Sunday, May 5, 2013

Firing Schedule

I posted on facebook awhile back and referenced a new firing schedule and some folks have asked about it.  Given that I am sick and have nothing to do I thought I'd go ahead and write about that.

This last firing was a very good one.  I've been tweaking my firing schedule to find a way to fire quickly but also keep my glazes from blistering.  Blistering has been the bane of my existence for the last couple years but I finally have it worked out.  I'm a believer in fast firing, for economic reasons.  Save fuel, save time, save money, make more pots. 

I learned to fire gas kilns in professor would start the kiln in the morning when he arrived and be finished by the time he went home at 5.  8 hour firings were the norm and I just always took for granted that everyone fired that way.   I was baffled(and still am) when I hear that people fire gas kilns for 20+ hours.  My thought is that you're either wasting gas or you kiln isn't set up right(or you have a hardbrick beast).  Of course there would be exceptions to this, some glazes need different schedules.  My ash glazes need a soak or slow climb at the end.  I fire our reduction kiln at the school in a similar fashion, I hit cone 012 in about 3.5 hours, cone 10 in about 9 hours altogether. 

Many people think that the pots will crack when fired so "quickly".  Industry fires bathroom tiles in as little as 45 minutes from raw.  Raku.  Need I say more?  At this point hopefully we all know that color comes in the cooling so all you need to do is hit your atmosphere and let your glazes melt, the cooling will do the rest. 

Now, when it comes to woodfiring I have a theory/personal belief.  Fire short or fire long.  I fire for flame, not ash, time is not required.  There is nothing nice about 2 day firings.  Long enough to get enough ash to cover the flame work, not enough time to let the ash do its thing.  So less than 18 or more than 48.  Just my theory.  (example: bandana pottery, 300 cu ft kiln, pots raw glazed and single fired in 16 hours). 

The best firing I've ever had was done in 6 hours.  The ash glazes I use nowadays can't handle that, they do need a longer firing to settle out otherwise I'll have a sea of blisters.  When I was firing slips I could go faster, after all...the slips don't need to melt, I just needed temp for vitrification and atmosphere for color. My kiln is approximately 75 cu ft, holds around 200 pots.  I currently fire in about 8 hours.  I fire to cone 9.5.  Which means I have close to 11 at the fire box and 9 at the flu.  I let the kiln cool for about 36 hours. 

I bisque all my work, it knocks several hours off the firing and also time spent gathering and prepping extra firewood.  I used to be an advocate for raw glazing (well, I still am but...) if you have access to an electric kiln it just makes sense for wood firing to bisque and save the time.  I've done 24 wood firings without having to worry about single firing, knocking off 5 hours per firing that's 120 hours.  I certainly didn't spend 120 hours loading and firing bisque kilns, definitely a more efficient use of my time.  This is what works for ME and my kiln. 

I don't track the temp per hour but here is the basic idea:
I have cones in 4 spots, top and bottom on both sides(cones 07,05,03,01  1,3,5  7,8,9,10):
Hit cone 07 at 3:45-4 hours.
Cone 3 at 5-5:15 hours- at this point I make adjustments to even out the kiln if necessary.  I want all cone 5's to fall together. 
Once cone 7 begins to fall I pull the passive dampers to slow the draft of the kiln, I want about 2 hours from cone 7-9.5. 
I salt once all 7's are down, about 1# per salting, every other stoke, total 7-8#'s.  I want to be done salting before 9 is flat.
I shut it down when the tip of 10 is horizontal, I call this "cone 9.5". 

One of the most critical aspects of this schedule is the beginning because in a wood kiln it is very easy to over stoke and put it into reduction.  I fire with the dampers wide open and the passive dampers shut, this ensures maximum airflow through the firebox.  I also have plenty of holes for air at the firebox.  I've seen a number of kilns similar to mine that don't have near enough ports for air in the firebox, this equates to longer firings and/or too much reduction.

Here are a couple pots from the last fring.

Questions?  Comments?  Slander?  Let me know!



Hollis Engley said...

Thanks, Brandon. Very useful information. And I get to cone 10.5, more or less, in around seven hours in my gas kiln. It seems to work.

Peter said...

Always good to read about how others fire kilns and what works. There is always a lot to think about when firing, and so many factors, including kiln, wood, glazes and clay body. My kiln seems to produce its best work in 11 to 12 hour firings, and my 10 hour pots look a bit undernourished, even though they got to temperature! I don't salt, but there is volatile stuff in the kiln at high temperature. I suspect this is from the kiln structure that has been naturally glazed from previous firings, and it gives a luster to work, particularly shino glazes. It could be that a few extra hours are needed to release that stuff!

Sad to read that it is difficult times for you, and you had a set back with your health. Pneumonia from dust exposure sounds bad, and I hope you are better soon.

Lisa researching ceramics said...

Hope you feel better! You have a lot of patience for 6 hours. Thanks for sharing your experiences. They're really inspiring.

Saga said...

Very cool colours :) Thank you for sharing!