Electric to Gas Kiln Conversion

Undergraduate Research in the Arts

Kyle A. Crowder May 16, 2014

Warren Wilson College Undergraduate Research in the Arts 2014

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Introduction

In my research I am looking at the feasibility of converting an electric kiln into a gas

fired salt kiln. The feasibility will be based on the expense of labor and money, and also the

product that is achieved through firing it. Depending on the functionality of the kiln, I will

further my research with experimentation in surface treatment and glazing for this specific type

of firing. I am very interested in the surface created by a salt glaze in terms of effects and

aesthetics. I hope to see this inform the work that I create now and for my senior thesis project

Background

Many ceramic vessels we use are fired. This simply means that the clay is heated in an

“oven” for a given amount of time until it is vitrified1. This is not the only method of firing to

achieve this type of arrangement for functionality but the majority of functional ceramics use a

glaze to create glassy surface that is smooth and clean for ease of use.

One can think of a kiln like an oven because kilns burn natural gas, electricity, wood, or

even oil. Each fuel has properties that change the effects one can create. For my research I am

focusing on natural gas as my fuel. The benefits of this fuel are that it is reliable and consistent

compared to wood. It is a Hydrocarbon2 so it allows for control over the redox3 of the

                                                                                                                         1  Vitrification-­‐  “the  action  or  process  of  becoming  glass:  the  high-­‐temperature  process  whereby  the  particles  within  

the  mass  fuse,  closing  the  surface  pores  and  forming  a  homogeneous,  impervious  mass  without  deformation.”  (Rice  1987)  

2  Hydrocarbon-­‐  “…contain  only  carbon  and  hydrogen  will  the  general  formula  of  CnH2n+2…”  (Wentworth  1973)  

3  Redox-­‐  Reduction  or  Oxidation  in  the  atmosphere  of  the  kiln,  in  other  words  the  amount  of  oxygen  allowed  

completing  the  burning  of  the  fuel.  

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atmosphere. The costs of the fuel are dependence on a fuel company and gas line repair can be

dangerous and should be done by a professional.

The kiln that I have built is a 10 cubic foot updraft4 cone 10, natural gas burning

atmospheric5 salt kiln. Cone 10 refer to the specific temperature to which one is trying to reach to

vitrify the clay body, for the porcelains and stonewares that I use this is about 2350 degrees

Fahrenheit. For my project I am working only with salt, or sodium chloride as my flux. This salt

added to the kiln when it is almost at peak temperature creates a glaze on bare clay.

A glaze is composed of mainly three parts. There is a glass former which is silica or flint,

a flux that lowers the melting point of silica and a stabilizer that bonds the surface of the clay to

the glaze and keeps the glaze suspended on the pot. These three parts are present in all glazes in

some shape or form. There are many other parts that are not essential like colorant, stains,

secondary fluxes, and opacifiers. Glazes are typically applied to the pot by dipping or spraying a

liquid mixture of the compounds.

The purpose of the flux in the atmosphere allows one to leave parts of a pot unglazed

when it goes in the kiln. The flux in the atmosphere replaces the flux that a glaze would contain;

the other two parts of a glaze are in the clay, a stabilizer in the form of aluminum oxide and silica

in the clay body. The flux acts on the surface of the pot and creates a glaze. Once one fires a kiln

with these fluxes, it is forever going to have residue that will affect future firings. I am

                                                                                                                         4  Updraft  Kiln-­‐  “A  kiln  which  has  a  draft  that  enters  through  the  inlet  flues  at  the  bottom  of  the  kiln,  passes  into  the  chamber,  and  exits  through  the  flues  in  an  arch  or  dome…”  (Olsen  1973)  

5  Atmospheric  Kilns-­‐  Kilns  with  a  flux  added  to  the  atmosphere  inside  the  kiln.  Compounds  typically  used  are  wood  

(calcium  carbonate,  CaCO3),  table  salt  (sodium  chloride,  NaCl),  or  soda  ash  (sodium  carbonate,  Na2CO3).  

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experimenting with only sodium chloride that is in the form of Morton’s Canning and Pickling

Salt.

This glaze is hard to describe in terms of color, it is typically a brown or orange color

with stoneware clays do to the iron in the clay, and with porcelain it is typically white because

there is no iron present in the clay. For simplicities sake the glaze will take the color of the clay

body because the clay body makes up two-thirds of the glaze.

Salt glazed pots first appeared in Germany during the late 13th or early 14th centuries.

(Rogers 2002) The technique is thought to have been a mistake by using old barrels which

contained brine in a wood kiln. The salt from the brine would have deposited a strange texture

most likely considered a mistake. Since then salt glazing has continued to spark interest in

potters across the world.

When building kilns it is important to know the difference between different bricks

because they are made for different jobs. Hard brick or high fire brick is used as a face brick,

meaning it is lining the inside of the kiln. This brick is more resistant to deterioration from the

atmosphere. Soft brick is the insulating brick. It is less dense and holds heat better but does not

stand corrosion for very long. This is where the decision to use an electric kiln for an

atmospheric kiln can become problematic. Electric kilns are only made out of soft brick so to get

around this one can apply a kiln wash6 over the walls to protect them.

Construction

                                                                                                                         6  Kiln  wash-­‐  is  composed  of  a  refractory  compounds  that  protects  the  brick  and  makes  it  last  longer,  recipe  that  I  

used  on  page  8.  

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Figure 1

Figure 4

Figure 2

The construction design was based on William Baker’s design,

Figure 1, a kiln builder and soda fired potter in Penland, North Carolina. It

starts with a layer of cinderblock on a level surface. Then bricks are stacked

to create a firebox7. The electric kiln is gutter of all elements and electrical

wiring and place on these bricks. With all of this together you end up with

about 11 cubic feet of space in the kiln. There

are two holes for the burners and on pull out brick that will act as a

salt port. The lid from the electric kiln has a 9x9 inch square cut out

for the chimney. A damper was constructed out of steel to allow

a18x24 inch kiln shelf to slide over the hole to act as a damper to

the chimney to control the flame. These types of dampers work very similarly to ones for wood

stoves or fireplaces.

Here you can see the flame’s hypothetical path through the kiln. The shelf right before

flue is there to help recirculate the flame. This creates a similar effect to what a down-draft kiln

would have. It would usually have

burners near the bottom of the kiln and

the chimney port would also be at the

floor level. The flame then, based on

brick arrangement would have to go up

the wall, into the middle, through all of

the pots, and then out the bottom. This typically is more even in terms of temperature. With a

                                                                                                                         7  Firebox-­‐  space  in  the  kiln  built  to  allow  the  flame  to  pass  into  the  kiln  without  being  stopped  by  bricks  or  pots.  

Fig. 3

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kiln this size the most important part is arranging the shelves to allow an even distribution. I have

found that staggering the shelves dramatically increases the evenness of the firing.

The next step was the natural gas and burners. I contacted Mark Ward of Ward Burner

Systems in Tennessee and laid out the right burners I needed to fire this kiln in about 12 hours. I

built an adjustable stand for the two MR-750 Venturi burners8. These are controlled manually

with ball valves and a Safety Shut-off valve9.

This is the most expensive part of the whole

set up. Natural gas can be a dangerous if the

right precautions are not taken to ensure that

everything is done correct and safely.

The last step was the chimney and

damper set up that I designed to be done very

cheap and quick. We had been given a quote

of about $2000 to install a stainless steel insulated chimney through the roof. I could not see that

much money go to something that was relatively temporary. It is import to think of the scale of a

project and make sure that one considers the longevity of a kiln when they are building it. I don’t

expect this kiln to last more than 20 or so firings because of the damage the salt atmosphere can

do to the soft brick. The chimney I built was made of zinc coated steel piping made for heating

and cooling ducts in homes. It’s cheap, only about $30 for all the pieces. The reason I even

                                                                                                                         8  Venturi  burners-­‐  Figure  4.  (Ward  Burner  Systems  n.d.)  

9  Safety  Shut-­‐off  valve-­‐  Control  shuts  off  both  main  &  pilot  gas.  Pilot  gas  is  taken  from  within  the  control.  Pilot  

activated  by  reset  button.    (Ward  Burner  Systems  n.d.)  

Figure  4  

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Figure  5  

included the chimney was because of the heat buildup and the smoke from the salting under the

roof. This is not a great solution, after 6 firings it has mostly fallen apart.

The damper is a control of the airflow leaving the kiln. This in turn controls the draft or

pull of the kiln. By not allowing much air to escape through the chimney, having the damper

mostly closed, creates a reduction atmosphere in an

updraft kiln because there is fuel that is not being

burned due to a lack of oxygen being pulled in the

kiln. Open the damper and oxygen is effectively

pulled through the burner port and a mostly complete

combustion of fuel is achieved in the kiln. It is

important to control this because they affect the color

of glazes. Oxidation will give you a pale celadon

glaze. Reduction can make a copper green glaze, into

a copper red glaze. These colors based on the

different isotopes10 of the colorants based on more or less complete combustion of fuel changing

the levels of carbon and oxygen interacting with the colorants in the glaze.

Firing the Kiln

The first firing happened Monday the 31st it was a failure… It was a hard night/early

morning watching the kiln stall at about 1900 degrees F. I struggled to get up the next day and

think about trying to do it again. I stated by investigating the burners. Turned out the orifice was                                                                                                                          10  Isotopes-­‐  atoms  of  any  one  element  can  have  different  mass  numbers  based  on  the  proportion  of  neutron,  

protons,  and  electrons.  (Wentworth  1973)  

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the wrong size. Because we had found these in the department, this was not a fault of Mark

Ward, it was the orifice of the burners I had found that were drilled at about 3/16 of an inch, built

for high pressure propane. I had assumed that because they were used on a natural gas system

before that they would work. After realizing this I changed out the orifice to the proper size of

1/8 of an inch. I tried it again on the following Wednesday. I could not even believe it, but it

made it all the way up to cone 10 within 13 hours.

Price Breakdown

The 3-D Department had old hard and soft bricks that I used so this cut my cost down.

Hard bricks run at about $3 dollars for a 9x4 ½ x3 brick and soft bricks are about a dollar more

per same size brick. I used scrap kiln shelves and cinderblocks. The electric kiln was donated by

the department but it is not rare to find someone looking to get rid of an electric kiln for free. I

also have a background in fabrication so small pieces like the burner stand and damper slide

were built for free.

• $370.00 for Safety-Eye automatic gas shut-off valve

• $500.00 for Natural Gas piping and instillation

• $38.00 for chimney

• $100.00 for 3 ft. x 100ft. roll of Kaowool

Total cost to build: $1008

Firing Breakdown

After one failed attempt and five successful ones I am very pleased with what has been

achieved in a kiln of this caliber. I have honed in on an ideal firing schedule that hopefully will

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grant consistent results in the future. I start the kiln in the morning, candling11 for three to four

hours. During this time salt port and peeps are removed and the damper is at about three inches.

In the first bump I close the peeps and salt port. I bump the gas to about a quarter of total

pressure and push the damper in to two inches. This must be slow because this is a very fragile

state for clay going up and cooling. I hold this until cone 012 is totally flat top and bottom. At

this point the gas is bumped again to almost full pressure and primary air at a half inch. The

damper is adjusted within half an inch usually increasing with the temperature. This lasts three to

four hours until cone 6 falls top and bottom. The gas and primary air can be totally opened at this

point. Salt is added after cone 9 is half over every 20 minutes or so. However much salt you are

adding should be evenly spaced within the last hour or two of the firing. Once cone 10 is half

over I shut the kiln off and close it up tightly and wait till it is all the way cool to open it. This is

not perfect but it’s a place to start. It should all happen within 12 hours but it varies on many

things.

Conclusion

Overall through the whole process, ending up with the result I did, I believe that this is a

worthwhile experiment. For only about $1000 I was able to achieve passable salt fired pots. I

don’t know how many more firings I could really do in it based on the state of the bricks after 6

firings. I hope that some of the resources and techniques I have laid out help and encourage

others to attempt this. It would be perfect for a beginner potter or a school, like Warren Wilson

College, without the kiln facilities large schools have.

                                                                                                                         11  Candling  refers  to  holding  the  flame  very  low  to  warm  up  the  kiln.  

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Results

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Recipes The Kiln Wash is sprayed or thinly painted on the walls of the kiln. This recipe comes from Gay

Smith who got it from Tony Clennell whom used it for a soft brick soda kiln in Washington. Tony’s Kiln Wash

2 parts kyanite (sillmanite) 2 parts zircopax (ulltrox) 2 parts alumina hydrate 1 part vee-gum For an atmospheric kiln one needs to wad their pots because the flux in the atmosphere will melt

everything together. This will keep your pots from sticking to the shelves. Wading Recipe

1 part alumina hydrate 1 part EPK (Kaolin) Flashing slips can add a great color variation to your work. This can be applied on bisque or

leather hard depending on the amount of clay in the slip. Testing is a good idea for these because when I was given the recipes, they said they worked on bisque clay but I found they bubbled and cracked when I did. Coffman Yellow

Nepheline Syenite 60 EPK 40 Soda Ash 1 Titanium Dioxide 5

Matt Long’s Red Grolleg 70 Nepheline Syenite 30 Yellow Iron Oxide 0.5 Titanium Dioxide 0.5

Bower’s Orange EPK 42

OM4 Ball Clay 42

Zircopax 10.5

Borax 5.5

One has to be careful with glazes touching the flashing slips, sometimes they bubble. These glazes I have included work very well and some have been specifically formulated for salt or I have found look really nice. Randy’s Green Flint 16

OM4 Ball Clay 10

Whiting 9

Zircopax 7

Barium Carbonate 13

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Dolomite 6

Custer feldspar 21

F4 Feldspar 18

Tin oxide 4

Copper Carbonate 6

Bentonite 2

White/Yellow Salt *(For Yellow Salt add 2% Red Iron Oxide)

Nephlyne Synite 60.7

Dolomite 20

Zircopax 15.2

OM4 Ball Clay 4.1

Bentonite 4

Shaner’s Oribe Custer Feldspar 31

Whiting 22.1

Silica 25.3

EPK 12.6

Talc 7.9

Bone Ash 1.1

Copper Carbonate 5.2

Snowfall Rutile Custer Feldspar 30

Whiting 19

Silica 24

EPK 16.5

Talc 10.5

Dark Rutile 6.2

Copper Carbonate 1.25

Bentonite 2

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Resources

Olsen, Fredrick L. The Kiln Book Materials, Specifications & Construction. Radnor,

Pennsylvania: Chilton Book Company, 1973.

Rice, Prudence M. Pottery Analysis; A Sourcebook. Chicago and London: The University of

Chicago Press, 1987.

Rogers, Phil. Salt Glazing. Philadelphia: University Of Pennsylvania Press, 2002.

Wentworth, Becker. General Chemistry. Boston: Houghton Mifflin Company, 1973.

Baker, William Converting and Electric Kiln to Soda. Wordpress.

http://design.williambakerpottery.com/2009/07/23/converting-an-electric-kiln-to-soda/

Web.

Bowers, Bruce. Converting an Electric Kiln for Wood or Gas Firing. Ceramics Arts Handbook.

Web.