First Firing of New Kiln

This month was packed with lots of activity as we constructed a groundhog style kiln patterned after a design known as the "manabigama" kiln, made a kiln load full of pots and bricks, and fired the kiln.

Click on the embedded slideshow, or click on this link to see the photos detailing the entire construction process.

I designed the kiln hoping for something that would fire quickly so that we can have the opportunity to offer this as a potential activity for school groups in the future, as well as being fuel efficient as possible so in order to be more environmentally friendly than a kiln that would take several days to fire.  To this end, I think it was a success.  The kiln fired in twelve hours.  Below is a chart of the notes I took on the pyrometer readings throughout the firing process.

We pre-heated the kiln with a small fire in the firebox for about 12 hours, and then let it cool down overnight.  We did this because some of the pots and saggars loaded into the kiln were still leather hard - not even bone dry yet, and we wanted to make sure we safely dried them out before heating them up past the boiling point of water.

The steep cooling curve was a bit of an accidental experiment.  We left the air ports open on the firebox for a couple of hours and the kiln cooled dramatically faster than intended.  Despite cooling really quickly through the quartz inversion range at around 570 Celsius, none of the coffee mugs had cooling cracks.

Here are some of the finished pieces:

First Mugs: the Good, the Bad, and the Ugly

Overall, I think I'm pretty happy with the way the ash glaze turned out (particularly the dark green one).  One glaze turned out a dark green-to-black, and the other was an opaque gray that I'm not crazy about.  In some places, for reasons I don't understand, the glaze had beautiful blue and white streaks in it.  This seemed to happen more where the cups received the most fly ash from the firing.

About half of the mugs that I made from the iron-bearing clay body warped excessively or split during the firing.  Some split, and then were healed again by the melting glaze.  Others, not so fortunate.  

I think in the future I'll be doctoring the clay body a bit with other materials to help it handle the firing better.  

Here are some of the results:

Kiln Firing Log

I just recently acquired a pyrometer and used it in its first glaze firing.  It was really helpful to have more real-time information to work with than just the pyrometric cones that I've been using.  

When I fired the kiln this last time, I periodically recorded the temperature as measured by my pyrometer.  Because the pyrometer doesn't extend as far into the kiln as it is supposed to, I think that the temperature measured by the pyrometer is approximately 100 degrees celsius cooler than the actual temperature, which I measure using pyrometric cones (at least from about 800 degrees Celsius and up). 

I fired the kiln from 5:15 or so a.m. until around 11:00 p.m. or so, and the pyrometer reached a maximum temperature reading of 1,100 degrees Celsius. 

The first three hours of holding it at 200 degrees Celsius was intentional to burn off the water in the clay slowly to avoid explosions.  From 200 degrees up to 600, the rate of increase was fast - about one degree Celsius per minute.  From 800 to 1000, rate of increase was slower, but still steady, and the last bit from 1000 to 1100 was painful.  As I write this, the kiln is still too hot to unload.  It is the first high temperature glaze firing with actual pots in it, not just test tiles. 

I can't stand the suspense. 

Ecokiln Test: Round Two

After taking out the primary heat exchanger and using just the secondary heat exchanger, we tested the kiln again.

Here's a picture of the kiln when it was firing.  The red flames are coming out of little spy holes built into the top and side of the kiln for the sake of monitoring progress during the firing.  This is normal during a firing, especially right after a stoking.

We reached cone 9 (about 1260 degrees Celsius) in about 17 hours. 

High Temperature Cones used: Cone 6, 8, 9, 10 
The melted black ones in the front cone pack are the low temperature cones for monitoring progress earlier in the firing  The low-temperature cones started dropping at around 6 - 8 hrs into the firing. 

Around cone 9, I tried to adjust the oil drip to get a little more clear burn, and something went wrong and the kiln dropped from a yellow-white color back down to an orange color - a drop of several cones worth of temperature.  At that point I decided cone 9 was good enough, and that it probably wasn't worth spending the extra wood for a few more hours to try to get the last little bit to cone 10.

The glaze tests generated some interesting results, and the some of the clay body tests yielded surprising results too.  A dark brown clay that we have here turned out to be some kind of nice iron-bearing stoneware.  It is really plastic and I thought that it would melt, but it seemed to handle the heat really well.  The TPI white clay seems to be a little over-fired at cone 9 - it might make for a nice cone 6 clay.

Line Blend between the white clay and dark clay.  I thought the dark clay would melt, but it turns out to be a nice iron-bearing stoneware clay, and not an earthenware after all.
The tile on the far right is what I am calling TBR #3 Dark Stoneware.

The little saggars I made from the white TPI clay for stacking the test tiles.  The white clay warped. 

Glaze Test Results:

The kiln was loaded with 3 sets of glaze tests.  The glaze tests are all made of a combination of rice husk ash (high in silica), wood ash (high in fluxes like calcium, sodium, or potassium), and a clay (either the TPI white clay like the kind that made the warped saggar above, or a gray fireclay from TBR, or the dark stoneware from TBR clay #3). 

Glaze Tests using TPI White Clay

Triaxial Blend of Glaze Tests using Wood Ash, Rice Husk Ash, and TPI White Clay (same as the warped saggars)

Glaze Tests using TBR Fireclay (from near the banana garden)

Triaxial Blend of Glaze Tests using Wood Ash, Rice Husk Ash, and TBR Fireclay

Glaze Tests using TBR Dark Clay #3

Triaxial Blend of Glaze Tests using Wood Ash, Rice Husk Ash, and TBR Clay #3 (the dark stoneware)

Test Firing Results with Heat Exchanger

I fired up the kiln with the heat exchanger assembled.  The results were mixed.  After 18 hrs of firing, the cone was at about cone 04 and having a tough time getting any hotter.  It seemed like the airflow just was not powerful enough to provide enough oxygen to keep the temperature climbing.

On some levels, the heat exchanger worked well.  The exhaust from the kiln was cooled to the point that you could hold your hand about 4 inches over the top of the chimney indefinitely.  The air entering the kiln from the heat exchanger was heated to the point where it was just a little cooler than the smoke leaving the chimney. 

It was disappointing that the kiln stalled out at cone 04, and it seemed like all the signs were pointing to an inadequate airflow. 

I decided to take out the primary heat exchanger entirely and try it with just the secondary heat exchanger.  The secondary heat exchanger was a set of 24 one-inch pipes that rests on the first layer of ceramic pipes.  Smoke enters the ceramic pipes via holes in the pipes, and then exits the chimney via the stainless steel pipes. 

I rebuilt the chimney and decided to test the kiln again.

Primary Heat Exchanger: The Guts

 Today I completed the assembly of the primary heat exchange on the small kiln.  The pipes were made with an extruder and a hollow die out of local clay, and the pipe connectors were press-molded from the same clay.   To fit them all in the bisque firing, I built a temporary extension on the firebox, and loaded up half of the parts into the firebox and half into the main chamber of the kiln.  Some of the pipes were fired up to as high as cone 6 (1200 deg C), while others went up to about cone 010 (800 deg C).  The ones that went up to cone 6 experienced some pyroplasticity and warped a little bit.

I plan to add in a secondary heat exchange unit using stainless steel pipes and some other extruded clay parts as well.  Will be working on this tomorrow.

Designing an EcoKiln

In August 2014 I began a new job at a beach resort in the Straits of Malacca as an artist in residence.  Part of my work is to develop an art program at the resort that we can market to school groups, and to use the arts as a platform for community development initiatives reaching out to the local nationals from nearby islands.

The EcoKiln Concept

The plan is to build two kilns - one for glaze research and very small production runs, and a second, larger one to use with school groups.  We are hoping to be able to offer school art classes a chance to learn about and participate in wood-firing techniques.

The small kiln is nearly complete, with just the finishing touches to go on the firebox and heat exchanger.  A heat exchanger is a device that recaptures heat that would otherwise be lost through the chimney, and redirects at least some of it back into the firebox.  In addition to gaining more efficiency, it also cools the smoke and reduces the fire hazard of sparks flying out the chimney.  This is important to us, as the resort is all built from wood.
 
As I've been researching these devices, I've come across three designs out there that have all contributed, in some way, to the design of the kiln I'm building right now. 

• Hugh Jenkin's Volcano Kiln
• Marvin Bartel's EcoKiln
• Iferica Ecokiln