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How Strong Is Unfired Clay?


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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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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)

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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.

 

Irene

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My 9th grade sone has finished his sculpture for his class with clay.  He cannot go anywhere to get it fired now with everything closed.  It is ready to be fired.  Can I try out firing it in our fire pit?  How long and what kind of wood/fuel?  He has to paint it before he turns it in.  It is due in 10 days.

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Hi and welcome @Leugh!

If you have no experience firing anything, pit firing a piece to the point where it actually converts from clay to ceramic would not be my choice of places to start. I’d consult with the instructor to clarify some expectations. If it’s a sculpture, it would be possible to paint it with acrylics dry, but it will suffer from durability issues and shouldn’t be considered permanent. 

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2 hours ago, Leugh said:

My 9th grade sone has finished his sculpture for his class with clay.  He cannot go anywhere to get it fired now with everything closed.  It is ready to be fired.  Can I try out firing it in our fire pit?  How long and what kind of wood/fuel?  He has to paint it before he turns it in.  It is due in 10 days.

If you tried firing it in a fire pit, chances are he'd have nothing to turn in.  Pit firing is very stressful for clay, so there's a high risk of it blowing up or breaking into many pieces.  

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On 5/5/2020 at 1:00 PM, Leugh said:

My 9th grade sone has finished his sculpture for his class with clay.  He cannot go anywhere to get it fired now with everything closed.  It is ready to be fired.  Can I try out firing it in our fire pit?  How long and what kind of wood/fuel?  He has to paint it before he turns it in.  It is due in 10 days.

How can the instructor expect work to be fired?!  I can't even require work, of any type, from my students.

Regardless, do not try and fire it, unless you have access to a proper kiln. 

On 5/5/2020 at 3:04 PM, liambesaw said:

If you tried firing it in a fire pit, chances are he'd have nothing to turn in.  Pit firing is very stressful for clay, so there's a high risk of it blowing up or breaking into many pieces.  

Yeah, as I mentioned earlier in this topic, years ago, I tried pit firing raw clay years ago, and almost nothing survived. 

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