While making terra sigillata is not a difficult thing to do, it certainly can be time consuming. By rethinking the science of the terra sigillata process, it is possible to eliminate some of that time and gain a better yield.
Defining the Terms
Acidic: pH below the neutral 7.0.
Alkaline: pH above the neutral 7.0.
Darvan 7: High alkalinity ionic polymer. It is a deflocculant (similar to sodium silicate) that weakens the electrical attraction between clay particles, acting as a thinning agent in slips and general dispersing agent for clay bodies and glazes.
Deflocculation: The process of making a clay slurry that would otherwise be very thick into a thin, pourable consistency.
Particle Size Distribution: Range of particle sizes found in raw clay.
pH (general definition): Scale used to measure acidity and alkalinity. Above 7 = alkaline, below 7 = acidic.
pH (as it applies to pottery): acidity (below 7.0) creates flocculation. Alkalinity (above 7.0) creates deflocculation. PH directly effects the ionic charge: acidity creates a positive charge resulting in the formation of flocs (flocculation.) Alkalinity creates a negative charge that repels adjoining particles (deflocculation).
Soda Ash: Soluble sodium carbonate used for deflocculation.
Sodium Cations: Positively charged sodium ions.
Sodium Silicate: Viscous liquid of dissolved silica in sodium hydroxide; high alkalinity.
Suspension: Keeps individual clay particles equally dispersed in a water solution based on the ionic charge.
Water Film: Microscopic layer of water that encapsulates individual clay particles.
Rethinking the Science
The process for producing terra sigillata is based on the principles of particle suspension, which are affected by temperature, alkalinity (pH), ionic charge, and clay particle distribution in the mixture. In revisiting the chemistry, I reviewed the works of D.D. Button and W.G. Lawrence, PhD at Alfred University.1 From these journals, I developed a method to decant terra sigillata in 15–20 minutes (rather than the typical 24 hours), and with better yield. This new thermal method uses Darvan 7 and relies on the effects of temperature to accelerate the process.
Effects of Temperature
Through their research, Button and Lawrence found that when the temperature fell below 68°F (20°C), the charge responsible for holding clay particles in suspension dropped sharply. This allowed larger particles to drop out of suspension quickly and settle, which directly affects how much terra sigillata you get. So do you get the same amount of terra sigillata with either approach, with the difference being time? Not exactly. The amount actually increases with the thermal method because of the higher levels of Darvan required to keep particles in suspension while rapidly cooling. This same study also revealed that as the temperature increased (up to 140°F (60°C)), the ability to hold particles in suspension likewise increased. Based on these findings, warm (100°F (38°C)) tap water should be used in lieu of cold water to accelerate suspension. To speed up the collection process, once mixed, the batch is then placed in the freezer to accelerate separation of large and small particles.
Effects of pH
A.L. Johnson and F.H. Norton, PhD reported on several different methods of deflocculating clay; using sodium silicate plus soda ash is the standard combination for making terra sigillata. The target pH is 8.0; below that will allow particles to settle too quickly, and above that will hold particles in suspension for longer periods.² Sodium, magnesium, and calcium will suspend particles. Calcium in particular will hold particles in suspension for long periods.
Tests with two samples were conducted. In sample #1, a test of 250 grams of OM 4 ball clay was mixed with 10 drops of sodium silicate to achieve a pH of 7.95. The sample was subjected to the thermal method by placing it in the refrigerator. It started to separate in 30 minutes and a visible sludge line formed in just over 2 hours. Note: Sodium silicate was the terra sigillata standard for decades until Darvan came along and was used with all clays—kaolin, ball clay, and iron-rich red clays. Sodium silicate builds a maximum charge in the first 20 minutes, but then dissipates over the next few hours, allowing larger particles to drop from suspension.3 This reaction is the basis of terra-sigillata chemistry, but calcium and magnesium interfere with it. Ball clay has naturally occurring levels of calcium and magnesium that develop a negative ionic charge capable of particle suspension apart from sodium silicate additions. To further illustrate the effects of calcium, 1% calcium carbonate was added to sample #1, which took nearly 30 hours to separate.
In testing the effects of calcium/magnesium on suspension, one tablespoon of white vinegar was added to sample #2, producing a pH of 4.15. There was no sodium silicate in the sample. The sample was then placed in the refrigerator and the separation occurred within 15 minutes. The acidity neutralized the negative ionic charge supplied by the calcium, which caused particles to settle out of suspension quickly. That reaction was further accelerated by starting with warm water (100°F (38°C)) and then rapid cooling to 60°F (16°C).
In more modern times, a synthetic ionic polymer called Darvan has become popular due to its long reaction time. Darvan 7 works by trapping calcium and magnesium ions, and releases sodium to achieve maximum suspension. The testing done on samples #1 and #2 were done to illustrate the effects of rendering calcium and magnesium inert. Darvan 7 accomplishes this neutralization by encapsulating these strong, negatively charged ions, halting their ability to create a negative charge in the water film. In turn, it then releases sodium cations, producing the alkalinity required for suspension. Remember, Darvan is used to suspend particles in slip, and over saturation in terra sigillata creates a charge that results in very poor yields.
In terra-sigillata chemistry, temperature plays a vital role in yield. Warm tap water (approximately 100°F (38°C) should be used in lieu of cold water to mix and to suspend the particles. Subsequently, reducing the temperature by placing the sample into the freezer accelerates the collection process (1). Remember, hot temperatures accelerate suspension and cold temperatures hinder suspension. Ball clay has sufficient small particle distribution to use the thermal method, with the base formula of 700 grams of water, 300 grams of clay, and 8 grams (130 drops) of Darvan. You can decrease water by 50 ml to increase the specific gravity or increase clay by 50 grams if your ball clay has less fine particles. This is how you control specific gravity. The particle distribution of the clay selected will determine final yield. Note: You may have difficulty with kaolin because of the large particle distribution. Darvan additions should remain constant, the prescribed level also plays an important role in particle suspension/separation. Use the amount of clay, water, and time in the freezer to control the final specific gravity (specific gravity for terra sigillata generally runs 1.16 to 1.18.)
Making terra sigillata using clays with large particle size distribution, such as Cedar Heights Red Art and kaolin, requires an additional 20% more clay (360 grams). As particle size increases, the amount of time in the freezer decreases. Red Art clay only requires 12–15 minutes, and large particle kaolin requires 10–15 minutes. Ball clay yields 15–20% of clay content; Cedar Heights Red Art yields 15%. Yield is subjective to particle distribution of the clay selected. The clay you choose will require monitoring the amount of time in the freezer before a visible sludge line develops. Once you have established that timeline: develop a protocol of marking the water line and sludge line on a clear, empty plastic bottle to decrease the preparation time. I have a plastic cup that holds 300 grams of measured clay, again saving the step of weighing. Once you establish a protocol, you should be able to produce terra sigillata in 30 minutes from start to finish, including shaking vigorously for 15–20 seconds to mix the ingredients thoroughly at the beginning of the process.
Thermal Collection Method
The basic method of terra sigillata and decanting collection demonstrated by Marcia Selsor, professor emeritus at the University of Montana at Billings, works well.4 The primary differences from her technique for the thermal collection method are:
- Start with warm tap water
- Increase pH with Darvan 7 additions to suspend particles
- Place in freezer to rapidly cool solution, which causes larger particles to drop out of suspension
- Finish decanting (poking hole in container above sludge line)
By incorporating the thermal collection method, you can produce terra sigillata in 15–20 minutes instead of the typical 24-hour period. In addition, adjust water, clay, and time in freezer to achieve final specific gravity.
the author Thomas Anderson has spent the last decade researching technical research papers from various universities on clay formulation.