clay?


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#1 JenniferG

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Posted 17 June 2014 - 09:19 AM

Say about 1/2 inch thick.. dried for a couple weeks.

 

Then painted with non food safe paint  (a decorative piece).

 

I made my couple pinch pots and was wondering if they'd hold up if they were not bisque fired nor glazed.. but rather painted with some paint.



#2 Rebekah Krieger

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Posted 17 June 2014 - 10:14 AM

do you mean you do not plan to fire it at all? 


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#3 Stellaria

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Posted 17 June 2014 - 10:34 AM

If you don't bump it against anything, or get it wet ever, it will last just fine, like any lump of dried mud you'd sit on a shelf.

#4 Celia UK

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Posted 17 June 2014 - 05:15 PM

If you just want to look at them, then fine. For any practical use, they'd be pretty fragile - almost certainly break if dropped on a hard surface, easily chipped if knocked with something hard.

#5 PSC

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Posted 17 June 2014 - 05:28 PM

You wouldn't want to hold anything in them, cause any use would make them crumble.

#6 Bob Coyle

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Posted 17 June 2014 - 07:40 PM

Paint them first with a thick coat of white glue. Let it soak in and set up and then paint with whatever pint you want. Not as good as bisque but better than painted raw clay.



#7 JenniferG

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Posted 18 June 2014 - 01:12 PM

Thanks.  I like the glue idea as well :)

 

Would the clay be stronger if I fired in my BBQ pit at 800F?  I can get it about that hot pretty easily.



#8 Benzine

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Posted 18 June 2014 - 04:01 PM

Thanks.  I like the glue idea as well :)

 

Would the clay be stronger if I fired in my BBQ pit at 800F?  I can get it about that hot pretty easily.

Nope, you'd probably just succeed in destroying the ware.  Clay doesn't lose it's chemical water, until over 1000 F, which is what initially makes it stronger.  Until that point, it will still absorb water, and remain brittle.  

I say it will destroy the ware, because unless you slowly increase the temperature, it would explode, spall, or at least crack.  It's possible, just difficult.  There was a video post here recently, of someone using a grill to fire some wares.  I don't recall if they were bisqued first though.


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#9 Bob Coyle

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Posted 18 June 2014 - 07:26 PM

Benzine is correct... 800F  won't do it. If you could completely surround the piece with glowing coals, you might get enough heat for a bisque, but unless it was taken up to temp slowly, it would blow apart.



#10 Benzine

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Posted 18 June 2014 - 08:34 PM

Bob, I learned that the hard way, by trying to pit fire greenware.  Spalling, spalling everywhere....


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#11 Mark C.

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Posted 18 June 2014 - 10:16 PM

They are not very strong-keep them up on a shelve -they are a bit stronger than dried mud with paint on it.

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#12 GavJ

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Posted 19 June 2014 - 02:30 AM

It would be stronger if you made adobe instead of just raw clay. I don't actually know if it would be workable enough, though.

Make maybe something along the lines of 70% clay, 15% sand, 15% (quite finely minced) fiber?  This is not what they use for adobe, but that's because they make big crude rectangular bricks and walls, not little pinch pots, so I'm guesstimating it closer to the raw clay.

 

You could then use very small amount of heat to burn off surface straw that would mess up your finish, then paint. And it should be quite a bit stronger than just greenware. IF you can actually still make what you want, which you might not be able to. If it is workable enough, though, I'd try to increase the proportions of sand and/or fiber until it just barely isn't, and work near there.

 

 

 

Also, firing clay in the coals of a barbecue is one of the few things I actually have real experience with and have done some decent amount of reading on (well, pits more so): low fire earthenware clays will generally hold up BETTER than fire clays or porcelain, due to usually higher thermal shock resistance. So your backyard clay might work in the barbecue stressfully quick firing while fancy white store bought clay might not, somewhat counterintuitively.  Tempering the clay will also help. Even the same recipe above would help quite a bit to prevent damage from rapid, short firing: the sand is just grog, and the fiber will burn off and allow porosity and quicker heat penetration and thus more even heating and less breakage.

 

When I have done this, it's still brittle (as I have been led to understand in another thread, probably mostly because I didn't hold it at heat long enough and/or heat slowly enough), but it's WAY better than greenware still. Probably takes 3-4 times as much force to break the same piece "fired" this way than its greenware equivalent (without tempering)



#13 TJR

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Posted 19 June 2014 - 07:52 AM

I say bisque it, then paint it if you wish. Someone will eventually knock it off a shelf. any high school can bisque it for you, or take it to a scrape and bake place.

TJR.



#14 clay lover

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Posted 19 June 2014 - 08:38 AM

TJR, what is 'scrape and bake' ?



#15 Mudslinger Ceramics

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Posted 19 June 2014 - 06:00 PM

Try a fine raku or sculpture paperclay mix from a ceramics supplier which will be strong enough if built on a wooden batt and not moved off it

......or get some dried out clay add 50/50 paper pulp or clean paper cat litter, add into hot water and mix thoroughly.  Next morning knead and wedge  before using.  Both these paperclay mixes can be quite strong if not moved too often and can be fired later when you have access to a kiln.

 

Paint a good layer of PVA glue to seal, let dry and then paint with acrylics or other decorative paints.  Paper fibres help bond the clay particles and make a stronger, but not permanent or unbeakable, clay body.

 

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#16 Stellaria

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Posted 19 June 2014 - 07:02 PM

Pretty sure the OP just has a couple pots they want to keep, and isn't looking to make further quantities of unfired ware?

#17 Diesel Clay

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Posted 20 June 2014 - 10:27 PM

Scrape and bake, granny green ware, paint your own pottery places. See also, "oh, my neighbour does ceramics"...
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#18 John Hertzfeld

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Posted 21 June 2014 - 05:17 AM

Don't know if you'd want to change materials, but if you aren't going to use the pots consider air dry clay, it is a product that hardens as it dries and shouldn't break if bumped off a table.




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