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Soda Firing
Soda firing involves the introduction of a sodium source into the kiln enviroment at cone 8. The soda combines with the silica, alumina, iron, and other elements in the surface of the wares to produce a glazed or flashed surface. Described and pictured below are two methods for introducing soda into a kiln.
The Gail Nichols method
The first method involves a soda mixture Gail Nichols developed using a mixture of calcium carbonate, soda carbonate, and soda bicarbonate. I make up a variation of this mix using 10 pounds of calcium carbonate(whiting), 5 pounds of soda carbonate(soda ash), and 5 pounds of soda bicarbonate. These dry materials are thoroughly pre-mixed and poured into a bucket of 5500 cc of water where they are mixed with a mixing prop and drill motor. This mixture becomes very hot and can burn your skin and hands, so wear goggles and gloves when mixing. In Gail's book, "Soda, Clay, and Fire", it is recommended that the soda ash is "light " soda ash, not "dense" soda ash. Either type works just fine, but the chemistry of the light soda ash allows for the mixture to set up like plaster, and is broken up into chunks for easy handling and introduction into the kiln. Mixing calcium carbonate with soda carb and soda bicarb reduces the fusing of the two sodas and has no glaze forming effects.
Pictured here are student works from a soda workshop. The benefit of this technique is that it takes an hour of soaking the kiln to volitize all of the soda mix, and vapor clouds associated with soda firings are greatly reduced. A heavy reduction for twenty minutes after the last introduction of soda, creates a variety of surfaces. These surfaces range from bleached out sections of glazed surfaces, heavy carbon trapping, to very dry, flashy, and sugary areas from high alumina clay bodies and slips.
Pablo's Red and Plumb Tenmoku on Steve Davis cups with manipulated flashing slips.

Flashing Slip cup and Plumb Tenmoku vase

At cone 8, the soda mixture is mixed and allowed to set up on a plywood board. Mark Coppos is preparing the soda mix and Steve Davis is introducing the soda mix. The burner manifold is removed to gain access to the burner ports. The soda mix is introduced via a piece of steel channel that reaches to the far wall of the kiln.

The soda mix is deposited directly under the flame path of the burners where it is volitized. This 9 inch tall area of the kiln is left open so that flames from opposing sides can evenly distribute heat and soda. The walls of this kiln are covered with used kiln shelves that take the brunt of corrosion from the soda and shield the 4 inch fiber blanket walls. A masonary drill and masonary hole saw are used to drill holes through the cordierite shelves. Porcelain buttons and 15 guage nichrome wire are used to hold the kiln shelves in place. They are inserted through holes in the shelves and pushed through to the outside of the kiln frame where the ends of the nichrome wire are twisted into the expanded metal skin.

Soda destroyed this extremely high alumina brick after 1 firing. I have spent many hours trying to develope a kiln coating that is resistant to the chemical attack of the soda. The soda vapors went right through the coatings and ate away at the underlining refractories. My current solution is to shield the kiln walls with some kind of sacraficial refractory such as used kiln shelves.
After the firing, a mixture of mostly calium oxide remains. After several days, the leftover mixture takes on moisture from the atmosphere and changes into a fluffy pile of white powder. At this time, the floor is vacuumed. I use coarse sand or grog as a filler in the floor to help reduce the chance of soda creeping into the floor beneath the floor tiles.
The shelves on the left were cut in half to open up the spaces between them. This allows wares to have greater exposure to soda vapors as they travel up from the floor. Half brick posts (soaps) are shown, but are not recommended. I had a major collapse in a previous firing because there was not enough support for such narrow shelves. A full, 4.5 inch wide hard brick is better. I now use 6 inch wide bricks as shown in previous pics and the shelves share these posts, achieving additional support. This is critical for the first, lower layer of shelves. The feet of vessels are kept within the surface area of the shelves. If they over hang, they will aquire too much soda and suffer glaze runs.
The frame of the kiln is 3 inch angle iron with expanded metal sides(#9-3/4 inch). The roof and floor are safety grate which is so strong that I can walk on the roof. Angle iron is used to hold the removable cable hook. The 2 masts that the lid raises on are dropped into square tubing that is welded onto the side of the kiln.
An 8 inch eye hook has had part of the hook removed so that it can slip over the gear screw of a 1/2 ton winch. A drill motor makes the lid move very fast up the 2 lid masts. This can be dangerous if the cable were to give, so a screw driver or metal rod should always be placed through both masts to keep the lid from falling on someone. The lid's square tubing that surrounds the mast should be much larger than the mast, or it will bind as the lid moves up and down.
Here are the middle and outside lid pullies (3 inch). They are bolted into 2 pieces of channel and are seperated by welded square tubing the captures the top of each mast.
These pics show the lid back when the kiln was built as a burnout kiln. An angle iron frame is incorporated into the structure to hold soft bricks for flues. The purpose of this construction is to transistion from a fiber blanket wall to a brick flue so that a damper shelf can move across a flat surface.
Where the safety grate has been cut to form the flues, a fiber gasket in placed in it's place. Then the fiber blanket is laid down in "wall paper" fashion. 4 inches of fiber blanket are placed into a 3 inch angle iron frame. Cone 10 porcelain buttons and nichrome wire hold the fiber against the safety grate and flue frame. Under proper tension, the fiber gasket will puff up where it will form a good seal with the soft bricks. With a good seal, little heat will creep over to the frame.
The pic on the left shows 2 layers of 2 inch fiber blanket, a 1 inch fiber gasket, and the soft bricks that form the flue. The pic on the right shows the effects of soda vapors over several firings. This fiber can rain down on pots during the firing, so it needs to be brushed prior to loading the kiln. Always wear a face mask and goggles when generating fiber dust. It is possible to hang light weight shelves from the ceiling, but you can also load shelves above your wares to shield them from the fibers.
Here are the finished flues with cordierite shelves for dampers.
This burner manifold is removed to gain access to the ports when the soda mix is introduced. I set the damper and red handled gas valve to wide open at 2,000'F, and adjust the propane regulator for the proper air fuel ratio. I do this the first time I fire the kiln, and then mark the site glass on the regulator with a marker. Then, in future firings, I will have a relatively efficient atmosphere. The atmosphere should be nuetral for most of the firing until around cone 8. At this time the soda mix is introduced. The atmosphere of the kiln is going to become more reduced as the temperature goes up. This is caused by the increased expansion of gases as the kiln rises in temperature. Since the propane is coming into the kiln under more pressure than the natural draft of air, the atmosphere will become more and more fuel rich. As a result of these atmosheric changes, adjustments must be made during the firing. An easy way to check the atmosphere of the kiln, is to take a peek at the underside of a damper. If it is covered in soot, then you are fuel rich. All propane sytems must use a regulator. A regulator that is a step up from a bar-b-que regulator and goes from 0-15 psi is a good choice. The actual pressure at the burner maifold is in inches of water column, even though the pressure is shown as 3 psi at the regulator pressure guage. This pressure drop is caused by the resistance in the narrow propane hose. I should install a pressure guage at the burner manifold between the burners and the gas valve. After the soda has been introduced, I turn the gas down a bit to hold the temperature steady around cone ten, and heat soak the wares for about an hour. This gives the soda mix additional time to "paint" the wares. At the conclussion of the firing, I leave the dampers half open to cool the kiln. The kiln is opened first thing the following morning.
There is no flame safety for this kiln, so wind shields are used in the early stages of the firing and a watchfull eye is kept. Here we are, unloading a workshop firing.
The Soda Ash Solution method
In this series of slip tests, soda ash was mixed with hot water to create a liquid solution of soda ash. This solution was poured into a weed sprayer and injected through holes in the side of the kiln. This takes about 10 minutes and requires 3 pounds of soda ash for this kiln. Any residual soda solution must be rinsed out for it solidifies upon cooling. A steel brake line tube can be fitted to the brass tube that comes with the weed sprayer. A compression fitting is used to attach the two sections of tubing. The steel tube is inserted into the kiln to about half way into the kiln. The solution is then introduced into the kiln by squeezing the trigger of the weed sprayer. With a count of 8, slowly pull the steel tube out to disribute the solution into the kiln. After the solution is introduced, the kiln continues to fire until cone 10 is achieved. There are 4 different clay bodies shown below. They are Aardvark Clay's Nara Porcelain, Woodfire, and Black Mountain. The last of 4 clays is Grogzilla from Clay Planet. The slips were applied on greenware and bisque fired to cone 04. They were then placed into the kiln for optimal spacing between the soda injection holes. They were wadded with a 50/50 mixture of kaolin and alumina hydrate. Wax resist or white glue can be used to hold the wads in place as the pieces are placed onto the kiln shelves. The burner system is a double blower set up that fires on 3 pounds of propane pressure. Parts and construction details can be found in the "Kazegama" link. A clear channel in front of each burner is left open down each side of the kiln walls in order to allow flame and heat to travel to the back of the kiln and then over the top of the wares on the upper shelf. I am disapointed in this technique for there is too much damage caused to the refractories and the pots from excessive soda spray which is very hard to control or regulate.

Johnson Flashing
Grolleg 50
EPK 10
Newman Red 10
Nepheline Syenite 30
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Steve's Orange
Grolleg 70
Newman Red 10
Nepheline Syenite 20

 

Van Gilder
Yellow to Gold
EPK 80
Nepheline Syenite 20
Soda Ash 2

Fake Avery #5
EPK 74
Newman Red 3
Nepheline Syenite 24
Slip #1
EPK 50
OM4 ball clay 50
Borax 6
Zircopax 12.5
Davis Red
EPK 70
Carbondale Red 10
Nepheline Syenite 20
Fake Avery 2002
McNamee 70
Imco 800 5
Nepheline Syenite 25
Tan
EPK 50
Taylor ball clay 50
Smooth Orange
EPK 40
OM4 ball clay 40
Borax 5
Zircopax 10

Orange
EPK 35
OM4 ball clay 50
Silica 5
Nepheline Syenite 10
Pots are spaced to accommodate the sprayer tube.
A variety of clay bobies and slips were tested.
Spraying soda solution through 12 holes on each side.
An old Kazegama frame was converted for soda testing.