[Potterylover]

Hi, I'm a newbie! About a month ago, I made a few small objects using terra cotta clay and let them dry all of this time in my studio on the second floor of my house. I live in Maryland and the weather has been varying between 30 degrees and 50 degrees outside. We keep the house at 68 F. So, today, I decided to do a bisque firing of this greenware and also, fired some smaller objects made from white clay that have been drying for about three months (I had forgotten about them.) My kiln is a Paragon Quickfire Kiln, which I have used for bisque firing before about five times, the last two firings including this one wasn't successful.

Today, when I fired these objects, two of the terra cotta objects exploded to pieces and so did one of the while clay objects. Again, these pieces ranged in size, but very small. I fired according to the kiln directions: fire to 500 degrees with a stilt propping open the hood, then removed the stilt to fire to 1950 degrees, cool to 1500 degrees and fire to 1950 degrees. At about 700 degrees, the first explosion happened, which was a small barely audible pop sound. At about 800 degrees the same sound again, so I turned it off and unplugged it. As it was cooling, there was a louder explosion, which caused "smoke" or clay powder to exhaust from the top of the kiln hood. Then, luckily nothing else happened.

One piece was completely shattered, made of terra cotta and the thickest of the pieces. Another had pieces that had exploded off of it. The last, a piece that was an inch long and a quarter of an inch think sheared in half length wise and broke apart.

All of the pieces seemed dry. Not sure what happened. Seems to me that it is most likely a combination of the drying not being complete and the kiln. (I'm going to be purchasing an electric digital kiln soon, which I hope will help with part of the problem.)

So, is there a good way to dry clay? How long should it dry for? I've heard that putting clay in a regular oven can help with drying...is that true?

Thanks for the help!

[clayshapes]

scahanin, on 22 January 2012 - 08:08 PM, said:

Hi, I'm a newbie! About a month ago, I made a few small objects using terra cotta clay and let them dry all of this time in my studio on the second floor of my house. I live in Maryland and the weather has been varying between 30 degrees and 50 degrees outside. We keep the house at 68 F. So, today, I decided to do a bisque firing of this greenware and also, fired some smaller objects made from white clay that have been drying for about three months (I had forgotten about them.) My kiln is a Paragon Quickfire Kiln, which I have used for bisque firing before about five times, the last two firings including this one wasn't successful.

Today, when I fired these objects, two of the terra cotta objects exploded to pieces and so did one of the while clay objects. Again, these pieces ranged in size, but very small. I fired according to the kiln directions: fire to 500 degrees with a stilt propping open the hood, then removed the stilt to fire to 1950 degrees, cool to 1500 degrees and fire to 1950 degrees. At about 700 degrees, the first explosion happened, which was a small barely audible pop sound. At about 800 degrees the same sound again, so I turned it off and unplugged it. As it was cooling, there was a louder explosion, which caused "smoke" or clay powder to exhaust from the top of the kiln hood. Then, luckily nothing else happened.

One piece was completely shattered, made of terra cotta and the thickest of the pieces. Another had pieces that had exploded off of it. The last, a piece that was an inch long and a quarter of an inch think sheared in half length wise and broke apart.

All of the pieces seemed dry. Not sure what happened. Seems to me that it is most likely a combination of the drying not being complete and the kiln. (I'm going to be purchasing an electric digital kiln soon, which I hope will help with part of the problem.)

So, is there a good way to dry clay? How long should it dry for? I've heard that putting clay in a regular oven can help with drying...is that true?

Thanks for the help!

[clayshapes]

Bizarre! I look forward to hearing what others have to say. I work with stoneware clay -- and I let my pieces dry 4-6 days (not weeks!), depending on the size of the piece and the weather. I have NEVER had an explosion in my kiln. I bisque and glaze fire about twice a month. I'm very curious to hear how others explain your disasters! (sorry this happened to you!)

[bciskepottery]

Sounds as if water in the clay turned to steam too quickly, was unable to exit the clay body, and exploded. There are two types of water in your clay, physical water and chemical water. You remove physical water by letting the pot dry on a shelf and by preheating your kiln load at 200F for a couple of hours. This pre-soak helps ensure physical water deep inside the clay -- which may feel dry to you -- is evaporated slowly. Chemical water starts to be removed at about 575 - 600F and continues through about 1100F. Chemical water is water that is formed as the heat of the kiln begins transforming the clay and molecules rearrange themselves, with some rearranging to water.

Preheat your kiln -- I usually preheat wares for a minimum of two hours and sometimes longer if wares are thick or damp. Then start your bisque firing program. Slowly build up to 500F to give your wares time to properly release water.

There is no need for a bisque load to be cooled to 1500F and then reheated to 1950F -- that is a glaze firing program.

[Mark C.]

The clay still had enough moisture in it to blow up. The climb rate was to fast for the moisture to escape both of these things together caused the problem
There are a lot of variables in drying out pottery-as well as firing it fast or slow
humidity will make a huge difference on how dry clay can get as well as temp.
You should get them dryer and fire slower at least till past 900-1000 degrees-then let it fly.
Mark

[Lucille Oka]

I read some of the operations manual for the Paragon Quick Fire as it pertains to firing ceramics. I have no doubt that your pieces were dry. I think the trouble lies in the firing method of your kiln. As soon as you switch it on it shoots up to 500°F. For ceramic greenware this is too quick.

Greenware needs a slow progression of about 60°F per hour for several hours until the interior temperature of your kiln is about 180°F. From 180°F you can proceed to ramp up the temperature a bit at a time. Your temperature rise was too quick.

According to the manual you have to turn the kiln on and off to slow down the temperature rise.

This kiln seems ideal for copper enameling where the quick rise in temperature is most desirable. But for greenware you must slow down the heat rise.

You can use a regular kitchen oven for a preheat but the need for a slow temperature rise is still the same.

[Marcia Selsor]

Both Mark and Lucille comment of the rate of firing which is probable. I don't know if you turn
things over or raise them on stick like pencils or chopsticks to assure the bottoms dry. I lived in Southern Illinois where it would take 4 days before I could trim pieces. I also lived in Montana where it could take 20 minutes! So much of drying depends on the local atmosphere.
I raise pieces on sticks when drying. For firing large work I made clay coils 1/4" diameter to set them on in the kiln to allow heat circulation and steam to escape.
Marcia

[Pres]

scahanin, on 22 January 2012 - 08:08 PM, said:

Hi, I'm a newbie! About a month ago, I made a few small objects using terra cotta clay and let them dry all of this time in my studio on the second floor of my house. I live in Maryland and the weather has been varying between 30 degrees and 50 degrees outside. We keep the house at 68 F. So, today, I decided to do a bisque firing of this greenware and also, fired some smaller objects made from white clay that have been drying for about three months (I had forgotten about them.) My kiln is a Paragon Quickfire Kiln, which I have used for bisque firing before about five times, the last two firings including this one wasn't successful.

Today, when I fired these objects, two of the terra cotta objects exploded to pieces and so did one of the while clay objects. Again, these pieces ranged in size, but very small. I fired according to the kiln directions: fire to 500 degrees with a stilt propping open the hood, then removed the stilt to fire to 1950 degrees, cool to 1500 degrees and fire to 1950 degrees. At about 700 degrees, the first explosion happened, which was a small barely audible pop sound. At about 800 degrees the same sound again, so I turned it off and unplugged it. As it was cooling, there was a louder explosion, which caused "smoke" or clay powder to exhaust from the top of the kiln hood. Then, luckily nothing else happened.

One piece was completely shattered, made of terra cotta and the thickest of the pieces. Another had pieces that had exploded off of it. The last, a piece that was an inch long and a quarter of an inch think sheared in half length wise and broke apart.

All of the pieces seemed dry. Not sure what happened. Seems to me that it is most likely a combination of the drying not being complete and the kiln. (I'm going to be purchasing an electric digital kiln soon, which I hope will help with part of the problem.)

So, is there a good way to dry clay? How long should it dry for? I've heard that putting clay in a regular oven can help with drying...is that true?

Thanks for the help!

This brings to mind the days of classroom projects where there was a mix of construction styles. My first question would be how things were constructed-hand built, or wheel thrown? This because all so often depending on the handbuilding process pieces may be of varied thickness requiring a much slower firing time than properly thrown wheel thrown pottery. I usually would water-smoke (prefire) a kiln for at least two hours with the lid open part way. This with the bottom switch on the lowest setting. This would drive out the atmospheric moisture in the clay. I would check the kiln with my hand before closing the lid(moisture in the atmosphere can be felt). After closing the lid I would not turn up for an hr, then turn next switch up to low. After an hr I would again feel the atmosphere from the top peep hole. If damp, let it be, if dry turn on the next switch to low. This checking let me get completely dry pieces to the firing. Then I would start my regular firing, at a slow bisque that would turn off about 6 at night.

[Chris Campbell]

An easy way to test for dryness is to hold the piece against your cheek ... If it feels cool it is not dry ... No matter how long it's been sitting.

[GEP]

scahanin,

You describe the pieces as "small" and that one of the exploded pieces was "the thickest," but exactly how thick were the pieces? In my experience, anything that is over 3/4 inch thick is in danger of exploding. Even if you let them dry for months, water has a really tough time escaping from a 3/4 inch thick piece of clay. Do you think any of your pieces was thicker than this?

Mea

[Wind n Wing]

Just a point of curiosity. You mention Terra Cotta Clay and White Clay, were they both the same cone for firing? If not that could also contribute to problems with your firing along with the other points that have been mentioned already.

[Arnold Howard]

scahanin, on 22 January 2012 - 07:08 PM, said:

My kiln is a Paragon Quickfire Kiln, which I have used for bisque firing before about five times, the last two firings including this one wasn't successful.

Today, when I fired these objects, two of the terra cotta objects exploded to pieces and so did one of the while clay objects. Again, these pieces ranged in size, but very small. I fired according to the kiln directions: fire to 500 degrees with a stilt propping open the hood, then removed the stilt to fire to 1950 degrees, cool to 1500 degrees and fire to 1950 degrees. At about 700 degrees, the first explosion happened, which was a small barely audible pop sound. At about 800 degrees the same sound again, so I turned it off and unplugged it.

How did the pieces from your latest firing differ from those in your first five firings? I suspect, as others have written, that the kiln fired too fast. The thicker the ware, the slower the firing needed. The QuikFire is very fast. I have fired it to 1000F in five minutes! One time I fired a self-supporting cone at full speed just to see what would happen. The cone never bent--it just bloated!

It would be possible to slow down the QuikFire with the PCB-1 Power Control Box. Or fire thinner ware. The QuikFire is popular mostly for glass and copper enameling.

Sincerely,

Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com

[Potterylover]

scahanin, on 22 January 2012 - 08:08 PM, said:

Hi, I'm a newbie! About a month ago, I made a few small objects using terra cotta clay and let them dry all of this time in my studio on the second floor of my house. I live in Maryland and the weather has been varying between 30 degrees and 50 degrees outside. We keep the house at 68 F. So, today, I decided to do a bisque firing of this greenware and also, fired some smaller objects made from white clay that have been drying for about three months (I had forgotten about them.) My kiln is a Paragon Quickfire Kiln, which I have used for bisque firing before about five times, the last two firings including this one wasn't successful.

Today, when I fired these objects, two of the terra cotta objects exploded to pieces and so did one of the while clay objects. Again, these pieces ranged in size, but very small. I fired according to the kiln directions: fire to 500 degrees with a stilt propping open the hood, then removed the stilt to fire to 1950 degrees, cool to 1500 degrees and fire to 1950 degrees. At about 700 degrees, the first explosion happened, which was a small barely audible pop sound. At about 800 degrees the same sound again, so I turned it off and unplugged it. As it was cooling, there was a louder explosion, which caused "smoke" or clay powder to exhaust from the top of the kiln hood. Then, luckily nothing else happened.

One piece was completely shattered, made of terra cotta and the thickest of the pieces. Another had pieces that had exploded off of it. The last, a piece that was an inch long and a quarter of an inch think sheared in half length wise and broke apart.

All of the pieces seemed dry. Not sure what happened. Seems to me that it is most likely a combination of the drying not being complete and the kiln. (I'm going to be purchasing an electric digital kiln soon, which I hope will help with part of the problem.)

So, is there a good way to dry clay? How long should it dry for? I've heard that putting clay in a regular oven can help with drying...is that true?

Thanks for the help!

Thanks to everyone for your guidance and help. I think the issue is my use of the Quickfire kiln and firing greenware as many of you have mentioned. The directions state that it is possible to fire greenware in it, but it has to be bone dry (which I thought these pieces were.) I do have a PCB-1 Power Control Box for this kiln, which I may try for firing some other greenware slowly and see what happens. The kiln isn't very practical for me overall, because even for bisque pieces, the kiln is too small and too short. I am happy to have all of the great pointers you've all given me...it has been a real learning experience. I have learned a lot from all of your posts.


Thanks!

[meisie]

Chris Campbell, on 23 January 2012 - 11:26 AM, said:

An easy way to test for dryness is to hold the piece against your cheek ... If it feels cool it is not dry ... No matter how long it's been sitting.

I've seen that you have said this before but my stuff is in my cellar. Unless I fire up the wood stove the cellar is about 50 degrees. I have some clay now sitting in my third bedroom (drying) which is not heated above 60 degrees. Won't that make a difference in the coolness feeling of the clay??

[rebbylicious]

meisie, on 23 January 2012 - 04:09 PM, said:

Chris Campbell, on 23 January 2012 - 11:26 AM, said:

An easy way to test for dryness is to hold the piece against your cheek ... If it feels cool it is not dry ... No matter how long it's been sitting.

I've seen that you have said this before but my stuff is in my cellar. Unless I fire up the wood stove the cellar is about 50 degrees. I have some clay now sitting in my third bedroom (drying) which is not heated above 60 degrees. Won't that make a difference in the coolness feeling of the clay??

I was going to joke "not if you live in wisconsin" everything is cold here!!!

[neilestrick]

Bone dry does not mean that there isn't still a bit of moisture in it. Pots cannot have 0% moisture because of the humidity in the air. So that last bit of moisture must evaporate out of the clay before it turns to steam. In a thick pot this takes longer, because there is more mass to heat up and the moisture has further to travel before it escapes the pot. But even in very thin pots it can cause explosions if the kiln is heating too quickly. You said that the explosions happened when the kiln was at 700 degrees. That's the air temperature, not the temperature of the pots. It takes time for the heat to penetrate the pots. That means that the temperature climbed so quickly that the air in the kiln got up to 700 degrees before the interior of your pots got up to 212 (when water turns to steam). That's way too fast. Typically, even in a fairly quick bisque firing, the explosions happen under 450 degrees or so. So slow it down, especially the first 500 degrees.