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Paper Clay Firing Schedule Needed


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I just got a used electric kiln and can't wait to use it.  Is there a good resource for firing schedules?I specifically need a cone 6 porcelain paper clay firing schedule. 

 

I would like to try a single fire schedule.  Has anyone had experience with single firing?

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Summary:
100â°C Atmospheric water turns to steam as the piece completely dries
300-800â°C Carbon, sulphur and organic materials burn off
573â°C Quartz inversion occurs
900â°C (∆011) Sintering begins
1060-1160â°C (∆06-3) Earthenware firing range
1170-1220â°C (∆4-6) Mid-fire range
1230-1350â°C (∆7-14) Stoneware firing range
220â°C On cooling only, cristobalite shrinks
 

First Stage: Atmospheric Drying

As a kiln is firing up and cooling down, the changes in temperature make some profound changes in the clay. The clay goes from this soft, totally fragile substance to one which is rock-hard, impervious to water, wind, and time. The change is nearly mystical in its complete metamorphosis, and might be deemed so if it were not so common.

When pottery is placed into the kiln, it is almost always bone dry. However, there is still water trapped within the spaces between the clay particles. As the clay is slowly heated, this water evaporates out from the clay. If the clay is heated too quickly, the water will turn to steam right inside the clay body, expanding with explosive effect on the pot.

By the time the boiling point of water (100â°C at sea level) is reached, the atmospheric water should have all evaporated out of the clay body. This will result in the clay compacting and some minimal shrinkage.

 

Second Stage: Burn Off of Carbon and Sulfur

Clay bodies all contain some measure of carbon, organic materials, and sulfur. These all to burn off between 300â° and 800â°C. If for some reason (such as poor ventilation within the kiln) these are not able to burn out of the clay body, carbon coring will occur, weakening the clay body considerably.

 

Third Stage: Chemically Combined Water Driven Off

Clay can be characterized as being a molecule of alumina and two molecules of silica bonded with two molecules of water. Even after the atmospheric water is gone, the clay still contains some 14% of chemically bonded water by weight. The pot will be substantially lighter, but with minimal physcial shrinkage.

This chemically combined water's bond loosens when heated. Overlapping the carbon and sulphur burn off, the chemically bonded water escapes from the clay body between 350â° and 800â°C. If the water heats too quickly, it again can cause the explosive production of steam inside the clay body. It is for all these changes and more that the firing schedule must allow for a slow build up of heat.

 

Fourth Stage: Quartz Inversion Occurs

Potters call it silica, but silica oxide is also known as quartz. Quartz has a crystalline structure that changes at specific temperatures. These changes are known as inversions. One such inversion occurs at 573â°C. The change in crystalline structure will actually cause the pottery to increase in size by 2% while heating, and loose this 2% as it cools. Ware is fragile during this quartz inversion and the kiln temperature must be raised (and later cooled) slowly through the change.

 

Fifth Stage: Sintering

Before the glass-making oxides begin to melt, the clay particles will already stick to each other. Beginning at about 900â°C the clay particles begin to fuse. This cementing process is called sintering. After the pottery has sintered, it is not longer truly clay but has become a ceramic material. Bisque firing usually is done at about 945â°C to 1050â°C (Cone 06 to 04 is recommended), after the ware has sintered but is still porous and not yet vitrified. This allows wet glazes to adhere to the pottery without disintegrating it.

 

Sixth Stage: Vitrification and Maturity

The maturation of a clay body is a balance between the vitrification of the body to bring about hardness and durability, and so much vitrification that the ware begins to deform, slump, or even puddle on the kiln shelf. Vitrification is a gradual process during which the materials that melt most easily do so, dissolving and filling in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body. It is also during this stage that mullite (aluminum silicate) is formed. These are long, needle-like crystals which acts as binders, knitting and strengthening the clay body even further.

 

Maturation Temperatures

The temperature a clay is fired to makes a tremendous difference. A clay fired at one temperature may be soft and porous, while that same clay fired at a higher temperature may be hard and impervious. It is also imperative to note that different clay bodies mature at different temperatures, depending on their composition. A red earthenware contains a large amount of iron which acts as a flux. An earthenware clay body can fire to maturity at about 1000â°C and can melt at 1250â°C. On the other hand, a porcelain body made of pure kaolin might not mature until about 1390â°C and not melt until over 1800â°C.

 

During Cooling

Quartz inversion happens again the quartz structure returns at 573â°C.

There is another event that clay goes through, this time as it cools. That is the sudden shrinkage of cristobalite, a crystalline form of silica, as it cools past 220â°C. Cristobalite is found in all clay bodies, so care must be taken to cool the kiln slowly as it moves through this critical temperature. Otherwise, pots will develop cracks.

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So a simple program that takes most of this into account:

 

150°C/hr to 100, hold 4:00-8:00 (allows complete burnout of atmospheric water)

150°C/hr to 400, hold 0 (nothing much happen so going quickly here is fine)*

80°C/hr to 600, hold 0 (slowing it down allows for burnouts of water and other materials and quartz inversion)

150°C/hr to 1100, hold 0 (nothing much again)

100°C/hr to 1220, hold 5 (or other changes around the peak depending on the cone and max temperature one wishes to reach)

120°C/hr to 935, hold 35 (slowed cooling and a hold around a change in iron)

 

*NOTE: I don't use paper clay but if I did I would consider slowing down the kiln around the time most of the pulp would burn out (~230°C)

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I do use paper clay in my Raku slabs. I let them dry for a few weeks. I fire them to 200 and hold for a few hours then fire a slow bisque schedule usually 8 more hours.Sometimes I take it to 200 and shut it off and let it set for a few hours. The whole cycles going up is about 12 hours.

 

 

Marcia

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the replies you have so far make me think they all believe you have a programmable kiln.  you have not said that.  is it a computer controlled kiln or one with a sitter only?

 

matthew, i am going to print this out when i get to the library so i can save it.  thank you.

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I will convert Matthew's firing schedule to Fahrenheit and see what happens.

 

When you do the maths, the start/finish temperatures in each stage are the usual C to F calculation, but the ramp rate is different..........  I think you leave off the add/subtract 32 bit, but no doubt someone will correct me if I'm wrong.   (Took me a long time to work it out, but I'm not at my own computer at the mo, so can't check on my spreadsheets for you.)

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I fire them to 200 and hold for a few hours then fire a slow bisque schedule usually 8 more hours.

Is that 200 c or f?

 

 

It will be °F - a little way below the boiling point of water - you don't want it above 212°F until all the water is gone from the ware.

 

It will also be °F because Marcia is  American.

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so Marcia what you are saying is candle under boiling point of water for a few hours and then slowly fire upto bisque temperature and then slowly cool down till you come to 200 and then shut the kiln off? or do you shut it down after bisque fire temperature has been reached. sorry for basic question. never done a firing before. however i do note that at school, the kilns are propped open an inch at the end when the kiln reaches 200. 

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OP: You mentioned "single firing to ^6" in your post.  Is this a piece of glazed ceramics or just raw paper clay?

Yes you can once-fire, but not all clay bodies and glazes will allow you to do this.  I would think that paper clay, having a higher organic content, would have some slight interference as those materials need to burn out through a membrane of glaze.

 

Kiln firing itself is an art form and many people fire differently, so I wouldn't be surprised to find tons of variations of schedules to fire your work.  You can try them all and see what works.  Personally I recommend blowing up a few firings to force yourself to learn.  There's only one way to find the limit of how fast you can fire a certain mass of clay, lol.  I've blown up plenty of work :)

 

I fire fairly slow and take a conservative approach to most work nowadays, but really I change the schedule depending on WHAT it is, how it's made and how the kiln is loaded.

 

In general, I like 5 ramps.  I learned to use 3 ramps that go progressively SLOWER on each ramp as the kiln gets closer to ~212*F/100*C where water turns to steam, this adds more time in the water purge/preheat in order to safely fire thicker, wetter ceramics that are being once-fired.  From there I go medium speed up to around 500* and then fire it off faster rate to whatever cone and give it a hold at the end.  Thicker work needs to be down-fired sometimes.

 

An example of this might be:

50/hr to 185* for 1hr

40/hr to 195* for 2hr

30/hr to 205* for 3hr

100/hr to 500* no hold

200*/hr to cone temp, 10 min hold

 

I keep the first 3 ramp rates the same every firing and change up the hold times at each stage depending on the needs of the work - sometimes this is 8hrs, 9hrs, 10hrs for questionably wet and thick ceramics.  i also change the rate for the last two ramps depending on the work.  The 100 and 200 are acceptable for most work, but thicker work needs to be slowed way down, more like 65*/hr to 500*, then 125-150/hr to temp.

 

Try out lots of different firing schedules and learn from each one.  Blow up your work and get disappointed, with mistakes you'll learn faster as you try to do it over and over again trying to get the results you want.  If you fire perfect every time you're not really learning from mistakes because there's no "need" to change and learn.  I know, I'm pretty brutal with my approach.  Hope this helps some.  Good luck.

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