Trouble Shooting First Kiln Firing

[birchhill]

Hello, just getting into pottery and trying to learn as I go.    Built a kiln out of old bricks we had lying around and some scaled down plans I found online.   Fired it up for the first time with some coil made items using High water Raku clay with a working range of cone 06-6.  KIln seemed to work great.  Put a 06 and a 6 witness cone in and within about 1.5 hrs melted the 06 flat.   We continued for a total of 8 hrs  never melting the cone 6 and then sealed off entrance and chimney.    The problem is with some of the taller items cracking.  On the flame side of the pottery there are cracks randomly through out the surface but, only on the flame side of the pots, smaller flat items on bottom shelf did not crack at all.    Trying to figure out what variable I should change with the next firing to prevent the cracking.   All items dried before hand about 3 days with the last day sitting next to a wood stove.  Thanks for any help with this!  attaching picture of side view of pot showing cracking on flame side and a picture of kiln

inside view:

outside:

 

at full burn  Tiles on roof to keep rain off hot brick:

 

side of pot:

 

 

cracks flame side:

[Callie Beller Diesel]

Hey Birchill, welcome!

I have a lot of questions for you, because it seems like you're coming at this from a non-standard angle and jumping in head first. I applaud your enthusiasm!

 

Fist thing, what were you trying to do in this firing? Was this supposed to be a cone 6 glaze fire or a cone 06 bisque, or a single firing to cone 6? What cone are your glazes rated to, if you were working with any? Were you just going for a raku firing?

 

To address some of the specific questions you've asked, you describe taking your kiln very quickly up to cone 06. If you heat that fast through quartz inversion (roughly cone 022) I'm not surprised you have a lot of cracking, particularly on the flame side of the pots. It has to go through that point slowly and evenly. On the plus side, it seems like the ware was fully dried beforehand, and I doubt there were issues from too much moisture. For a point of reference, a programmable electric kiln will reach cone 06 usually in 6-8 hours on a slow cycle for bisque. So if this was a first firing, it was waaaay too fast.

 

Another set of things that could be contributing to your cracking is the design of your kiln. Scaling down kiln plans will mess with things because proportions matter. Without knowing measurements, relative height of your chimney, placement of firebox, size of stacking space, presence and size of the bag wall, it's hard to say how much of this is the kiln design itself. Each of those things will affect how efficiently and evenly your kiln fires. We've established yours is fast and hot, so the chimney draws, and it could stand to slow down a bit. Some of that will be your stoking rate (I'm assuming you're firing with wood as I don't see burner ports), but there's about a million adjustments in construction that will affect how easily you can control your kiln. The big thing though, is I think it's unlikely it will ever hit cone six because there's not enough insulating value in a kiln made from construction bricks to get that hot. It needs to be firebrick. Those construction bricks may melt with enough heat. You've established your kiln will hit earthenware temperatures, and will probably work on a temporary basis if you want to do some of that, or some raku firing, if you can slow things down a bit. You may experience brick spalling or dunting. The mortar will likely give you grief as well, unless you've used a refractory castable.

 

I know that seems like a complete downer, but if that doesn't scare you off, all these things can be addressed. We can probably get you pointed in a direction that'll get you some functional pots.

[neilestrick]

I agree with Diesel. If you haven't already bisque fired your pots, you've got to slow down the first half of the firing. You should be taking at least 5 hours to get to cone 06, or even slower. You need to go very slow in the first 300 degrees to prevent steam explosions, and slow around 1000F degrees to prevent cracking during quartz inversion, and slow at around 1900 to make sure you get everything burned out of the clay that needs to get burned out.

 

Typical wood and gas kilns are built with 9" thick walls. Anything less than that will be problematic. What kind of bricks did you use? If they are normal house bricks, you'll melt them, like Diesel said. And if you used regular brick mortar than will also degrade and fall apart if you get too hot. We usually use a mix of fireclay and sand for kiln mortar in wood and gas fired kilns.

 

There's no reason you couldn't use the kiln in its current state as a raku kiln, though.

[birchhill]

Thanks for the quick and in-depth replies!   My goal for this first firing was to see if it was possible to hit a cone 6 with this kiln design.  I had hoped to get a natural ash glaze and smoke affect by sealing the kiln off at end of firing and depriving of o2. It was really a test run just to see what would happen and if the kiln would hold together.    The kiln is made out of solid hundred year old bricks and I did use regular mortar which already has begun to substantially crack.   Before next firing I will be welding a metal frame around it so even if all mortar fails it wont collapse.  I understand it will only handle so many firings, the hope being by the time it fails we will have more experience with building things from clay and what we need in a wood burning kiln.   These are the plans I used, reducing by 50%

http://ceramicartsdaily.org/firing-techniques/kiln-plans-and-diagrams/an-experienced-wood-firer-shares-kiln-plans-for-a-small-instructional-wood-kiln/

  It sounds like i fired it way to fast.  I think with this next firing I will start the fire out front of fire box  and slowly work it into box to facilitate a slower warm up.  I also, will stop after I hit cone 06 since 6 looks unattainable for melting ash.   Thanks again and gladly welcome any other input.

[neilestrick]

Ash won't melt at cone 6 anyway. It may fuse to the clay, but won't melt and get glassy or runny. You need about cone 9 for that.

 

The mortar will calcine and crumble away, so keep an eye on it.

[Guest JBaymore]

Hi Birchill,

 

As has been said.... cone 6 is not sufficient to get a melt out of natural ash.  Red brick will be slagging and melting on the hot face if you manage to attain that high a heat work.  You MIGHT be able to get a firing or two to cone 9 before it is totally problematic.  Another big issue at those temps is the concrete mortar.  Mortar mix has a lot of calcium in it... and that is a flux on silica (it has that too).  It will start to melt like the brick....... but will crack up and start falling into the kiln well before that.

 

I've build kilns for high fire shizenyu (natural ash deposit glazing) for myself and clients out of a single layer of 4 1/2" of high duty hard firebrick backed with varying outside layer type of thing like local clay or home made castable or even just clay-ie and sandy dirt about a minimum of 6-8" thick.  Works fine...... just not designed for any longevity.  Bricks stuck together with 50% fireclay / 50% silica sand.

 

A tad more chimney height and closing down the primary air hole will help you get higher temps out of that unit.  Or if you can't close down the air because that is your stoking port (looks like that) ....... maybe use the "wood wall" technique.  That entails using the wood to close off a lot of the larger P. air opening and pushing single pieces in as they burn away and adding new in their place. 

 

As the temp of the exiting flue gases increases, the draft increases, and it takes a surprisingly small opening to get sufficient primary air.  Example...... on my 5 chamber noborigama I might have about four openings that are 2" x 4"1/2" wide open at the higher temps when still stoking the main firebox.  Another example .... on my college's anagama at the sustained holding at cone 12-14...... we typically have four openings about 2" x 5" and one 9" x 2 1/2".

 

best,

 

................john

[Mark C.]

Red brick will met at high temps as noted above.These bricks are made for house building not kilns.

The clay pots need to be more dry (longer time to dry) and the fire need to be much slower.

Let us know hoe they hold up at cone 6?

[birchhill]

Thanks for the replys and great info! I was a little confused about the air intake portion.    I experimented with various size opening as I would stack loose brick in front of intake to restrict opening then remove when loading wood.    It seemed i would get more fire height above chimney with bigger opening then with restricted opening.   At times flames would be 3' above chimney top but, when I would close down the air intake to a smaller size it would reduce flame out of chimney top.  Maybe Im thinking about this all wrong?   IM equating fire height out of chimney with an increase in internal kiln temp, is this right?   Is there a simple ratio of flue size to air intake?  or is it way more complex then that.   Thanks again!

[Guest JBaymore]

Unfortunately......... way more complex.

 

Think of it this way...... any flame burning anywhere but INSIDE the ware chamber is heat energy that is not being used to actually raise the temperature OF that chamber.

 

Just because you see all those dramatic pictures of anagama with 20' of flame coming out to the top of the chimney does NOT mean that is what you should want.  There are lots of reasons for those pictures..... not the least of which is ..... "it makes a great picture". 

 

Another reason is that anagama style kilns are "primitive" types of kilns... with poor fuel aeration because of the design...... and the technology was abandoned by potters a long time ago for good reasons.  The resurgence of the anagama in the past 25 years or so ("The Western Anagama Revival Period") is because of the aesthetic effects.... not the superior kiln design.  The inefficient design causes effects that people like.  The 'trade off' is poor efficiency and a difficult to fire kiln.

 

Wood firing is one of the most complex and difficult ways to fire clay work.  It requires very precise fire management to get a good climb as well as good effects.  You are going to have to do some "trial and error" stuff with that kiln to get this figured out.

 

WOOD FIRING PRIMER STUFF:  For a given sized hole into a wood kiln, as the firing progresses and the temperature of the flue gases in the chimney increase, the amount of air being drawn into that hole steadily increases.  If there is just enough wood fuel to utilize that increasing air flow, and there is good mixing action,.  then the kiln continues to climb as heat is realized and all the air is used for combustion. 

 

BUT... if there is not enough fuel to use all the air that is coming in, then draughts of COLD (relative) air are getting pulled into the chamber...and running thru the wares.  Not good.  And you are heating up that un-necessary air... and sending it out the chimney.  Defect city.

 

Now if the amount of air at any point requires so much fuel to use it all that the rate of CLIMB is then too high for the wares (at any given point)...... then that is not good either.  Defect city.  To affect the rate of climb... you don't just cut back 'pounds of wood per hour'.... you have to cut back the ever increasing air supply.  Do this by making the hole(s) smaller.

 

If however you are stoking more wood at any point than the air hole(s) can supply, then you are getting incomplete combustion. You are not realizing the heat energy from the wood.  If the chimney temperature is high enough to support combustion at the exit point.... when the fuel rich flue gasses hit air... they ignite... and you get a plume of flame that looks like an F-15 afterburner.  If the chimney exit point is colder than that... you get light to heavy smoke (particulates and CO and complex aldehydes and such.... not great stuff to put into the air). 

 

This unburned fuel circulating thru the wares also is casuing "reducing conditions" in the chamber.  If this is happening at a time you QWANT that to happen..... good stuff.  If not.... Defect city.

 

If you need to have a certain amount of wood going in to maintain the climb rate you want... and there is not enough air to get the fuel to burn in the chamber... you have to open up more of an opening into the kiln.  If you already have it wide open and the kiln is still not climbing...... you need to increase the chimney exit point temperature (to induce more draft).  Many ways to do this.... but more than I can get into here. 

 

Simple (but expensive) way to up the draft rate for any given chimney is to increase height and diameter (cross section).

 

Hope that helps.  Been firing wood since 1969.  It is my main finish firing solution.

 

best,

 

..................john

[birchhill]

Thanks for the reply!  It makes sense any flames past flue entrance is lost heat.  I guess if the standard bricks cant handle excessive heat there may be no point in pushing the limits of the kiln.   Really all I know at this point is I exceeded cone 06 enough to melt the cone into a flat blob but did not touch the 6.   So, if I stick with this operation range, what kind of limits will I have as far as glazes and durability of pottery in regards to making functional ware?  Also, some questions on drying pottery, I have read some people cover theirs with plastic to slow down the drying and some dont.   I want to avoid drying errors as a variable here.  What is recommended method for drying greenware?

I just have been placing it in a cool room of house for several days then moving next to wood stove after that.  I also set it up on pencils for air flow underneath

[birchhill]

Great info!    I was definitely getting jet after burner affect out chimney.  So, that seems to indicate over loading of wood or a need to increase air intake size?

[Guest JBaymore]

Not being there and not seeing the details of the kiln first hand... can't say for sure.

 

However the typical "newbie" (not meant derogatorily   ) kind of wood firing mistake is to stoke too much wood.  And when the kiln is not getting hotter as fast as they'd like after they do that,... to stoke even MORE wood.

 

So probably that is the starting point for your thinking.

 

best,

 

...................john

[neilestrick]

If your clay can handle the rate at which you are drying them, then you're good to go.

 

Any idea how hot the kiln actually got? Did you have any intermediate cones, like 2 or 4? Ideally you want to do that, so you have an indicator of the climb, especially if you're not using a pyrometer. I'd use cones 08, 06, 02, 2, 4, 6.

[birchhill]

Here is a picture of the  2 cones that where in the kiln.   An 06 which is melted flat and a 6 which seems un-touched

 

Is there anyway to guestimate temp from how much the 06 melted?

[neilestrick]

No guess from the 06. In the future you'll want to set the cones in a little clay boat- flat bottom with a little lip around the edge, because at higher temps the low cones will be puddles.

[Guest JBaymore]

The cone 06 has a characteristic that tells you something here.  Note the crumply, wrinkled, and gray/black surface quality and the sort of "bloated" feeling.  Also note that you can not still see the "corners" where the pyramids formerly existed.

 

A low level cone like this has a lot of iron oxide in it.  Even the "low iron" ones.  Red iron oxide ( Fe₂O₃ )is converted to black iron oxide in reduction conditions.  Black iron oxide ( FeO ) is a flux on silica, which is also in the cones.  It is also a colorant... and results in blacks in reduction, amongst other possible things.   The color of the surface and the wrinkling is indicative of this early reduction fact.

 

Another impact of early reduction on the low cone formulations is outgassing.  Which causes the bloated look. 

 

When reduced, a cone like this will melt at a lower amount of heat work than a cone in a non-reducing environment.  So the melting of this cone is coming from a state of early reduction in the kiln..... hence the fact that it melted. 

 

If it were a more normal looking surface and the sharp edges were "missing"... then you'd say that it likely was maybe 3 cones higher than what you are seeing.  But in this case I am guessing it was about cone 06 or LOWER where that cone was located.

 

That does not mean that in other locations in the chamber it was not HOTTER than that.

 

This also maybe confirms the "overstoking" idea that I mentioned earlier.  Another possible piece of evidence.

 

best,

 

..................john

 

PS:  Note that cones do not measure absolute temperature.  They measure the impact of heat energy over time... called heat work.  The faster your kiln gets hot....... the HIGHER the absolute final end point temperature will be when a cone bends.  And vice versa.  See the rate columns on a cone chart.

[birchhill]

Thanks. That's a lot of good info. I most likely was overstoaking it. I was routinely loading to capacity with thinly split Osage orange wood. The exterior brick was reading 900 on a stove thermostat . Thanks again for the replys and information. Up to this point my only experience with pottery has been YouTube videos and what I could read online.

[birchhill]

At the end of the firing while fire was still burning I sealed up entrance and chimney. I hoped to turn the pots black from o2 reduction. It did not happen other then the 06 cone turning black. I absolutely love the color of Maria Martinez pottery. How does one achieve this?

[Mark C.]

Goggle is your friend in this quest.

 

Bone dry  animal dung is helpful as well-you can see them stacking it in film

[birchhill]

Thanks for the video!   I just removed a couple of the bricks that supported shelf levels and they did turn black.  I also broke one in half and its the same shade all the way through.    IS this because of the iron content of the brick?   It was a dark red brick.   Did I achieve enough o2 reduction but only turned bricks black and not the pottery because it is made with a light gray clay?

 

[PeterH]

You may find the latest freebie interesting, especially the article on emulating wood-fired

results at cone 6 (oxidizing).

https://t.e2ma.net/click/y4twu/qe4oos/6yeb6g