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.