Ceramic Design Studio
-NEWS & EVENTS
Pit Fired Ceramics - Instructions for Potters©
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
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.
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.
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.
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).
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.
Let pieces thoroughly dry before bisque firing.
2. Terra Sigillata|
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.
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
Let pieces dry thoroughly before bisque firing.
3. Unpolished surface|
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
1. Colored Effects
Colored clays: Inlaid colored clays often give an interesting contrast of design to the
random flame painted patterns from the pit fire.
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.
Stains or oxides can be put on the pots before or after bisque firing.
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|
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.
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|
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.
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|
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
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.
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.
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|
Shards or pieces of metal can be placed on the pot’s surface to mask it from flame and
6. Marking Pen and Paint Effects|
Experiment with black and colored ink markers to see which survive the fire.
Use copper markers and copper containing paints (aerosols are convenient) for “controlled”
7. Cages and Containers |
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.
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
8. Crackle Surface |
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|
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
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|
|1||Bottom layer of pit is clear of rocks or glass|
Why do we use these combustibles?
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. |
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. |
“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|
Iron reds, yellows, oranges
Further precautions: |
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. |
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:|
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. |
Place pots in a portable barbeque with heated charcoals or low propane flame and heat to about
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. |
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|
|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. |
Remove the ash from the interior of the pots. |
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. |
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. |
The pit site should be cleaned and left as natural as you found it. Remember ashes are very caustic.
Wear a mask. |
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
Reproductions of this article are prohibited without expressed written permission from Eduardo Lazo.
© Copyright 2003 Eduardo Lazo. All rights reserved worldwide.|