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Babs

Opening Kiln Early

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I guess most of us have cracked a kiln a little early when it is still warm and most of us know the risks involved in this.

Is there any damage  to the kiln if it is cracked when still hot?

My new kiln is a soft brick top loader, and though I cracked the old one early on many occasions, is this practice causing damage to elements or kiln?

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Impatience of course......different glazes, different platters.....nature of the beast... or excitement  "one more sleep!!" 600c at themo, would like to drop it to the 220 mark.... but I'll just go and do something else just wondering..  Sometimes it's because I am going to town, not something I do every week and I would like to take those specific pots to the Gallery

Justification of malpractice I know.

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DON'T DO IT! :angry:

 

THE WORLD AS WE KNOW IT WILL END! :ph34r:

 

KEEP YOUR LITTLE HANDS UNDER CONTROL!

 

THE KILN POLICE ARE WATCHING! :blink:

 

WOULD YOU DO THAT TO A CAKE YOU ARE BAKING IN AN OVEN!!!!! :angry:

I LIKED gooey choc cakes, in my past life...  cookie dough much tastier than the baked stuff.

I know, :rolleyes:  I know off to garden for a bit

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I try to wait until it's under 200 but have opened it up when it was 250 before. I actually have a system once it reaches 300 something I pull the top plug, 299 or less the next plug down gets pulled, 199 bottom plug and I crack the lid a couple inches, anything below 175 is fair game. I keep a towel down there and if the shelves are a little warm use it to lift it out when I am really impatient as even when the controller reads 175 the shelves lower in the kiln can feel hotter. I WISH I had the patience and the time a lot of the time to let it cool to room temperature but I have only managed it a couple of times.

 

I am pretty sure it's better to wait for the elements and bricks but so far I have not seen any cracking of the bricks other than the first fine ones that appeared with the first couple of firings and the kiln is still firing within the same time and range as when I first got it 105 firings ago. Maybe I've just been lucky though...hmm maybe I should take a dish of something down for the kiln gods tonight to appease them for my misuse of their territory. Or maybe they like living in the edge?

 

T

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Thanks guys ,you have given me strength.

I went and spent the day glazing pots, tomorrow I have to go off property to work. I was hoping to be superwoman and unload, glaze and load today,

80C now that is low. I usually haul them out just under 200C like an oven bake!!!

I'm going to go with the 80C Mathew but when the ambient temp is around 30-35 it seems to take forever.

Might mix a glaze after Dinner if the fishing widow scenario sets in, not complaining here. Or I could get tempted.

Might be spoiling this new kiln a tad

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Temperature (â°C) and Events

100  Atmospheric water turns to steam as the piece completely dries

300-800  Carbon, sulphur and organic materials burn off

573 Quartz inversion occurs

900 (cone 011) Sintering begins

1060-1160 (cone 06-3) Earthenware firing range

1170-1220 (cone 4-6) Mid-fire range

1230-1350 (cone 7-14) Stoneware firing range

220 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 Sulphur

Clay bodies all contain some measure of carbon, organic materials, and sulphur. 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 no physical 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, raw glazes to adhere to the pottery without it disintegrating.

 

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 (aluminium 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 clays 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 glazed 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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You have convinced me Mathew, I submit to reason. I nurse it up past those crucial temps, but I get the NEED to see my pots on the cooling cycle. I will never be impatient again, gonna print your reply and stick it near my kiln.

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Matthew: that is very interesting for all of us. Thank you for sharing! I never ever open the kiln (front loader) before temp is down to at least 80°C. I don't want to work for hours and hours on a piece and then maybe loose it while opening the kiln too early. Patience is a virtue!  ;) 

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It is only critical if you have it in the pot  :D Not the most educated opinion but I think most clays have enough feldspar to cancel out Cristobalite formation.

 

"Cristobalite forms spontaneously (within bodies) at temperatures above 1100C from very fine quartz found in some clays, from finely ground silica powder and from molecular silica liberated during the formation of mullite from kaolin. If feldspar is present in the body then any available molecular silica is taken up in the formation of silicates, and thus cristobalite does not form. Even if it does then it too is taken into solution. A good cristobalite-avoidance strategy in formulating a body is to use enough spar or naturally fluxed clays to be sure that any potential cristobalite is drawn into body glass (check with dilatometer test) and then re-establish fit with fine quartz. In this way quartz is compressing the glaze at 573C rather than cristobalite at 220C. A typical cone 10 porcelain with 25-30% feldspar will show no evidence of cristobalite on its expansion curve (as measured in a dilatometer). Conversely, high iron often non-vitreous stoneware bodies can generate high cristobalite levels."

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Some possible reasons to open kiln early

Your rocket ship to space takes off in the next 30 minutes and you will not see your kiln load for 20 years until you return from Pluto

The nearby volcano is erupting and lave is flowing towards our studio and you only have 15 minutes to get out

Wildfire is coming your way and you have to evacuate

The dike has failed and the flood Is 5 minutes away from your studio being flooded

Your piece in kiln is for the presidents dinner party held in next few hours

The last reason is you just heard about the asteroid which will end life on earth hitting in next few hours and you need to see your new dog bowl in kiln.

Now if your reasons are lesser than those listed above I would consider just waiting until the pots are below 200 degrees

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There is a video I want to share but cannot find it anywhere legitimate. I tried uploading the video myself but got blocked by youtube straight away. Tried to dispute the copywrite claim but probably wont be accepted on my educational reasons  :o

 

A guy takes a porcelain pot out the kiln when it hits cone10 and dunks it straight in a bucket of water. Pot survives, all very surprised. If I can get the BBC to accept my claim I will post it. Otherwise it takes a long time to search and find it but if you want to it is a good watch the whole program (pm and I have a link). I thought if it was unlisted I would get away with it but seems that I can't.

 

Ceramics - How It Works was the program if anybody is interested.

 

I have also seen a pot taken out the kiln at 550c and split straight in two. Also porcelain.

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Makes my eyes hurt watching him do all that with no glasses.

 

I have been opening my lid at 250F since I started, and have yet to have a pot crack on me because of it. I have had lots of pings, but that was due to the clay body and the glaze not fitting not because I opened the kiln early. I tested this by leaving pots in the kiln until it was 100F then taking them out, and listening to them ping all evening.

 

I don't take the pots out at 250F I just open the lid so it cools to room temp in an hour or so.

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Yea pretty much that but it goes straight from kiln to bucket. If some pots are perfectly happy doing that then as long as you have happy pots I don't see a problem. Not so sure about shelves and props, as they are consistently re-fired then maybe this could help promote Cristobalite formation. Still I am cautious enough not to crack it open it before 250c and fully open under 200c.

 

Some pots can ping years later, at least every so often I hear one go in the studio.

 

 

Marc, I always feel like we could get hit by an asteroid any second. Swinging round a nuclear fusion ball on a rock makes it tough not to.

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