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Firing Log

ancient kiln | 21st century logbook

February 3, 2010

Advice + Revisions = Success with Short Throat Kiln

Filed under: Firing,Kiln,Pieces,short throat kiln — odin @ 10:49 pm

I received advice from Steve Harrison (link 2, link 3) and Claudia MacPhee, see below for their comments, regarding the short throat kiln I built. Specifically, they gave me some suggestions regarding the smokey lid and honestly, after reading the suggestions I was sort of chagrined at how silly I had been. To sum it up, the large front air inlets I like rob power from the lid’s inlets and thus allow smoke to escape from the lid. Kind of like a hole in a straw makes it hard to drink a milkshake.

Secondly, my chimney was also a bit short, so I gritted my teeth and bought another taller ladder, and enough bricks to add ten rows to the chimney. The expensive part was the ladder — $90. I also dropped my grate one brick level so that the firewood rests closer to the coals.

The upshot is that the kiln fires to cone 10 without having to resort to careful stoking of kindling at the end and it doesn’t smoke while being used in boury box mode. In order to melt cone 11 flat, I still have to stoke small sticks by watching the chimney till the flame disappears, then stoke a stick, watch the chimney flame till it disappears, stoke a stick, etc. etc. I don’t need to go that high, but I like to know I can. I think the extra high temperatures sometimes cause interesting cracking patterns on the surfaces of the pieces.

I still like want this kiln to perform somewhat like an anagama — just smaller with a shorter firing period. So periodically I open up the front inlets, brave the smoke, and beat and stir the coals vigorously. It seems to be working. For example, the piece below was glazed only on the inside. Despite the fact that the outer surface was completely raw, the fire exposed side developed almost the same glaze color as the white liner glaze except with the added bonus of areas of ash based olive green (bottom right photo shows the front side). I think with a little practice, I may be able to avoid glazing completely and just rely on the fire. Click thumbnails for the big picture.

Steve Harrison‘s (link 2, link 3) Suggestions:

I think that the problem with the smoke coming out the lid is mostly to do with the short chimney, as you identified, but I also think that it is to do with the big air inlet/stoke holes at the bottom. Once I have enough ember to get the wood on the hobs to ignite, I close up the bottom hole completely and only have a smidgin of air going in under the embers through the mouse hole. This concentrates the draught in through the top holes and draws the smoke and flame down into the throat arch and chamber.

Claudia MacPhee‘s Suggestions:

Thinking about how my little kiln fires makes me really wish I could go out and run a load through it now! Alas, too cold….Just let me know what parts you want to see and I will try and get some photos up on my blog.

I made the bourry box exactly like the one in the diagram in the book. Counted the brick and didn’t use any ‘innovation’. I put in the brick hobs and cut my wood exactly 26″ long. A bit of a pain, but works well.

Usually I start a small campfire in the bottom of the box about 9 pm. Two big pieces with a bunch of paper and small stuff between them. I feed it from the bottom until it is going good with a few coals, then pile on more whole logs through the top. Don’t get too excited about a huge fire at this point. Mostly just keep it going until there are lots of coals then heap if full of wood, close the damper and put the three bricks in the front.Primary and secondary air vent are closed all this time. It is usually around midnight and I go into the house for a nap. Set my alarm for 4 am., get up and make a pot of tea. Fill up my thermos and get a cup full and go to check the kiln. There will be some coals left and I unbrick, pull out the damper and stoke it up, again on the bottom. Just keep filling the box with all the junk wood I have collected from around the yard and odds and ends from bucking the slabs exact. About every ten minutes I throw more stuff in. At this point I use aspen (tons of ash), pine/white spruce and hunks of willow. Willow is the hardest wood I can get. It grows very slowly up here with lots of cold and little rainfall so is tight and heavy. The willow I use has been dead for many years and is bone dry. It makes a really good, long lasting coal bed.

After about fours hours there are enough coals to go to the hobs. This is one of the coolest parts of the firing. I will take an armload of slabs, three to four inches thick, open the top lid and quickly place them on the hobs-it is like magic! They burst into flames and are pulled down into the kiln! After this point I keep the hobs full. The point here is to never let a bunch of cold air flow between the pieces of wood. So as it burns up put more pieces on the opening holes. Hard to describe. I try to lay the slabs in with the cut side up. There are always at least two layers. Also start using the primary air. The bottom three bricks are put in and clammed up.

I take a clock and a book out for the firing. Put new wood on every 10 to 15 minutes. Fill her up, go off and feed the chickens, fill her up, go inside to get some food.

My first firing I grossly under estimated how much wood I’d need. By 6 pm I was using my time between loads to buck up more slabs. Had enough time to do an entire wheel barrow full between stokes. That never happened again! Also I have never used the pyrometer to fire. Every body gets so neurotic with those things. I try to take the “Laid back” part seriously. I live in a beautiful and quiet place. I sit there and listen to the birds, read my book, hear a boat out on the lake or a plane going by.

After I go to the hobs I’ll begin the side stoking. Now this is the pyro dream, a technique from the Aussies known as ‘flamethrower’. You pull the brick, place a handful of small, long wood in and just hang on to it for a bit. It blows up and the flame goes in huge circles.

So the rhythm is-fill hobs, pick up sticks, pull brick, put in wood, hang in there for a couple minutes, drop them in, replace brick, sit down, drink tea, read book or stare into space for ten minutes. I have mouse holes but have never used them. Also have never needed secondary air. Usually keep the primaries about half open, just enough air to keep it from ‘chugging’. My first firing I had cone 11 (highest one I had) melted. Got some awesome hare’s fur on some bowls near the front. Didn’t side stoke that one so the pots near the back when in for round two next time.

I am dedicating my life to melting as much of the Coast Range as possible…lots of goodies to melt in this part of the world, plus a nifty bunch of volcanic ash, local copper and glacial silt that makes wonderful glazes. Chinese stoneware glazes, especially the iron ones really do it for me, I love them lots. I put in lots of glazed pots, but want flashing on them rather than a ton of ash. By this time I know where to put pots for what I want the kiln to do to them. Believe me, you can get all the ash you want at the front end. This is getting much too long. If there are any more questions, just ask. I have found this a really fun kiln to fire, and can do it by myself. I have children your age, so if an old lady can do it, how hard can it be??

Good luck!

Claudia

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