Thursday, May 14, 2015

Safety in firing kilns for clay and glazing

Any potter must use kilns which fire to very high glass-melting temperatures.  Electric and gas kilns are used frequently, but also wood firing kilns are used...as well as pit firings and raku firing.  I've been involved in all of them, but am just a participant, not the main person who knows all the ins and outs.


But most potters don't fire their wood-burning kiln every day.  More likely every other month...or perhaps once a quarter, or even once a year.  These kilns are huge, usually long with pots packed in very tightly.  Then there's a fire box, and various bag-walls of fire bricks that channel the heat back and forth throughout the chamber before it finally goes up the smoke stack.  The design of the kiln is quite interesting, and usually is based upon one that has existed for centuries and been fired successfully.

For Sepia Saturday this week, we get prompted to consider safety and blast furnaces.


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This is a chart of the temperatures for clay and glazes, with cone numbers which are what are used to designate the firing range.

Our studio fires bisqueware (the first kiln firing) at cone 05 - 06 or 1855 - 1911 degrees Fahrenheit.  This takes our clays to a solid state akin to what your regular red flower pot is like...it's still porous, but hard enough to be handled without easily breaking.

On the following chart the "What Happens to Clay" column is a bit different for the clay bodies we use, all purchased at Highwater Clays in Asheville, NC.  We work with mid-range clays which mature at cone 6, or 2269 degrees Fahrenheit.  There is a porcelanous white clay, as well as a stoneware-like clay, which provide different colorations with throwing or hand building.  All our glazes will fit these clay bodies, and also mature at cone 6...and we don't use any lead glazes in the studio.


Cone numberOrton Cones
Final temp in F
Color of FireWhat Happens to ClayTypes of Ware and Glaze
15 2615 White
14 2552
13 2462
12 2435 Porcelain matures Porcelain
11 2417
10 2381
9 2336 Stoneware clays mature Stoneware
8 2320
7 2295
6 2269
5 2205 Yellow
4 2161 Red clays melt China Glazes
3 2138
2 2127
1 2109
01 2080
02 2052 Buff clays mature Earthenware
03 2019
04 1971
05 1911
06 1855 Red clays mature
07 1809 Orange Low fire earthenware
08 1753
09 1706 Low fire lead glazes
010 1679 Low fire lead glazes
011 1641 Cherry Red
012 1620 Lustre glazes
013 1582
014 1540 Dull Red
015 1504 Organic matter burns out Chrome red glazes
016 1465
017 1405
018 1353 Overglaze colors
019 1283 enamels
020 1180
021 1143
022 1094 Dehydration begins

Our studio only fires to these two temperatures, not doing any custom work with low firing or luster glazing (cone 012 - 09).

The electric kilns in our community studio at Black Mountain Center for the Arts are computerized, so we don't have to look through peep holes to see when the cones have bent over, showing what the temperature actually is.  When firing in a wood kiln, the cones tell you the tale.  So those colors of the fire will also be letting the practiced potter know how hot it is in there.

When I was a clay chemistry student at the University of Florida in the 80s, I worked with various chemistry formulas to develop a clay body which could mature at cone 6 or mid-range.  They had been firing at cone 9, a high-fire clay, and wanted to become more energy efficient. My clay body was pretty, and seemed to be vitrified (hardened) but after using my bowls for a few months, they started to crumble.  A lesson learned.  This was a major effort done by many universities (and commercial ventures of course), and now most students in community studios also are firing at cone 6.

There are many chemicals that exist in clay...it's not just mud.  And the amount of each chemical lends specific characteristics.  So relying upon a commercial outfit to mix up a great batch of a certain clay gives us the freedom to choose the characteristics we want to work with...and not have to measure out all the chemicals ourselves.

For safety in firing the kilns at BMCA, we follow certain procedures.  First only certain assistants can open and empty the kilns after they start cooling.  The firing's usually take around 12 hours, and 12 hours of cooling before they can be emptied.  Our Studio Manager is the only one doing the loading of the 2 kilns, which is good because he is pretty impartial about whose pots are loaded, basing his decisions on sizes of pots that are waiting to fire, as well as how long they'e been waiting...in order to make a full kiln load.

The kiln room is contained completely within concrete block walls, with a concrete ceiling within the whole studio, behind a metal fire door, so it's basically a fire-proof room.  It has one vent with a fan which pulls air and chemicals from the kilns as they are released during firing...venting to the outside.  Another fan cools off the controls of the kilns, so the computers can work properly.  

As one of the volunteer assistants, I've learned how wearing the heavy leather gloves help while emptying a kiln which is still around 200 degrees hot.  That's cool enough for a pot to come into the room, but still a bit too warm for finger touch.

I am pretty disorganized this week, and can't locate photos of a raku firing, nor those of a pit firing.  These are both done outside, and don't use cones to finish up the firing. 

OK, that's my pottery sharing for this Sepia Saturday.  I wonder what the rest of the Sepians are talking about over HERE.




29 comments:

  1. Geez - and I thought my self-cleaning oven got hot!

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    1. It certainly does. Which reminds me, I need to run mine one of these days!

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  2. So, does firing at cone 6 now produce pots that don't crumble?

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    1. Yes, using a different formula than mine, of course. The Highwater people also have cone 9-10 clays, as well as some earthenware ones for low firing. Getting a true vitrification means it won't absorb any moisture any more...and will kind of ding when you tap it against another piece of pottery.

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  3. There are so many folks that I know that don't run a fan on their controllers. wonder how long they last, my skutt manual said to use a fan and I always have.

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    1. I have other friends who don't as well...I guess they don't read the manuals. Or maybe being outside the controls don't get as hot as inside.

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  4. How awesome :) I didn't know cone 6 was such a recent innovation! When I studied in the early 80s there was just either cone 9-10 or 04 for earthenware. I have used cone 6 since 1994, very happily.

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    1. I got my degree in 83...so it was being looked into that early. Don't know when the more successful formulas for cone 6 clays came out, though.

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  5. I had no idea there was so much involved in all this!

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    1. It's interesting, at least to me! Probably like statistics to a lot of others. Being creative and artistic sometimes requires some chemistry too.

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  6. Interesting. Only the other day I was thinking about the day my father came home from our works in the late 50s having badly burnt his arm. A kiln had just been broken open, and he put his arm in to test the temperature and in a fraction of a second his arm was badly burnt. As you are interested in clay and kilns, here is a link to a blog written by a local brick collector, with photo and history material supplied by me:

    http://eastmidlandsnamedbricks.blogspot.co.uk/2015/02/william-drury-lowe-denby-terra-cotta-w.html

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    1. Those are great images of the industrial kilns which produced "sanitary ware" which were the pipes prior to PVC...and probably are still needed for many larger pipes. They look like they fired with coal, then you mentioned they switched to gas. I recently learned that some bricks are now called "architectural bricks" which refers to their color existing throughout the brick, rather than just on the facade. Oh, I also enjoyed the bridge on the river Kwai...that looks like it replaced one which had been blown up, which was the point to the movie.

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  7. There was a documentary on British TV the other day in which six modern "celebs" were sent back to live in a virtual Victorian age and this episode was set in a pottery works in Stoke. It really brought home the dangers of working with these high temperature kilns. Your post adds some wonderful detail and information.

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    1. That's such good timing! I'd love to see how "celebs" coped with the challenges that were taken for granted in Victorian times.

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  8. Jeez! The only thing I've ever fired was a thingie I made that looked like a cross between a penguin and a wren; it got fired in a school kiln. I had no idea how complicated this process is!

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    1. Well, I know potters today who make lovely birds which have no known brothers out there flying about. Any time you feel the urge to pic up clay again, please do so!

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  9. I've written about blast furnaces today but the company involved also operated brick making facilities on another site. The science behind the clays used is fascinating as are the kilns that are used. Great post, Barbara,

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    1. Thanks Bob, I'm off to go see what everyone else has been writing now!

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  10. My experience with firing pottery is limited to school days as well, but thank you for this very interesting excursion into your world.

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    1. The joy of pottery in North Carolina is that many folks here like it...and there are also lots of potters around too, so we can talk shop often. I'm a bit surprised when I land in other places like Florida or Connecticut or Indiana where the everyday person doesn't have (or want) a hand made piece of pottery. I smile at their foreign made mugs from sweatshops...with pity!

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    2. I meant the manufactured mugs which are foreign made...there are some beautiful hand crafted wares made many other places!

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  11. My husband is a potter (who hasn't potted for too many years). As a work-study student at OSU he was the one who loaded and fired their gas (and a few electric) kilns. He was also involved in salt glazing. I don't think there are many places that fire with gas anymore. The innovation of computerized kilns is new to me. Amazing!

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    1. That's great to hear, Nancy. Once a potter, always a potter. A friend at our community studio can't throw because of arthritis, but he builds things by hand that are amazing. I've never salt glazed, but I sure do love the works that come out of those kilns.

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  12. I find the science behind any art craft, whether pottery or cooking, a fascinating subject. Your explanation of clay chemistry and kiln temperature answered some questions I've had about fired ceramic qualities.

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    1. This is just the tip of the (ahem) iceberg. I took a chemistry class which broke down clay and glaze bodies into molecular levels...which I've mostly forgotten. There had been a student graduate just before I got to UF who went on to help devise the makeup of the ceramic tiles for the space program for the underside of the shuttles for the return entry...which got very hot as you know.

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  13. My only experience with pottery and firing pottery was back in the early 1950s when my aunt spent several months teaching my compadres in my Girl Scout troop. It was pretty rudimentary, and she had only kept her small portable kiln. We thought we were so smart with our kiln glazed small sculptures and hand thrown pots (small). -- On another note, you remind me how much I miss my potter friend who went to the big city to develop specialty glazes for tiles. A fascinating thing that you potters do.

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    1. Glad you had that experience and remember it. There are some great opportunities for the chemists in pottery. (Read my comment I just wrote to Mikel).

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  14. My mother had a wood-burning stove with two ovens and a cooktop and she always managed to get everything at just the right temperature for roasts, or boiling vegetables. And it kept us supplied with hot water every day. Your kilns sound much more complicated!!

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  15. I've watched glass blowers and blacksmiths work and the sheer look of the fire is enough to make me not want to try it.

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