Friday, April 20, 2012

Firebox from Hell.... or was it?

Hell is sometimes depicted in art and literature as a place that looks like heaven for potters that have wood fired kilns, especially of the anagama type.  Surrounded by cheerful grinning devils, hapless scantily clad naughty folk stoke for days, and days, and days, and days.....  There is usually a most impressive fire going, with a hazy orange red flame, showing a reduction atmosphere, and the surrounding industrial landscape is of the "good old days" when there was a major potting industry, and no clean air act!!

Of course there is nothing hellish about a kiln with a fire that burns well and brightly, it is a cheery sight, and the wood pops and chuckles in a most agreeable and companionable manner.

Artists would have done a more convincing job of hell, if the fires had been the sort that you try to start on a chilly day, when the sky is leaking, and the wood is damp, and the matches break when struck, or give a feeble flame that dies with a "pop" in the slightest of breeze.

The depiction of hell would be even more convincing, if the feeble flame had actually finally taken, and singed the paper apologetically, caused the small dry stuff to flicker, and then settled back to a dismal smoky charring of the bigger stuff.

Convincing, even more so... if the fire had reluctantly commenced as above (goaded and cajoled by the angry verbal expostulations of the potter), but the chimney had failed to draw. In fact, if the whole system had gone into reverse, with the chimney acting as an air intake, gulping a sad column of air downwards, squeezing it into the chamber, then over the bagwall to the fire in the firebox, and then.... propelling all the smoke out of every stoke hole, air intake, and small gap in the firebox into the face of the potter!

Clearly, those masters of the paint brush who depicted a cheery blazing hell were not potters!

Those who have read the previous post, will know that I have been reconstructing my wood fired kiln.  I pulled the internal firebox out of the kiln, constructed a solid chamber floor, then began the process of making an external firebox.

True to my character, I did not settle for a simple solution...  I half made a number of constructions, and kept thinking, "...there is another way of doing this!"  It was a bit dispiriting.  Some of the chopping and changing was due to the realities of dealing with recycled materials, almost always of the "wrong" size, shape, or quality to what was hoped for at the time.  I found that one make of heavy fire brick, that I had actually bought new a few years ago, had a nasty tendency to break in half, almost without me even touching them.  In fact I am sure that some did break when I merely looked at them!  I could have done a stage act... the man who breaks bricks, by mind power alone!  Also, my motley collection of firebricks seemed to be in about 100 individual sizes.  So, some morning commenced with me tearing down the work of the previous day, and other mornings resulted in progress.

Something nagged at me the whole time though.

My rough drawing of a Bourry Box down draft fire box.  Hobs would support the ends of the firewood, but are not shown in this drawing as it would need one from another angle.... sorry about that!

An "ideal" firebox would probably be of the down draft type, known as a Bourry box.  This variety of firebox, normally has no grate, but the wood is held aloft on hobs.  Combustion occurs below the wood with this system, and the incandescent embers, below the wood, play an important part in the process.

One problem with a Bourry box, is that they are said to work best with timber that is cut to the right length so that it can be supported by the hobs.

A firebox that can take almost any length of wood, has a conventional grate, more like what you would have at home, if you still had an open fire.  A simple firebox like that can work OK, but the ones I have made for past kilns, have always required frequent stoking, and lots of bending or kneeling.

I have been musing about grates that are more like a flight of stairs, with air allowed in between each tread.  The wood is introduced at the top of the flight of stairs, and slowly makes its way down them as it burns. Throughout its life, there is a good supply of air.

On past wood firings, I would often build a miniature version of a grate like this in the mouth of my firebox in the early stages of a firing, and always took note of how well it worked.

As I fooled around with the firebox, I did some reading online, and found, to my joy, that there are potters using a system like this.  I see that some have built "train kilns" that look a bit like steam locomotives.  The fire box in "train kilns" is where the cab would be on a steam locomotive, and these had a semi self feeding ladder style grate.

So my firebox gained a ladder grate.

The ladder grate and firebox under construction.

I built the thing rather tall, and too close to the kiln for easy loading.  In fact it "got away from me" a bit as I built!  I roofed it with a rather tired fiber lined kiln door, that is about 3 feet square.

When firing the air passages can be blocked with a small brick or opened as needed.
Once finished, I was tempted to tear the box down again.  I thought of a "better" version that was T shaped, with two ladder grates and a profile that would allow vastly easier access to the kiln.  However, I made myself do a test of the horrible looking thing that I had made, so that I could determine what worked, and what did not.  I was reasonably confident that the new firebox would make the kiln get up to at least earthenware temperature, but.....???

I decided on a firing to just above 600 Celsius, and an empty kiln.

The usual preparations were made, pyrometer probe inserted through the kiln chamber roof, and the firebox made ready with newspaper, scrap cardboard, and some small wood.

I lit a match.... well, darn it, several!  Nothing wanted to light.  I was starting to feel rattled when, finally a match lit and stayed alight long enough to transfer a feeble flicker of flame to some paper, and....

I had constructed a firebox that worked in reverse.  It was really terrible, not so much of a whiff of smoke could be persuaded to work through the kiln to the chimney.  Everything poured out the back of the firebox.

I was unable to use the firebox side stoking position, for, to open this made the situation worse, but I was able to insert material through the flue-ways from the back of the box, and keep the fire going.  With eyes streaming tears as I lowered myself to a crouching position to avoid the worst of the smoke, I asked myself, "why keep on stoking"? ... but I did!

By removing 3 spy hole bricks from near the top of the kiln door, I managed to encourage some of the smoke to get into the kiln chamber.  The pyrometer very slowly crept towards 75 degrees, but would go no further.

After three quarters of an hour of adjusting dampers, plugging and unplugging air intakes, coughing, streaming, and stoking, I sat on a garden seat, and watched the whole miserable thing puff away.  I took some photos of the smoky box, and the smokeless chimney, and, as I continued to watch, there was a sudden gathering of yellowish vapour at the top of the chimney stack.  For a second or so, it was feeble, then it gathered in volume and the smoke turned into an impressive stable plume.  I took another photo, then rushed back to the kiln.  The temperature was now at 125 C and the pointer of the analogue dial was climbing visibly.  Around the firebox, the air was clean and clear of smoke!

My spirits lifted.

The kiln started to work at 5 past midday, and was at 800 Celsius by 3.15 in the afternoon.  I terminated the test at this point, when friends arrived.

The step grate really worked.  I found that I could stoke with logs of up to 6 inches in diameter, and it responded rapidly when smaller wood was added.  The side stoking port worked well, and was pleasant to use, especially when compared to the internal firebox that this kin had in the past, where stoking was done kneeling, and every few seconds with small wood!

The air intakes were... interesting!  Opening too many could cause a partial reverse circulation of air in the firebox, and some smoke out the wrong end.  It was also possible to get strong reduction in the firebox, but no visible reduction in the kiln.  Reduction in the kiln could, however, be achieved by careful use of the chimney dampers.

Anyway, a mad week really.  I don't know why I do these things.


Linda Starr said...

I think it's the challenge of it all and the internal competition with one's self to see if it can be done, at least that's the only reason I can figure why I do some of the things I do with or for clay. Ha.

gz said...

Ever thought of a little pre-heating of the chimney?

Michèle Hastings said...

Gwyneth said exactly what I was thinking as I was reading about your lack of draft. I would give it a go next time at the beginning. Happy for you that it worked! I have a friend with a boury box kiln that loads wood from the top. It's the easiest kiln to fire. Open the lid, drop in the wood... no flame shooting out, you don't even smell like wood smoke when your done!

Peter said...

Hi Linda,
Good to hear from you. You are right, and I guess we would all still be eating bananas and living in trees if there wasn't some deep fundamental drive in some of us that says "I wonder if there is another way....?"

Hi Gwynneth,
Yes... ! The problem with this chimney is that I didn't build a mouse hole into it or a passive damper that I could pull right out to put some hot stuff in, as the original kiln, with its internal fireboxes did not need it. All this is one of the real hassles of modifying an existing structure. However, I am going to at least try the oily rag down the chimney thing next time, and see if I can have more success that way!

Hi Michele,
It took me nearly two days to get rid of the smell of smoke... I could have claimed it to be a new personal fragrance for men...! Anyway, as in Gwynneth's comment above, you are both right, but... my existing chimney makes it difficult.

A couple of other thoughts I have are, 1) getting a small fire going right up against the bag wall of the chamber right at the start... or 2) blowing a really strong air jet through the lowest air intake of the firebox. The air from that goes well forward, and was designed to help clear any ash that was near the firebox throat.

I am also going to do a bit of a rebuild of the firebox, and make it lower before the next test.... this will reduce the siphoning effect.

gz said...

glad to be of service, as they say!

I've been trying your Red Tenmokulaze by the it supposed to be black?!!

Peter said...

Yes, the "Red" Tenmoku should be black (!!!), but it can get a red break on thin areas if all goes well. Not sure how it got the name, it is a recipe I inherited. Sorry for any confusion, but have found it a useful glaze both for itself, but also as an under glaze or for something tucked away inside mugs or teapots. I have a black tenmoku that is black, and the red tenmoku does look a brown black beside it.
Agggggh! :)

cookingwithgas said...

We preheat the chimney with pinecones that the trees throw away- it too needs preheating to get the smoke up and away. That or Charcoal does the trick for us.
Out worse time was a storm that came up during firing that sat over the chimney tossing great winds down the chimney.
We grabbed a ladder and I handed Mark brick as we built up the past the wind- exciting???!! Maybe- fun???maybe not! Stories to tell- yes.
I hope it all came out well.

Peter said...

Hi Meredith,

Pinecones, now that's a great idea. I could toss them into the chimney like flaming molotov cocktails at the start of a firing! What fun, and they would smell nicer than an oily rag (and may even be carbon neutral!!). Anyway, Thanks for your comment, when I struggle with getting my fire going, I'll now think of you two heroically battling wind and rain and building the chimney higher.