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Bone Dry

Beginner Bone Dry

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#1 Jodae

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Posted 11 February 2014 - 06:06 PM

So as  beginner, I am going through the growing pains of not enough education in ceramics. My pots are really thick walled and heavy, just due to the learning process and insecurities with clay and its fortitude.  I have had many pieces "die" in the kiln due to many factors - air bubbles, not enough wedging... I have had the kiln operator at our local guild leave pieces out of the firing for me - thank god, due to the fact that they are not "Bone Dry". How important is the fact they are not "Bone Dry" before bisque firing and how do I tell if they are or not? Im new to my local climate, the other members tell me  their experience is about 1 week of drying time is good....But I am a scientist, love details and facts, figures, and would love a quick lesson on how to tell if the moisture in my clay has evaporated enough in order to go to bisque firing....

 

Thanks for any feedback for my "beginner" questions.



#2 neilestrick

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Posted 11 February 2014 - 06:20 PM

First, air bubbles and poor wedging do not make pots blow up in the kiln. Both will make it more difficult to center and pull the clay, but neither will cause explosions. The only thing that causes explosions in the kiln is moisture. Air only expands about 1.7 times as it heats up (if I remember right. Not much, anyway), which is not enough to blow apart the clay. But water turning to steam expands 1700 times, which is more than enough to blow it up. So yes, you do want your pot to be bone dry before it goes into the kiln.

 

But even bone dry pots can blow up, because they still have a little bit of water in them. It's impossible for the clay to have 0% water in it if it's sitting in a room with 40% humidity. So when the pot goes into the kiln, the remaining water must evaporate out before it turns to steam inside the clay and blows up the pot. In thin pots, this happens easily, as the heat doesn't have to penetrate very far to warm up the clay and quickly evaporate out the water. But in thick pots it takes longer to heat up the clay, and the kiln can get hot enough during it's initial temperature rise to cause the water to turn to steam before it has a chance to evaporate out.

 

So, try to get your pots down to 1/4" thick, and even in thickness. Dry them upside down so they dry faster and more evenly. Fire them upside down, too, to heat more evenly. If they are too thick, the only way to keep them from exploding is to slow down the firing, or do a preheat  at 200F to dry them out.


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#3 Jodae

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Posted 11 February 2014 - 06:45 PM

Thanks so much for the speedy reply, I'm looking at my pinch pot sphere here, thinking that I am still too thick on my walls, but as confidence and knowledge increases, the thickness will decrease! Might seem like a crazy question, but could I use an old toaster oven to pre-dry at 200F before bringing it in to the local guild for bisque? Would it be evenly heated enough? Maybe I'll try thanks to the local thrift store's toaster ovens galore and post the results here !

#4 Marcia Selsor

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Posted 11 February 2014 - 07:43 PM

Just turn them up side down to allow the bottom to dry. It is good to keep flipping pieces as the dry. When you think it is dry, hold it to your check and should NOT feel cool. You can lick the bottom and the mark should quickly disappear.
Those are just some simple ways to know. But if they are really thick, they may not work as well.

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#5 Benzine

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Posted 11 February 2014 - 08:16 PM

Just turn them up side down to allow the bottom to dry. It is good to keep flipping pieces as the dry. When you think it is dry, hold it to your check and should NOT feel cool. You can lick the bottom and the mark should quickly disappear.
Those are just some simple ways to know. But if they are really thick, they may not work as well.

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#6 neilestrick

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Posted 11 February 2014 - 09:13 PM

No need to go through the work of oven drying them. Just make sure you let them dry for a few days before they go into the kiln.


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#7 Nancy S.

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Posted 11 February 2014 - 09:25 PM

Another good way to tell if it's dry is to put it on a sheet of paper when you think it's dry. If it leaves a wet mark, let it dry another day.



#8 Pres

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Posted 11 February 2014 - 10:31 PM

Ive written before about having classes do pinch pots in the dark. Try doing your pinch pot only with your hands, don't look at the pot work the thickness evenly by feel of the clay between your fingers and thumb. With some practice your pieces will become thinner. Practice to the point that nothing is so precious that it can't be scrapped as future pieces will be so much better.


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#9 flowerdry

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Posted 12 February 2014 - 05:34 PM

Welcome to the forum, Jodae.  This is a great place for information.  Please let us know a bit more about yourself...Male/female...part of the country, etc.

 

Marcia, where can I get one of those checks that determines if pots are dry?


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#10 Norm Stuart

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Posted 12 February 2014 - 06:24 PM

I've always used the rule: if my tongue sticks to the ware, then it's dry enough to bisque.

 

I use a pre-heat on glazed items in lieu of licking each one.

 

Welcome to the forum, Jodae.  This is a great place for information.  Please let us know a bit more about yourself...Male/female...part of the country, etc.

 

Marcia, where can I get one of those checks that determines if pots are dry?



#11 Dharsi

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Posted 12 February 2014 - 09:47 PM

First of all, I don't recommend this practice with anything you care about! But that said :) last weekend I was doing some stamping on mugs and couldn't find my favorite stamp so just for giggles I made another and stuck it on top of the hot wood stove, Much to my surprise, it was dry in minutes still in one piece and the squiggle I had added to a post was still attached. I throughly expected the thing to crack or fall apart, or for the squiggle to pop off, but when it held up to use I got a little more bold and tossed it *in* the wood stove to see if I could blow it up.  Still nothing.   I made a few more and will bisque fire them the next time I do a load.  

I wouldn't recommend this with a larger piece or anything you cared about, but I'm trying to say experiment.  If you are a scientist then you know what I mean when I suggest that you try new things, push the tested boundaries and take notes.  One thing you can count on in ceramics is that nothing is always predictable or the same, and just when  you think you have it all figured out, ###### happens.  

The wonderful thing about clay is that it is reusable over and over, if you don't fire it.   I've learned valuable lessons by making and reclaiming pieces over and over.  I think I used and reused my first 50 pounds of clay for 6th months before being happy with enough pieces to bother firing up my kiln.  

My pieces always feel cold to me no matter how long they have been sitting on the shelf, so I go by how they look and feel in my hands.  

The only piece I have ever had blow up on me did so because I was doing another experiment.  I decided to try spraying glazes on greenware.  I usually bisque everything.  The piece was definitely dry when I sprayed it but I put it right in the kiln and programmed a fast glaze fireā€¦ dumb dumb dumb but hey, that's how we learn right?! I guess for me, the joy is in the journey.



#12 Pres

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Posted 12 February 2014 - 10:37 PM

You might try the cheek test. Put the piece against your cheek, if it feels cold leave it there for a while-if it warms, it is bone dry. If it stays cold still to wet to fire.


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#13 perkolator

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Posted 13 February 2014 - 06:44 PM

i do the "cheek test" with the back of my hand.  if it feels cold, there's still moisture inside.  If you are force-drying, there is always the risk of cracking and is usually due to either too rapid or uneven drying.  airflow (or better - warm air) is the best for drying out work as opposed to straight heat -- temperature, airflow, and surface area all effect rate of evaporation.

 

ultimately, yes, you could definitely put a wet piece of clay in the kiln and fire it without issue - the problem is that you will have to adapt firing schedule to accommodate the piece(s), and have a longer hold/preheat.  most likely with the firing schedule of your studio and peers' work, the schedule used doesn't work out in your favor and you get broken pieces.  In my studio, we typically once-fire everything (all sculpture).  Because students like to wait until the last minute to finish or glaze work.  It is not uncommon on kiln loading days to encounter work that's questionably past leather-hard, or literally just got a quart of glaze soaked into it, lol.  Luckily, I fire conservatively enough to seldom blow up a student's work.  I know, we're crazy and like to rock the boat with our technique, but it works :)

 

like Neil said above, your clay will ALWAYS have water in it until you fire the clay.  Even if you throw a tiny little shot glass and let it dry for a year, you will still have water inside due to the moisture in the air. If you dry your clay out in an environment with ZERO water in the air, you will STILL have water in your clay that could potentially make the piece explode....this is the CHEMICAL water that's molecularly attached to your clay particles.  This water is present until the clay gets fired past something like 500*F, and is a permanent change.  If you have a very thick piece of clay being fired at too fast of a climbing rate, the piece can and may explode during this window where the chemical water is being driven off.

 

 

Want to do a test?  Everyone always claims you can't put wet work in a kiln without it blowing up, if you go past boiling point (212*F) because the water turns to steam and blah blah blah we all know this explanation.  Try taking a piece of fresh clay (like say a 3"x12" solid pug stick) and put it directly into a kiln holding at 300-350*F, well above the boiling point of water.  Come back after a few hours to look for exploded bits of clay everywhere and you likely will NOT find it.  Sure, the piece may crack like crazy to let moisture out, but it won't explode the way you think it would.  It's actually quite fascinating IMO and might be very helpful when it comes to firing certain items.



#14 Bob Coyle

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Posted 13 February 2014 - 07:34 PM

You didn't mention where you were located. Drying will be much different in Alabama in the summer compared to Arizona.

Here in Santa Fe 1/4 inch pots dry in two days whatever the season. The cheek test works OK . Here if it feels cold it is still losing water.



#15 neilestrick

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Posted 14 February 2014 - 12:45 PM

Any time I have been near the kiln when a pot has blown up the kiln has been at around 500F. I'm wondering if it has to get that hot for enough water to turn to steam all at once to cause the boom.
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