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Pit Fire Ceramics - Advanced Techniques©

By Eduardo Lazo, MFA

View Eduardo Lazo's work at his Web site at http://www.EduardoLazo.com

 

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.

 

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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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