|Monthly Tech-Tip |
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.
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Terra Sigilatta is produced using a simple sedimentation process. A clay is slurried in lots of water and allowed to settle. The fine, unsettled portion (or upper part the the sediment) is extracted and concentrated to produce the terra sigilatta slurry. It is applied to the surface of terra cotta ware (and optionally burnished). When fired it can produce gloss even though no glaze (or glass development) is present. This low-fire process has a long history among indigenous cultures. Read about it at Wikipedia or search on google.com.
Potters commonly experiment with the raw clays and ball clays at their disposal in search of a desired fired effect that couples with application properties they can tolerate or adapt. Redart, for example, is a common material in North America that has been found to readily sediment into layers of differing particle sizes. Ball clays produce terrasigs of widely varying character even though their plasticities and fired maturity as raw materials can be quite similar. Deflocculants are often employed to help accelerate the sedimentation process. The difficulty in achieving a gloss surface can give potters renewed respect for ancient and indigenous cultures that perfected the process.
Terra Sigillata can flake or crack off ware during drying and firing (especially if applied more thickly). Thus it needs to have a drying and firing shrinkage similar to the body. And EBCT test can be used to determine the compatibility. If it is flaking off during drying (as evidenced by the EBCT bar bending toward the body during drying), then add some bentonite to increase drying shrinkage. If the bar bends toward the terrasig it may be necessary to calcine some of it to reduce drying shrinkage. If it flakes off during firing (the bar bends toward the body) then add some feldspar or a frit to make it more vitreous. If the bar bends toward the terrasig during firing then you will need to add something refractory to reduce its maturity (e.g. calcined alumina is available in super fine particle sizes).
There is an interesting scientific side to the process (pointed out to us by Juan Figueroa Dorrego). Sedimentation of free falling particles in water follows Stokes Law. The law predicts the speed of the particle sedimentation based on size. According to an article by Sonja S. Singer (Industrial Ceramics - London, 1963 - Chapman & Hall Ltd.), in 20 hours a 2 micron particle will settle 20-25 cm. That means that the height of the container and timing are important (to prevent finer particles sedimenting on the bottom).
The gloss on this low fire red clay is not the product of a glaze or any kind of glass development. It is from a layer of incredibly fine clay on the surface (a process called Terra Sigillata). It is the product of a lengthy research project by Monika. She investigated many materials, techniques, clay bodies and firing schedules.
The clay is Plainsman Snow fired at cone 04. August 2022 by Nina Berinstein. Very thin and light pieces can be made without danger of warping during firing (because this clay as zero fired shrinkage). This piece was treated with Mod Podge, this has sealed it against water penetration and not affected the appearance or texture.
Tera Sigilatta at Wikipedia
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).
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
|By Tony Hansen|
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