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Primitive Firing

The use of some traditional firing techniques is still popular among modern potters and sculptors (who are accustomed electric and gas kilns, often with computer controllers).

Key phrases linking here: primitive firing - Learn more

Details

Most potters and sculptors fire in electric or gas kilns, these are often computer-controlled. However, the use of traditional techniques, some very ancient, is still very popular. It is in this sense we use the term "primitive firing" (not because it does not use technology!). The scope of techniques is as wide as the world and its history. Each culture developed its methods around the clay and fuel and utilitarian and aesthetics needs it had. In many places in the world these techniques have survived to the present day with little change (e.g. India, Africa) and are well documented in books, sites and Youtube.com.

Many ancient cultures produced pottery at large, even industrial scales, using elaborate kilns. However primitive firing methods enabled anyone to fire ware with almost no facilities or resources (e.g. in an open fire, a pit or a basic containment method) using primitive materials (e.g. coarse, non-plastic clay). While potters today consider cone 06 a very low temperature, to ancient potters that would have been a dream temperature! Today potters in the developed world inject the materials, tools and some techniques they have available now into old firing techniques. Obvious differences are the clays people have today, the manner in which they form the ware and the fuel they use to fire. In all, the result is a safer, more reliable and consistent process yet producing ware of similar appearance.

A common technique is the use of burnishing to produce a smooth surface (few primitive cultures had glazes) and surprisingly dense and good surfaces can be achieved. These are especially important on food surfaces of the otherwise porous matrix of the clay.

When utility is desired (e.g. ware that can be used over an open flame) terra cotta clays are normally employed. These generally must not be exposed to higher temperatures, the open porous matrix, although not strong, is the key reason the ware can withstand temperature shock. Often grog is added (whereas ancients would have used sand) to impart thermal shock resistance at a hotter temperature (where ware fires harder and stronger). Kilns or containment can be made using the same clays from which the ware is made.

Primitive ware is sintered, not vitrified. While potters today bisque fire their ware at 1800-1900F, primitive cultures would typically have final-fired at much lower temperatures. Even as low as 1000F, clays can sinter to a form that no longer slakes in water (a typical open wood fire can easily achieve temperatures hundreds of degrees hotter than that). If partial containment is employed and fuel varied (e.g. use of sawdust, hard wood) even higher temperatures can be achieved. Many books and websites deal with the intricacies of this type of firing.

One area where modern technology is really needed in primitive pottery is the development of clay bodies using materials native to the area. Using an account at insight-live.com you can learn to test and characterize (create a comprehensive report) the traditional bodies used. Then you can find materials in the area and characterize each of them. Finally, using that material knowledge, you can develop and optimize a clay body. Optimal clay bodies have minimal drying shrinkage, maximum plasticity, maximum drying performance and dry strength, minimum soluble salts, maximum fired strength, minimum fired shrinkage and porosity, maximum thermal shock resistance, the desired color and surface (the traditional body may have had none of these properties optimized). Armed with such a clay body, made from native materials, you will have higher value. Please contact us if you need help using Insight-live to start a such a project.

Related Information

Pierre Aupilardjuk and John Kurok firing their work


The work of Pierre Aupilardjuk and John Kurok from Rankin Inlet, Nunavut, Canada. They are visiting the Medalta International Artists in Residence program in Alberta during the fall of 2016 to demonstrate their firing technique. First they sculpt the pieces from a medium temperature stoneware (lightly grogged body), then dry and apply terra sigilata. Finally they bisque them. Next they carefully stack items into a 45 gallon drum with fine wood chips filling all spaces. The drum has an expanded metal mesh a few inches up from the bottom, this creates an air space (holes around the outside of this airspace allow air in from the bottom). They light the wood on fire at the top and put the lid on. This burns for a couple days (a hole in the lid permits enough ventilation to enable the wood to slowly burn and smoke. The black finish is glossy and clean.

Barrel-fired piece by Kathryn Newsom


Barrel-fired piece by Kathryn Newsom

She bisque fires P300 and M370 pieces and then puts them in a burn barrel with various organic materials (sawdust, corn husks, seaweed, pine cones, etc.) along with copper carbonate and copper wire. She builds a wood fire over top of the pots and lights it. The smoke paints the pieces. Each piece is unique. Kathryn does not sell online but you can reach her at knewsom5@gmail.com.

Ancient pottery with bloats - Why?


Some archaeologists view this the bloating in these jars as a fault. But they might not be. The bloating could have been their way of gauging the maximum density (and thus strength) at which the terra cotta clay can be fired without warping or sagging. A requirement for these jars might have been water tightness so a few of these bubbles could have been proof of suitability.

That being said, Brian Weibe offered a more down-to-earth explanation. This form began in the Hellenistic period 2nd century BCE. Although potters could make exquisite hand-burnished ware (Eastern Terra Sigillata) making these store jars was all about efficiency. The 'utility' clay was augmented with sawdust and any industrial combustible (and probably hastily mixed by foot as they do today in Palestinian potteries in the West Bank hill country) to assure fast maturity with minimal time or fuss. The result is a genre that is easy to spot (by these barbaric throw lines and little attention to symmetry or style. They were easily broken and needed to be replaced inexpensively.

The difference between vitrified and sintered


The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature (because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely). That is what vitrified means. But it has a high fired shrinkage, poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder (VeeGum), all that cements zircon ceramics together is sinter-bonds between closely packed particles (there is some glass development from the Veegum here). Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, low thermal expansion and maintains its strength and hardness at very high temperatures.

Adobe brick clay - What is it actually?


Adobe brick clay

Adobe brick clay is simply the local clay that is used to make the adobe bricks. Almost always red burning but not always terracotta or earthenware. These clays are almost always suitable for pottery (with more processing). Here is an adobe brick clay from Sinaloa, Mexico. Locals have been making adobe bricks from this for 500 years. Adobe clays are generally just terra cotta bodies. But this one is not. These bars are fired from cone 2 to 7 (bottom to top) or 1125-1225C. They would be firing them to less than 800C, far lower. This clay actually qualifies as a stoneware but they use it as an earthenware. This is not as bad as it sounds. At 800C almost all clays are highly porous and pretty soft - that is actually an advantage for the method of construction used (e.g. watery mortar, easy cutting, good adherence of plaster, insulating value). These same bricks are used to build the kilns and they service well since temperatures never get high enough that they warp or crack.

Bisque temperature can be lower than you think


These bowls are made from a talc:ball clay mix, they are used for calcining Alberta and Ravenscrag Slips (each holds about one pound of powder). The one on the right was bisque fired to cone 04 (about 1950F). The one on the left was fired to only 1000F (540C, barely red heat), yet it is sintered and is impervious to water (strong enough to use for our calcining operations). That means that there is potential, in many production situations, to bisque a lot lower (and save energy). Primitive cultures made all their ware a very low temperatures. Tin foil melts at 660C (1220F) yet can be used on campfires for cooking (so the temperatures of primitive wares would have been low indeed).

Links

Glossary Kiln Firing
All types of ceramic are fired in a kiln to cement particles together to produce a hard and water and temperature resistant product.
Glossary Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
Glossary Terra Sigilatta
Terra Sigilatta is a sedimentation process used by potters to remove coarse particles from a clay powder. It enables burnishing and gloss surfaces without the use of glaze.
Glossary Sintering
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
Glossary Raku
Gas fired rustic ceramic ware is cooled from red-hot in a closed container with organic material. The zero-oxygen atmosphere produced reduces carbonate metal decoration to its metallic form.
By Tony Hansen
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