Firing the PrattMWP salt kiln

Salting and cooling-

At ∆8 we begin to salt the kiln.  We continue until we have reached ∆9 or 10.  Some potters adjust the damper during salting. We keep the damper setting still.  We can discuss other options later.

We use a salt mixture of 2 parts salt, 1 part sawdust, 1 part fine ash. There are many recipes.  We play with this recipe from time to time. We do not use sodium bicarbonate or similar materials.  On occasion we throw in some borax, but I like the addition of ash better.  It is light and more likely to carry through the kiln with the vapor than other material.  The sawdust adds reduction material and ash.

The mix is scooped into a length of angle iron that will fit through the salting port, the opening of which is the size of a soap (a straight brick cut in half length wise).  Slide the angle iron into the kiln and turn.  You will develop a rhythm or schedule for the saltings- two troughs each side, every 8 minutes, for instance.  Continue until you have a good glaze buildup and/or reach ∆9/10.

After a salting or two, pull a test ring to gauge the glaze buildup.  We use a rod (rebar) to hook the ring from inside the kiln.  Drop the ring into a bucket of water.  When cool dry it off and check the glaze.
As the kiln ages and "seasons," it will require less salt to get the glaze buildup.  I have read some different notions about this, but I find it to be so.

After we have reached the temperature and glaze desired, we shut the kiln down.  Turn off all burners.  What you do now will have an impact on the glazes.  We "crash cool," or open all ports and damper to get the kiln to cool down quickly.  The idea is to get the glazes to "freeze," resulting in a higher gloss.  You can play with this to see what effects you can get.  If you do crash cool, drop to 2150˚F or so. Do not go below 1950˚!  The chamber should look bright orange at its coolest.  This will take a shorter time than you think.  After cooling, seal the kiln tightly, closing all ports and damper.  Then, patience. Open the kiln when you can comfortably place your hand in the upper part of the chamber.  Wait a good 24 to 36 hours to do this!  There is a common rule of thumb for heating and cooling.  Take as much time to cool as you took to fire.  We fire rather fast so we keep to the 24-36 hour rule.  More exactly, after 24 hours, we open the damper and spy ports and wait another 10 -12 hours.  The results are worth the wait!!!

Next post about salt firing- some alternate approaches using salt.

Firing the PrattMWP salt kiln

Ramp and Reduction-


After a bite and coffee it is time to get back to work.  But first a word of caution-  the early stage of the firing is the most dangerous for two reasons-  your safety and the pots and kiln remaining intact.
  


First, the pots- the initial heat up is when clay is most vulnerable. Moisture is the main concern.  As mentioned before, we fire bisque ware to reduce our worries in the early part of the firing.  This lessens the chance of steam building up in the pots and causing blowouts.  If you have green-ware or freshly glazed pots in the kiln, the front end of the schedule should be slow.  A note- if you do fire green-ware, which is a good idea for fuel savings, the culprit behind blowouts is not air pockets, but firing too fast.  You could fire clay with more pockets than swiss cheese and be fine if the schedule is slow enough.  It is steam that you want to avoid.  Steam is pressure. It is created by firing too fast.


Second, safety- the early part of the firing, when the chamber is dark, is the most dangerous.  If a burner blows out, unburned gas will pour into the kiln until something, most likely another burner, ignites it, often violently.  I have a few stories of such events.  All had good endings, just pottery or kiln damage.  Do not walk away from a kiln early in the firing!  Be there to monitor the burners.  When the kiln chamber reaches red/orange heat you can relax a bit.  Then, if a burner blows out, the heat from the chamber or firebox will ignite the burner.  Don't count on it though.  Be there!  Some other time we can talk about safety burners with baso valves and pilots, et. al.- good options, but they can be dangerous as well. Nothing is better than being near the kiln during a firing.


Ramping or turning up the kiln is mostly a matter of increasing the gas and air.  Keep the flame a nice lazy blue as you increase the fuel.  Turn the fuel up first, then add air to get the flame right.  Do this in stages. We turn the opposite corner burners up at the same time. How much depends on the strength of the burners. Aim for a schedule that will give you red heat in four to six hours, again assuming the ware is bisqued.


Oxidation and reduction might be explained as "clean" and "dirty." Oxidation is complete combustion of the fuel- plenty or air- leading to a clean blue flame. Reduction is incomplete combustion- not enough air- leading to a "dirty" yellow flame.


The damper is a key element in ramping and controlling the atmosphere.  Our damper is in the stack, about rib high.  Some designs have the damper in the horizontal flue between the kiln and the stack.  I prefer placing it in the stack.  It is easy to access and view.  The damper plate, made of a strong refractory material, usually a kiln shelf, controls the amount of hot gases exiting the kiln. The stack creates draw.  The damper plate controls the amount of draw. In doing so, it effects the amount of air mixing with the fuel in the burners.  Draft or draw pulls the flame from the burner through the kiln.


Our kilns are a natural draft design- the stack and damper control the pull of hot gases through the kiln.  Some kilns are force air or forced draft.  These have a blower attached to the burner which pushes the air/gas mix into and through the kiln.  Updraft kiln designs are usually forced air with no stack but still require a damper.


When firing, you open the damper to create more pull or draw; close it to decrease draw.  The damper works in tandem with the burners controlling heat gain.  Importantly, it also effects the atmosphere of the kiln.  In general, opening the damper draws more air into the burner flame, making it cleaner.  This creates an oxidizing atmosphere in the chamber.  Closing the damper decreases the amount of air to the burner flame, creating a dirty flame (unburned fuel) resulting in a reducing atmosphere in the kiln.

The damper can offset what you are doing with the burners, so gaining some knowledge of its function is important.  For our purposes we will keep things simple.  In the early part of the firing keep the damper open as much as possible.  This will keep the chamber oxidizing (clean- little or no unburned fuel).  Close the damper some (to a set position) when you are ready to start reduction.

Reduction should begin between ∆ 010 and 06.  I have read articles that say you can reduce as late as 2100˚f.  Shinos require an early reduction to get carbon trap.  We are looking for a good body reduction to set up the glaze color from the vaporization of the salt. It is called reduction, or a "reducing atmosphere," for what the free carbon or carbon monoxide (from unburned fuel) does to the clay and glazes.   Clay (everything, really) contains some amount of iron.  Iron in its oxide state- what we put in glazes and what is found naturally occurring in clay is written Fe2O3.   Free carbon or carbon monoxide is unstable at high kiln temperatures.  It wants to attach to an oxygen molecule(s) to become C02.  There are two sources.  One is the air around the kiln.  The other is the oxygen molecule found in the iron oxide contained in the clay or glaze.  It robs an oxygen molecule from the iron or other metal oxides and thus "reduces" the iron from an oxide (Fe2O3) to a metal (FeO) on its way to becoming C02.

Reduced iron is a grey (metallic) color.  Break a pot that has been fired in reduction.  You will see a dark cross section.   The pot shard will have a brown surface, however.  This is from re-oxidation of the iron in the pot's surface. As soon as the kiln is shut off, air rushes into the chamber and creates an oxidizing atmosphere.

Damper in to create enough back pressure that you get some flames at the front upper spy holes, about 6 -8 inches.  This tells you that the chamber is reducing.  Continue to fire the kiln with some flame showing for the rest of the firing.  We will adjust things a bit later.

The kiln chamber is constantly changing.  As the kiln heats up pressure builds.  This can change the visual cues.  The reads may change so be ready to make adjustments at any time.  Keep the kiln ramping while getting the same flame at the spy ports.  Continue until ∆8.

Basics of Firing a Salt Kiln

Firing any fossil fuel kiln can be broken down in a few different ways.  We will organize it as startup, ramping/atmosphere control and firing off- roughly beginning, middle and end.

Starting a kiln: lighting burners-

Before lighting any kiln make sure all the burner valves are off.  We have a separate main valve gate so this is especially important when we first open the main.  The burners (natural gas in our case) are nothing more than carburators.  They mix air with gas. Our burners are homemade "alfred" type, using basic pipe fittings.   I will post images soon.  You can find the design in books on kiln construction. They have a small chamber containing an orifice for the gas and a primary air port, or gate,  to let air in to mix with the fuel. We also have used manufactured venturi type burners which operate in the same way.  They are easy to find and operate.

To light the burner, use a small propane bunsen torch as a pilot. (Use the appropriate safety gear.  I will assume this through out these posts.  There is plenty of information about safety in books and magazines.)  Make sure the primary air is closed tightly.  Light the torch and place its flame in front of burner you are going to light. Slowly open the burner gas valve until a small amount of gas is released.  We call this cracking the burner.  When the burner ignites the flame should be lazy and yellow.  Leave the flame this way for around ten minutes or so. This will warm the burner, making it safer to introduce air to the flame.

After ten minutes, usually less in the summer, the burner can be adjusted. Give it more air to get a hotter, cleaner flame.  This is done by slowly opening the primary air port.  Give the burner enough air to turn the flame blue.  You may see some yellow at the tip.  This is fine.  You want an "oxidizing" flame.  This is a clean burning flame that has sufficient air to burn all or most of the gas.  Do not give the flame too much air.  This will result in the flame being cooled by it own air supply.  It can also result in "coughing" or back burn where the flame recedes into the burner chamber.  You will hear it.  The burner will also get very hot so be careful. Shut the burner down for a minute and start over.  Repeat this process until all the burners are on. The timing for lighting burners depends on the pots in the kiln.  If you are firing greenware the kiln should be fired slowly.  We fire bisque, so lighting all the burners (4) can be done in a shorter time.

Please note that I refer to sensory cues. You will not learn to fire a kiln (or troubleshoot) by being given numbers, or inches, or the like. I will give some quantitative reads at times for sure.  But all kilns (and burners) are different so a good general knowledge the process is key.

Once all the burners are going with a nice lazy blue flame, you can relax for a bit, have a bite or a cup of coffee before ramping the kiln.

Our firing schedule is roughly this-

8:00 or 9:00 am- start the first burner.
By 9:00 or 10:00 am- all burners on.  Start ramping.
Ramp kiln to ∆ 010 - ∆ 08 (1650˚f -1750˚f, roughly). Start reduction*
Ramp and reduce until ∆ 8.  Start salting.
Soak and salt until ∆ 9 is down, which is ∆ 10 pointing at nine o'clock.
Shut off all burners, and crash cool to a bright orange kiln chamber.

Usually this takes 10 to 12 hours, sometimes a bit longer.  We try to salt after dusk so the vapor is not as noticeable.  If passers by call the UFD, they have to come.  We always call them before salting to give them a heads up.

* There are two schools of thought on reduction.  Some potters do a "body" reduction at this temperature (for a given amount of time) and then ramp in a "neutral" atmosphere until doing a "glaze" reduction at ∆ 8,or 9.  We favor ramping in a reduction atmosphere. There are pros and cons about this in terms of being green and "wasting fuel." We try to strike a middle position.  The kiln results are the priority. A bad firing is neither green or efficient.

Next: Ramp and Reduction

  

Firing the PrattMWP salt kiln- some notes...

Our salt kiln is built mostly out of k23 and k26 insulating fire brick.  Although this type of brick is susceptible to faster deterioration,  coatings or not, we feel that the time and fuel savings are enough to warrant using them.  From a teaching standpoint, the ability to fire our kilns using a shorter schedule is especially helpful in a continuing education setting.  Our students do not always have the time to tend a kiln for 24 hours.  There is no need for this anyway.  A good firing schedule does not have to be long- just set up for a safe and and successful outcome.  A good steady ramp and knowledge of some basic elements that impact ceramic materials are needed.  

Over the next few posts we will discuss heat gain or ramping and controlling of the kiln atmosphere.  These are easy to describe but nothing improves firing skills more than experience, trial and error. Learn to read the kiln.  Take good notes.   Pay close attention to cause and effects.  Lectures or written instructions are a good start but never a substitute for doing and seeing.  You fire with the senses!  All the pyrometers and Co2 analyzers will not take the place of using your eyes, ears and nose.  Weather is often questioned as a factor.  It is to some extent.  Barometric pressure can have its effects.  Outside temperature is of no consequence.  For this discussion, we will assume the kiln is protected from the most problematic of weather conditions, the wind.  


We fire our salt kiln with natural gas. Some other time we can get into the differences between fuels.  Most potters use natural gas, propane or wood.  Oil is still used, but it seems rare.  Still, the basic ideas we will talk about are the same regardless of fuel.    


One last thing-  your head should be inside the kiln!  Some of the reads are "external" but they are symptoms of the atmosphere in the chamber, surrounding the pots.  And the reads will be different from kiln to kiln.  They can also be different from firing to firing.  Good notes and kiln logs will help in seeing  the nuances and effects of loading patterns and kiln designs.  This stuff is amazing.  How nature applies its laws to the firing and how materials react to heat, oxygen and carbon, we might take for granted.  Over the years one can lose the mystical sense of any firing.  But it is sheer poetry (in motion!).  It isn't the outcome.  It is the input.  Good craic(!) as they say in Ireland.

Firing the PrattMWP salt kiln

Loading the salt kiln


Previously, we talked about prepping the salt kiln for firing.  This involves cleaning any debris from the last firing and getting the kiln furniture ready.  Wash the shelves and make sure the posts are in condition to make it through another firing.


Loading the kiln is relatively simple.  It is a bit more time consuming. The pots must be set so they will not need excessive clean-up after the firing.  Since the shelves are coated with a wash (see earlier post) that will be unaffected by vapor, the pots can be place directly on the washed and dusted shelves.  But as an added precaution, we place a clay and alumina wadding directly on the foot of the pots.  There are many recipes for wadding out there.  Some inexplicably call for the addition of flour.  I think it is added as a binder.  All it does is cause the remaining wadding to go sour and stink.  No need for that.  Follow any of the recipes you might find.  Leave out the flour and you will be fine.  Ours is simple.  Kaolin, Alumina, grog and sometimes pyrophylite.  We use something close to equal parts by weight EPK and alumina (hydrate or oxide) with enough grog and water to make it stiff but workable. Add some pyrophylite if on hand.


We make marble sized balls of wadding, dip one side in a water based wax resist and stick that side on to the bottom of the (bisqued) pots.  On occasion, we brush the bottom of some pieces with a wax resist that has alumina mixed in.  But this approach is more for porcelain than stoneware.

The pots are wadded as we load.  Pots should be stacked a bit more loosely than a reduction or electric firing.  Leave enough space for the vapor to flow around the pieces.  With experience you get to know what part of the kiln gets more vapor and load accordingly.  Loading does alter the flow of flame and vapor through the kiln.  Shelf placement has a greater affect.  Most kilns have at least two banks of shelving.  Make sure the shelf heights in banks are staggered as much as possible.   You don't want to create heat zones or sinks.  Remember that the vapor is in the kiln chamber for a very short time.  It is simple fluid dynamics- baffle and flow.  Keep the vapor moving inside the chamber as long as possible.  Adjust the bag wall if necessary.  For example, adding height to the fire wall may push heat to the crown.  Think of fluids moving through the  kiln and around the  pots.  But this is a bit more involved than we will get into now.  More on that in another post.

Our kiln is a downdraft.  The fire boxes are on both sides with the salting ports directly over head in the front wall about midway up the kiln.  The ports are high enough that the salt will have a distance to fall and vaporize (few feet), but not too high for placing the salt in the kiln.  We use a five foot piece of inch and a half angle iron as a trough that we slide into the port, parallel to the side wall of the chamber, and rotate, dumping the salt over the firebox.  But, again, I am getting ahead of myself here! More on this later.

Place cone packs throughout the kiln.  We use ∆ 8,9, and 10.   Use more cones if you want to see heat gain or need a marker for knowing when to start reduction.  You can never have too many cones.  Our kilns have more spy ports than you might see in other designs.  In addition to the usual ports in the door (3), we have two in each side wall.  You need to be able to see what is going on in the kiln!  Some potters believe that the vapor affects the cones end point.  I am not convinced that this is true in any measurable, significant way.  So we don't protect the cones.  The cones and the test rings are the only tools that give an accurate sense of heat work.

Place draw rings in front of the cone packs at each front spy ports.  Both cone packs and rings should be in far enough to get a good read on the chamber temperature and vapor gain, yet still visible with the rings within easy reach of of the draw rod.  Make sure the rings are sized to fit through the ports.  Remember they will shrink some in the firing.

We brick up the door with straight (high duty, fire) bricks all laid up in a bonding course, or short side facing into the chamber.  I learned this from Peter Sohngen some years back.  Only one end gets glazed.  I am not sure which is a better door, one of bonding courses or a door of stretcher courses (which still need a bonding course every 5 rows or so).  In either case, you will have to replace bricks often enough.  I am planning to experiment with a door design that would use straights tapered ends in a bonding course door.  We will let you know how that goes.

Make sure the cone packs and test rings can be seen at the ports.  Leave a few bricks out at the top of the door for early draw when you start the burners.We usually place an offering of sake (or schnapps or whatever is on hand!) and rice in bowls on the kiln top and start the firing.  In the next post of this series we will talk about starting the firing, the optimal firing schedule, controlling the ramp and atmosphere, and salting the kiln.  Happy New Year to all!

Happy New Year

The fall anagama firing seems like quite awhile ago.  The end of semester work load (mainly on Janelle's shoulders- with help from Vincent) was quite a crush thanks my needing a new left hip.  All has gone well. Many thanks to Janelle and Vincent for filling in.  They are the best.

The was our fourth firing.  We continue to learn a great deal each time.  This firing was no different.  First, we were determined to take this firing to 40 hours wanting more heat work and ash deposit.  We came within a half an hour of our goal. We were able to extend the firing just fine without over-firing the front of the chamber.    As mentioned in an earlier post, we saw the effects heat work in action.

Second, we took a different tack toward getting heat to the rear of the kiln.  Previously, we raked coals from the fire box toward the end of the firing to create room in the fire box for air to sweep over the coals. Create a hotter firebox, then draw this heat to the rear.  In addition to the open airports, we were opening the stoking port to give the kiln as much air as possible.  Air = heat!  This contributed to more heat alright- concentrated in the front of the kiln.  And hot coals at our our feet made stoking tougher and more tiring- not to mention the need to put the hot coals somewhere.

We found that raking hot coals is not necessary at all.  In fact, it makes little sense when the idea is to get as much heat from your fuel as possible.  A lesson I learned when younger (rake as few coals as possible) is taken to the logical end.  So we have made life a bit easier by being a bit smarter and patient.  It's these little things that experience brings.  I especially like solutions that repair more than one problem, in this case- no coals to deal with and moving heat (work) to the rear of the kiln.

We stirred up the fire box regularly to keep the fly ash moving and to keep the surface area of the burning wood exposed to as much air as possible.  This also was a better way to manage the coals.  So patience in stoking and letting the firebox work was the key.

We also used quite a bit of pine bark at the end for color.  I am not sure of the result- willhave to do it again.

We didn't use the Troy body this firing since the clay shipment didn't come in time.  We plan to experiment with it more next time.  Maybe the bark will have more of an effect on the lighter body. I like a darker body, but it would be nice to have a bit of light burning stuff to play with.  I find the bodies that seem popular now like the B Mix seem to be on steroids, loaded with plasticizers.  The Troy body is similar.  Good for simple full forms and great for surface marks and tracks.  I find it difficult to make pieces that demand more from the clay, some tooth (the folded walls I like are too thick- maybe with more time I will get it).

I want to play with some things like lithium wash and soda ash to see what happens.  Making a slip out of the Troy body might be interesting also.  And the old stand-by of Rutile and G.B. is always good.

The Master

We will be posting images and comments on the wood firing shortly. I ran across some images of our man Vincent Clemente, retired professor of art here at PrattMWP, and the dean of potters in the Mohawk Valley. Thought I would share them. He is demonstrating for my sophomore throwing class. At 86, he is still an integral part of our wood and salt firings.

For more information on Vincent, see the May, 2008 issue of Ceramics Monthly.