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Pit Fired Ceramics - Instructions for Potters©

Introduction

Contemporary pit firing techniques often yield remarkable surface effects on decorative ware. Vibrant and exciting colorful designs are imprinted on unglazed clay forms by dancing flames carrying fumes from selected combustible materials and chemicals.

Pit firing has as many variations as there have been potters doing it throughout the ages. My focus here will be to introduce you to a methodology that results in multiple color development on the clay surface. These are techniques that I have used over the past 20 years. Basically, low bisqued pieces are placed within a carefully constructed bonfire framework such that the pieces are flame painted (fumed by chemicals and vapors from the combustibles) as they are exposed to oxidation and reduction atmospheres. Clay objects are prepared for the pit using preferred “basic” or “advanced” methods. The pit can be constructed in many configurations around variations on a theme. I will describe two of my favorite pit constructions (one for groups and one for single person firings) that consistently yield multiple color effects. A quick review of basic preparations is followed by more advanced techniques for special results.

The Fundamentals

Fabricating the Ware
Ceramic pieces of all sizes are usually made of a stoneware body that contains grog and/or sand. White stoneware clays work very well, such as Claymaker’s Stout Stoneware, Laguna’s Danish White, or Aardvark’s Hopkin’s White. However, colors will differ when the stoneware contains some iron (Laguna’s Soldate 60 or Amador). The iron in the clay body induces warm orange and salmon colors from salts used in the pit fire mix. Porcelain and Laguna’s B-mix can also be used but cracking is often a problem in medium to large sized pieces if the ware is not thoroughly dried and deliberately preheated.

Are we only using high fire clay? Yes, because you want the clay to be “open” during the pit fire to readily accept the fumes and permit multiple hues and shades of color on the clay surface. The palette of colors obtainable is described in Table II below. You can pit fire earthenware clays but the window of opportunity is small because the pit fire does reach maturing temperatures for low fire clays.

Pieces may be hand build, wheel thrown, extruded or cast. The surface may be smooth or textured. Forms that have a lot of surface area will “show off” the flame painting. Round or globular forms best survive the stresses of the firing process. That is not to say that tall pieces do not, but if they are exposed to uneven temperatures the possibility of cracking increases. To avoid breakage of protruding parts on a piece (such as a spout or handle) one must have a notion of how the pit will settle to protect these extensions from significant superimposed weight. The size and number of pieces dictates the size of the pit.

Preparing The Ware
The surface of the pieces may be primed in any combination of three ways.

1. Burnish
a. Smooth the piece with a rubber rib immediately after forming the pot. This initiates the process of pushing the sand/grog into the clay body to leave an even silky clay layer for burnishing. Be careful not to indent or make unwanted ridges on the surface.

b. Let the piece dry to a late leather hard stage, then burnish with a smooth tumbled stone to further submerge the sand/grout and to “polish” the surface; use small circular motions and avoid marring it with abrupt changes in direction. Use only the weight of the stone, do not press the stone into the clay as this may cause surface cracks during firing (sometimes a desired effect).

c. When the piece is almost bone dry, burnish again. Apply a light coat of lard, Crisco, or cooking oil to approximately three square inches of the pot’s surface at a time. Let it soak in for a couple of minutes, then polish with the stone to a mirror high gloss finish. When properly done, you will see transparent layers of burnished surfaces.

d. Let pieces thoroughly dry before bisque firing.

2. Terra Sigillata
e. Cover almost bone dry ware with three to five layers of terra sigillata and when leather hard, buff to a sheen. Different effects are possible when you buff with chamois, cotton cloth, plastic, or soft brush. I prefer to use soft plastic bag material wrapped around a soft sponge.

f. I also prefer to use terra sigillata prepared from the same clay as the clay body used in making the piece. (Although ball clay, Red Art, or Gold Art are also very popular.) Terra Sigillata is made by mixing 5 cups of dry clay with one gallon of hot water to which I add a deflocculant such as 2 teaspoons of Calgon (sodium phosphate, sodium carbonate, no longer sold in California), or 1½ teaspoons of sodium silicate, or 2 teaspoons soda ash (sodium carbonate), or 1% Darvan #7. Blend with a Jiffy mixer until the mixture is very thin. Let this stand for 20 hours. This is considered to be the optimal time because gravity causes the heavier particles to settle out while the finest particles (less than one micron) remain in suspension due to deflocculation and atomic vibration. Three visible layers will form: a seemingly transparent “water” layer, a “ready to use” terra sigillata layer, and the dregs at the bottom. I siphon off and use the top “water” layer (as it contains the very finest of clay particles, even though they cannot be seen with the naked eye) along with the middle “terra sig” layer. Place these two extracted layers into a separate container and let stand another 20 hours. Again, siphon off all but the heavier clay layer at the bottom. Place the top layers into an open drying container and let the water evaporate till you get the proper consistency (that of skim milk or 1.13-7 specific gravity on a glaze hydrometer). While the terra sigillata may be brushed or dipped, I prefer to spray it onto pieces using a HVLP spray gun for a super even coating

g. Let pieces dry thoroughly before bisque firing.

3. Unpolished surface
h. Pieces are left “rough” when the intent is to emphasize a textured surface. Texture is applied during the formation of the piece. Some of the most beautiful pieces from a pit are often those that have not been polished.

When thoroughly air dried, the pots are bisque fired in an electric or gas kiln to burn off organic and volatile materials, to drive off chemically bound water and to “harden” the clay. I prefer a porous “soft” bisque of cone 018-014 because this porosity facilitates multiple color development and more intense colors. At cone 08 or hotter you loose the sheen of the burnish or terra sigillata. (Dried and preheated greenware can also be pit fired but there is greater risk of breakage due to fast, uneven, and sudden temperature changes. I have had success by placing greenware into a saggar as explained below.)

You are now ready to fire your pots in a pit of your design, unless you desire to experiment with more advanced preparation techniques.

Advanced Preparations of the Ware

One can “manipulate” the surface effects by various treatments. Here are a few of my favorites:

1. Colored Effects
a. Colored clays: Inlaid colored clays often give an interesting contrast of design to the random flame painted patterns from the pit fire.

b. Colored slips or terra sigillata: Stains (less than 8%), cobalt carbonate (less than 0.5%), and copper carbonate (less than 10%) can be used creatively to offset pit colors.

c. Stains or oxides can be put on the pots before or after bisque firing.

d. A variation of Paul Soldner’s Raku Halo effect can sometimes be obtained by brushing on designs with a mixture of 50% copper carbonate with 50% red iron oxide in a water solution over a white slip or body. Temperatures must approximate 1650°F with heavy reduction followed by re-oxidation.

2. Wire Effects
e. 24 to 32 gauge copper or steel wire can be wrapped around a pot to create linear patterns that can result in black, red, green, or “shadow” lines depending on the metallic content of the wire and local firing atmosphere. Interesting impressions are also possible using copper or steel mesh kitchen scourers that are unfolded and placed or wrapped on portions of the pot.

f. Hardware cloth or chicken wire fencing can leave interesting metallic patterns of black or grey to contrast with the pit flame painted fumes.

3. Cloth Effects
g. Cotton string or sisal twine soaked in 10% salt (ordinary or sea) water, dried, and tied onto the surface gives green, black, brown, or maroon colored line patterns.

h. Pots can be wrapped in old tee-shirts similarly salt soaked and wrung. Burning in the pit enhances the orange color from salt and/or the reds of copper carbonate. If reds are desired, it is important not to disturb the ash that forms from the tee-shirt combustion because it creates the required local reducing atmosphere.

4. Saggar Effects
i. Clay saggars: Pack pot and combustibles in a bisqued clay container to trap the combustion gasses in the same fashion as you do in a gas kiln saggar firing. Muted “soft” colors emerge.

j. Therma-foil (temperatures up to 2000°F, available from LL Kilns - hotkilns.com): This material can be used for varied saggar effects that impart metallic flashes. The saggar can be lightly stuffed with color enhancing combustibles and chemicals.

k. Heavy duty aluminum foil: Pots may be sprayed with ferric and sodium chloride solutions imparting red to orange hues. Note: aluminum foil starts to melt at 1100°F. Do not place these saggars over or touching another piece.

l. Slip coated paper saggars: The ware, combustibles, and chemicals can be placed into a paper bag then encased in paper soaked in clay slip.

5. Masking Effects
m. Shards or pieces of metal can be placed on the pot’s surface to mask it from flame and fumes.

6. Marking Pen and Paint Effects
n. Experiment with black and colored ink markers to see which survive the fire.

o. Use copper markers and copper containing paints (aerosols are convenient) for “controlled” coloration.

7. Cages and Containers
p. Sometimes you may want to create a more contrived local combustible environment for your piece and not depend so much on the haphazard pit construction for effects. If you are confidant that your work does not need much pre-heating, then place your work, chemicals and combustibles in a cage made of wire, cardboard or supermarket paper bags.

q. Very delicate, thin, or greenware pieces may be placed in a cage made of wire or steel to protect it from being crushed by heavy wood or by other pieces. Fumes easily pass through the cage.

8. Crackle Surface
r. The clay surface will crack in random patterns if you press too hard during burnishing or apply more than eight layers of Terra sigillata. The crackles will often take on different contrasting colors during fuming.

9. Low Fire Glazes
s. These may be used in a pit but care must be taken that glazed surfaces of pots do not touch other ware as they will adhere to one another. (Sometimes only the insides of pots are glazed.) Ash and other combustibles will also attach to the glaze but these often can be scraped off with sometimes interesting results. Temperatures in the pit reach earthenware temperatures.

These “advanced” techniques can be used in any combination you choose. Be aware of potential impact on surrounding pots, be they yours or someone else’s.

Constructing The Pit

The pit is dug to provide wind protection and to retain heat sufficient to permit the fuming process. The pit may be circular or trench like. When firing by myself or with one other person at the beach, I use a perforated oil drum or the fire pit rings provided by the State. When firing in a group, I dig a trench large enough to hold all the pots. The following tables describe two very different layering techniques in the construction of these pits. (Note: any combination and layering that suits the potter can be used. There are no rules, only guidelines.)

Table I Small Pit in a Fire Ring or Perforated Oil Drum - For 1-2 potters
Layer Combustibles Thickness in inches
13 Cow pies surround the entire mound from the ground up 4
12 Top layer is composed of leaves, twigs, and weeds 6
11 Top dry seaweed layer 6
10 Pots intermingled with kelp, sea grasses, small driftwood 4 - 16
9 Seaweed and Magic Dust layer 2
8 Leaves, twigs, and weed layer 5 - 10
7 Pots intermingled with kelp, sea grasses, small driftwood 4 - 16
6 Seaweed and Magic Dust layer 6
5 Driftwood, ground level 4
4 Seaweed roots and Magic Dust layer 6
3 Cow pies or damp seaweed to tamp the fire 6
2 Coal bed 3
1 Bottom layer of pit is clear of rocks or glass

Why do we use these combustibles?

1. Vegetation from the sea is impregnated with sea salts containing many trace elements that give unique coloration to the ware. As an alternative you can soak other combustibles in table or sea salt and dry prior to pit firing.

2. Cow pies serve three functions: When dry, they easily burn to earthenware temperatures. They impart various colors on their own when they burn: blacks, greens, grays, browns. They cover the ware with a clinging yet easily removable ash. This ash blanket promotes a local reduction atmosphere necessary for copper reds.

3. “Magic Dust” is a term I coined to describe a mixture of one third copper carbonate and two thirds table salt that is used to “enrich” the pit fuming atmosphere. Magic Dust is sprinkled on combustibles but not directly on the ware. Salt will adhere to the clay and eventually “eat” right through it. Care must also be taken with seaweed pods as they contain a high concentration of salt that will transfer to the pot.

N.B. Warning: Wear a mask, protective goggles, long sleeves and pants when sprinkling Magic Dust. Copper is a respiratory tract irritant. Repeated inhalation of dust can cause sinus congestion, ulceration, and perforation of the nasal septum. Contact with eyes can cause conjunctivitis, discoloration, and ulcers on the cornea. It can also irritate and discolor the skin. Wash immediately.

The pit is dug 14-20 inches deep into the ground, care being taken to remove all rocks and glass fragments. Coals are laid in the bed, lit and allowed to reach a glowing stage. Damp seaweed or dry cow pies are placed on the coals to tamp the flames. This layer takes several minutes to ignite giving you time to construct the rest of the pit which can be 3-5 feet above the ground. The pit will burn actively for an hour or so. Because of the liberal use of cow pies, be aware that this pit creates a lot of smoke for 15-25 minutes. Do not disturb the ash layer covering the pots when you are trying to promote local reduction for copper reds. This pit construction permits pockets of reduction within an overall oxidizing atmosphere. Some of the colors obtainable are listed in Table II. When the pit reaches about 1100°F green copper fumes will be easily seen rising from the flames. Warning: Do not inhale this smoke. Salt becomes active around 1400°F. Colors will swirl within the pit and chlorine vapor will be visible. Do not inhale the smoke. The pit will have an orange glow and will easily reach 1860°F. When the pit cools to about 500°F, you can elect to cover it with steel sheets to protect the ware from the wind and yet not influence the fumed colors. (How do you know when the pit is 500°F? Gain “tactile sense” by placing your hand near an opened kitchen oven heated to that temperature.) The pit is not disturbed and allowed to cool naturally to the point where pots can comfortably be handled.

This one person small pit construction is used to maximize the natural and stunning sea salt effects from fuming. Many colors are possible as well as impressions of the sea plants. Fuming chemicals are used to enhance color development.

Table II Color Palette Chart

Combustibles and Chemicals Expected coloration on cone 018-014 bisque
Hardwood sawdust Black, dark gray
Hardwood coals Black, smoky gray, blue-grays
Cow pies from grass fed cows Gun metal black, jet black, golden yellows
Cow pies from grain fed cows Dark greens, grays, blacks, browns
Driftwood Blue-grays, aqua shades, gray-blacks
Seaweed roots Browns, rust, honey
Kelp leaves Yellow, orange, peach
Kelp pods Orange, brown
Saline grown leaves, twigs, grass Golden yellows, greens
Table salt Orange, yellows
Sea salt Salmon, orange, yellow, gold, peach
Copper carbonate Green, black, maroon, reds
Cobalt carbonate Blues
Cobalt carbonate Blues
Ferric chloride Iron reds, yellows, oranges

Further precautions:

1. Ferric chloride solution (concentrated or diluted) can be sprayed or brushed directly onto the ware before placing in the pit. Ferric chloride is caustic. Do not breath it, do not get in on your skin or in eyes, do not swallow it.

2. Cobalt carbonate (less than 0.5%) solution can be sprayed or brushed onto the piece before it is bisqued or on the inside walls of saggars. Cobalt is a mild skin, eye, and respiratory irritant. It can cause skin allergic reaction. Inhalation can cause asthma like symptoms, lung damage, and pneumonia. Absorption causes vomiting, diarrhea and “hot flashes.”

Pre-heating can be done in three ways for this small pit:

1. Place pots around the pit rim while the coal bed matures. Turn the pots every few minutes to ensure even heating as you move them gradually towards the hot coals.

2. Place pots in a portable barbeque with heated charcoals or low propane flame and heat to about 300°F.

3. Place pots in pit with an extra thick layer of tamping materials so the pots will be in a steaming atmosphere before the combustibles fully ignite.

4. The trench style of pit fire does not require preheating as the pots are in a natural draft that gradually heats them from the top or one side.

Table III Trench Style Pit Fire Used For Groups With Many Pots

Layer Combustibles and Chemicals Thickness in inches
8 Leaves, twigs, kindling 6
7 Hardwood planks or small firewood pieces 12
6 Small intertwined branches 24
5 Cow pies 4 - 6
4 Pots interspersed with small twigs or driftwood 10 - 20
3 Magic Dust, minimum one cup per square yard liberal
2 Hardwood sawdust 8
1 Bottom layer of pit is clear of rocks or glass

This trench can be deep or shallow. (The largest I have used was 6 feet deep by 25 feet long by 7 feet wide.) If a shallow pit is desired, hardwood pieces or split logs of firewood are placed vertically along the sides of the pit to “extend” the trench walls from the ground up. This pit is lit at the top when the trench is deeper than 2 ½ feet. If the trench is shallow construct it so that the wind will blow from end to end and then light it from the end receiving the wind. When that windward end burns, it will enable pre-heating of the ware to take place as the fire follows the draft. I do not place pots that I want preheated too close to the lit end of the shallow trench. Stand back and watch the fire roar. The same colors, temperatures, and precautions that were described for the one person pit apply.

Post Firing

1. Remove the ash from the interior of the pots.

2. Do not wash the pots. You do not want salt deposited on the surface to permeate the body of the piece. Pit fired pots are decorative, not functional.

3. Carefully shake off or wipe any ash residue adhering to the surface. Single edge razor blades or credit cards are useful tools in removing carbon or wire fragments that stick to the pot. Be careful not to scratch the delicate surface.

4. The pit site should be cleaned and left as natural as you found it. Remember ashes are very caustic. Wear a mask.

5. Pots need to be treated to prevent or retard absorption of water from the atmosphere. The salt in the pit fired clay will react with water vapor and corrode the surface of the pot. For treatment of burnished or terra sigillata pots I like to apply two layers of Johnson’s or Butcher’s Floor Paste Wax for a semi-gloss sheen. Use a soft shoe brush or cotton cloth to bring out the shine. When first applied, the wax will seem to streak and discolor the surface. This will vanish in a few hours. The wax odor will persist for about a week. If a more reflective surface is desired, I use Future Acrylic Floor Finish diluted with water to a 25% acrylic solution. I also use Future, Watco Clear Liquid Wood Wax, or Jasco Silicon Grout Sealer on rough or unpolished textured surfaces.

Pit firing requires a lot of effort. It is not without hazards: physical, chemical, and toxicological. Yet, when properly done, it is fun and very rewarding. There is fulfillment when fateful chance weaves its magic through the pit’s flames producing unique (one of a kind) ware.

Should you have any questions about preparing your work for an upcoming pit fire event, please feel free to write to me at EduardoLazoMFA@aol.com.



Eduardo Lazo is a ceramicist, teacher, author, and consultant. He has been a studio artist for 20 years and lives in Belmont, California. His current ceramic interests lie in two forms of vapor glazed ceramics - Pit Fire (low fire salt) and Kosai Ware (precious metal fuming). Eduardo has an MFA from California State University Los Angeles. Contact via www.EduardoLazo.com.

Reproductions of this article are prohibited without expressed written permission from Eduardo Lazo. © Copyright 2003 Eduardo Lazo. All rights reserved worldwide.



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