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| Story According to Rachael-2002 |
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Most sodium Silicate used by potters is manufactured by National Silicate Corp. For this paper the sodium silicate is at a density of 1=2E33=2E or 8% Na2O and 26% SiO2=2Eth (look for 2E listed on the label). Most “waterglass is 1 mole Na2O to 1 mole SiO2 in H2O. Other deflocculants would include Lye (not recommended) Wood Ash (not recommended) and Sodium Hexametaphosphate (not recommended) and TSP (not recommended but less of a problem than the other materials - use caution!). Follow all warning labels on the products. Wear dust masks and eye protection note the warnings listed on these products and in this paper!
The method described below uses handheld power tools and dangerous materials. Use caution when using all equipment in the studio. Mixers spin at high speeds and you should always keep your hands out of the bucket when mixing materials. Follow directions printed on the tools and read the operation manual. When mixing dry ingredients such as clay, wear a dust mask. When using deflocculates wear eye protection!)). It is always a good idea to test new ideas before you use them on your best ceramic ware. Test..Test..Test. Please be aware that you use these ideas and methods at your own risk. Historically terra sigillata has been used for thousands of year as decoration on pots and to a lesser extent as a sealer for porous ware. Prior to the invention of glaze, terra sigillata was used to seal cooking pots that could be used on open fires. For decorative uses, some examples of terra sigillata are the roman red and black pottery and the black polished ware of the Native Americans in the Southwest United States. In contemporary ceramics terra sigillata is used as a decorative technique for forms that emphasize the surface and is not used for functional ware. Terra sigillata can be applied to pots by dipping, spraying, or most commonly by brushing the material on the surface of the form. Since terra sigillata is very thin when used, several coats may be necessary. The surface of the form is then burnished a hard object (spoon, stone), with your hands, or a soft cloth and the surface will become shiny. By polishing the surface and compacting the clay surface you can with practice make a surface that is almost glaze like. Most of these surfaces must be fired to a low temperature; come 04 is usually the limit for a shiny surface. The upper and lower limits for terra sigillatas is in the range of cone 04 through 08. I recommend firing to cone 06 for the formulas contained in this paper. The following are recommendations for making terra sigillatas: All clays can be used to make terra sigillatas. The ratio to follow is 800 grams of water to 200 grams of clay. Deflocculating of the mixtures, based on the 800 water to 200 clay, is .25% (1/4th of 1%) soda ash + .25% sodium silicate* for white clay. For red clay use .50% sodium silicate = .25% soda ash. The percents are based on the dry weight of the clay. Base white terra sigillata: Water 800 OM4 150 EPK 150 Talc 150 Zircopax 50 (Woody Hughes and Christine Federighi variation, increase Talc to 300, Zircopax to 100) Changes to the base terra sigillata would include substution of the OM4 for Gold Art clay. This will reduce the pastel effect of using the very white OM4 and improve the color quality for Mason Stain use. When using Gold Art remember that terra sigillatas do not shrink much because of the very fine and consistent particle size of the mixture. Terra sigillatas made with Gold Art are the most likely to shiver on the fired surface. Gold Art terra sigillata for use with Mason Stains: Water 800 Gold Art 200 Frit 2124* 5 (deflocculating based on white clay amount) Black Water 800 Red Art 200 Mag Diox 10 Cobalt Carb 20 Copper Ox 20 Black Copper 10 (deflocculating based on red clay) This black terra sigillata is a deep black color and takes a good polish. If you fire this formula* Red Water 800 OM4 100 Red iron 100 (deflocculating based on red clay) Red Art* Water 800 Red Art 200 (deflocculating based on red clay) Changes in Calgon: In the past year, the makers of Calgon have changed their formula for the product. The new Calgon is no longer suitable for use as a deflocculant. Colorants used in terra sigillatas: Common used colorants used in terra sigillatas include: Chrome, vanadium, cobalt carb., Albany clay, iron chromate, copper carb., black copper carb., zirconium, zircopax, talc, crocus martis, iron, rutile, cobalt carb.,and Mason Stains. If you use metal carbonates as colorants you must add the colorants to the mixture and let them settle with the clay. If you use Mason Stains, the stains can be added after the terra sigillata is completed. In all cases, using colorants or not, mix the terra sigillata well before use. To make terra sigillata: Measure out the water to be used. Make sure the water is hot, but not boiling. Mix in the deflocculant and let it set a few minutes to dissolve. A small, tall plastic container is best to use. I use a one-quart plastic container that has a tight lid. An important part of making terra sigillata is to allow the clay particles to fall a distance through a column of water. The taller the container the better. Measure the clay components to be used and add to the warm water mixture. Let the clay dissolve in the water before adding the colorants or the frit. Stir and mix the dissolved clay very well. Run the mixture through a sieve. Place in the plastic container and wait one day. The next step in the process is to remove the water that has settled on the top of the mixture. I use a turkey baster and siphon the water off the mixture. Be careful when you siphon the water not to disturb the mixture or to remove any of the clay particles. When as much of the water is removed as you can, without disturbing the clay layer, tip the container and remove all of the very loose liquid/clay mixture. This is the terra sigillata. The bottom layer will be darker in color and have a tendency to stick to the bottom of the container. The majority of the clay will remain in the original container and will not be used. Discard the remaining clay. Don’t recycle this clay, discard it. Terra sigillatas will continue to settle in the container. Siphon off the excess water that forms on the top of the container until the desired thickness of the mixture is obtained. Do not remove the settled bottom layer a second time. This is important for terra sigillatas containing metal oxides, this is the color and should be mixed well before use. Remember, terra sigillata is very thin, almost like milk. Using other clays for terra sigillatas: You can obtain different or varied results in the use of terra sigillatas by varying the type of clay used in the sigillata. For example, many of the formulas can be changed by substituting the type of clay used. Instead of using OM4 for the white formula, use Gold Art. Gold Art is courser clay and will result in a softer color with a warm hue. Perfect for colors such as blue with mason stains. The major clays used in terra sigillatas are: OM4 ball clay which is very fine white clay, Red Art, a very fine textured red clay, Gold Art which is warm white course clay, Barnard, a dark greenish fine clay, and Albany slip which is no loner available, and makes dark brown fine textured clay. Adding Mason Stains: Mason Stains, or other brands of stains are easy to use in terra sigillatas. If you use the white base, the resulting color will be a slight pastel of the stain color. Because of the cost of mason stains, add the stain after the terra sigillata has been made. For a dense bright color, up to 10% of the dry weight of the terra sigillata can be used. To calculate the dry weight, weigh one container of water to the same level as the terra sigillata. Using the exact same container, weigh the terra sigillata. The difference in weight is the dry weight of the clay. Applying the terra sigillata to the clay surface: Since the terra sigillata is a fine grained, densely packed clay layer, several coats of the thin terra sigillata will need to be applied. To apply the terra sigillata you can spray, dip or brush the surface. The terra sigillata is best applied when the surface of the object is slightly dry. The body will absorb part of the water from the sigillata and polishing can begin immediately. If you apply several layers of terra sigillata, it may be best not to polish the surface between applications. If you polish and then add additional layers, especially different colors or even types of terra sigillatas, additional polishing will wipe off the additional layers. If the surface is allowed to dry completely, spray the surface with water and a small amount of vinegar. Allow the water to be adsorbed by the clay and then polish. Terra sigillatas can be applied to wet clay, dry clay, bisque. But, the best surface can be achieved when terra sigillatas are applied to late leather hard surfaces and polished with a hard polishing tool such as a stone or a spoon. Allow the object with the terra sigillatas to dry completely before firing. This is important because steam build up behind the dense clay surface can cause separation and failure of the terra sigillata. Other uses for terra sigillatas: A fun thing to do with terra sigillatas is to make clay chalk with the mixture. You can use these chalks to draw on the surface of the form. Allow the terra sigillatas to dry out and form a clay like consistency. Roll the clay into thin “pencil” like forms and allow drying. You can then draw on the surfaces of unfired terra sigillatas or directly onto bisque surfaces and glaze over them. The most dramatic effects can be achieved with the black and white terra sigillatas. If you should have any questions or comments about this paper, please contact Jerry Bennett at: jerry@jerrybennett.net
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