Terra Sigallata Preparation and Application
Is a very thin, deflocculated clay slip containing only the finniest of clay particles less than a micron (1/1000 of a mm in size). Applied properly and polished it produces a glass like shine to the surface of a clay pot, sculpture or tile. This surface lends its self beautifully to nearly all lowfire techniques. The glossy quality, a result of flattening the clay platelets so that they reflect light will remain through firings up to cone 02. As higher temperatures are reached and greater shrinkage occurs the glossy surface begin to diminish as the clay platelets orientation is disturbed. I have fired a terra sigillata made from OM-4 Ball clay to cone 10 with beautiful results, just not the glossy quality achieved at lower temps.
Terra Sig can be made from nearly all clays but coarser clays will produce less sig per batch. There are many recipes for Terra Sigillata available and they are very simple, clay, water and a deflocculent. The process for making it varies from the ďhillbilly methodĒ to the scientific approach; they all seem to work to varying degrees. It may also be available for purchase at your local pottery supply; Clay Art Center in Tacoma, WA www.clayartcenter.net/ produces a verity of beautiful sigís.
This is the recipe I have used with great success. Credit goes to Vince Pitelka, whose exhaustive research has produced a plethora of information we can all benefit form. Go to Vinceís website: http://iweb.tntech.edu/wpitelka/index.htm where he has generously posted information on terra sig and many other clay related topics of interest.
Super-Refined Terra Sigillata Formula
In a 5 gal. bucket add the following using the method described below.
OM-4 Ball Clay 11. 00 lbs. or 4 989.516 07 gram or 5000.00 grams
Soda Ash 0.027 lbs. 12.246 993 99 gram 12.25 grams
Sodium Silicate 0.027 lbs. 12.246 993 99 gram 12.25 grams
Specific Gravity reading of 1.15
A 5 gal. batch will yield much less than 5 gal. of Sig.
Polishing & Burnishing:
Burnished clay also produces a glassy excellence but the surface quality will begin break down and loose its glossy quality when fired past cone 012. The burnishing process is much more labor intensive than simply brushing and polishing terra sig.
After sanding and cleaning the bone-dry piece apply 3-6 coats of terra sig with a very soft brush. Too many coats, or too thick will promote cracking and pealing. When the liquid is absorbed and the surface dulls you can begin to polish. If you choose to go the route of burnishing, you may coat the sanded and cleaned work with light oil; a trick I learned from Juan Quezada. The oil (baby oil in Juanís case, but other oils work as well) will soak into the work incorporating the fine clay dust from sanding creating a smoother surface to begin with. When burnishing with a hard surface tool you can apply moisture to the area being burnished with a slightly damp cloth. This provides enough moisture to work the area to a highly compressed and glossy surface before moving to another area. The temptation to apply terra sig over the burnished pot is great but beware by burnishing the surface so smooth the terra sig has little to grip to and may peal. I have had beautiful results raising the grain of the burnished piece by polishing the surface with the same slightly damp cloth prior to applying the thinnest possible coating of terra sig and polishing with a plastic bag. The surface each time Iíve done this has shown cracking in the terra sig following the bisque fire.
Terra sig only requires polishing and everyone has their favorite material to polish with from simply using your hands to space age microfiber. Iím somewhere in between using plastic, but always open to new products.
Burnish your terra sig. may be counter productive, it takes more time requiring the use of oil to keep the sig from drying too quickly and will crack the surface of the terra sig. This can be a beautiful effect in less itís not what you are looking for.
The terra sig surfaces are soft a prone to scratching so handle them carefully. I bisque my work at cone 010 if I want a glossier finish, many believe but the lower bisque leaves the body more open allowing for more carbonization. Iím not convinced I can detect it with my eye although it does make sense. Iíve seen beautiful work emerge from the kiln that was bisque fired at cone 04.
Can I apply terra sig on my bisque ware? Yes, however apply very thin coats, 2 to 3 at the most. It takes longer to absorb and your polish should be very light. Make sure you re-bisque fire the work prior to the atmospheric firing. This can be a very good technique when working with very thin and delicate pieces.
Once the work is fired and cleaned up you can chose from a number of room temperature sealants (liquid & past waxes and shoe polishes to spray acrylics) that will give you the finish you desire; mat, semi-mat, gloss to high-gloss.
The following pictures will give you a visual aid for the process I use for preparing my work for the application and polishing of terra sigillata.
Ken Turner Pottery
June 23rd, 2013