Terra Sigillata Explained: Clay Particles, Deflocculation, and Surface

What is Clay?

Clay comes from the ground, but what is it really? Why is it different from regular dirt or soil?

Understanding Soil and Clay

Soil is primarily made up of mineral elements that have weathered and broken down into smaller particles. Clay represents the smallest particle size of these minerals, which originate from nearby mountain ranges.

What Makes Clay Unique?

Clay minerals have a unique structure, resembling stacked pages. Most clays are phyllosilicates, meaning their crystals are formed from silica tetrahedra and alumina/magnesia octahedra.

The Structure of Clay

Due to this structure, when clay breaks down through weathering, it doesn’t shatter into chunks. Instead, it cleaves and bonds into flat layers or plates.

The Role of Water

When water comes in contact with these clay particles, it slips between the layers. This action weakens rigid electrostatic bonds, allowing the particles to separate and flatten more flexibly.

Clay particles, in the form of microscopic plates, carry negative charges on their faces and positive charges on their edges. This allows the plates to slide, align, and attract each other.

The Plasticity of Clay

This interaction creates plasticity: particles can move without falling apart. This is why clay becomes plastic or malleable, unlike dirt and sand, where water merely fills the gaps between particles without attraction and alignment.

Now, complex shapes can be formed through throwing and molding, allowing for intricate designs. If you keep adding water to clay until it reaches a creamy or paint-like consistency, you have created what is known as Slip.

What is Terra Sigillata?

Terra Sigillata is one of the oldest forms of surface decoration in ceramics. The term, which means "sealed earth" in Latin, was first used for a medicinal clay from the island of Lemnos.

Historical Significance

It was called "sealed" because cakes of clay were harvested, pressed together, and stamped with the head of Artemis. The clay's unique mineral content was believed to protect against poisoning, making it highly prized as a medicine and antidote. Doctors would prescribe this clay, instructing patients to dissolve it in liquids and wine.

Evolution of Terra Sigillata

Over time, Romans began to use the term Terra Sigillata differently to describe red-slipped tableware. These vessels were valued for their luminous, refined smooth surface, reduced porosity, and fine detail.

People noticed that the same properties that made the clay 'medicinal' also made the vessels dense, less absorbent, and more hygienic. Coating vessels with Terra Sigillata reduced seepage, improved food storage, and limited odor and residue.

Creating Terra Sigillata

Terra Sigillata is an ultra-fine mixture of clay and water (Slip) that is painted on the surface of clay. It is a liquid suspension of the finest clay particles.

2-Cup Terra Sigillata Recipe

• Water: 800 ml (3⅓ cups)

• Dry clay: 14 oz by weight

• Deflocculant: ¾ tsp (start slightly under, adjust drop by drop)

  
  

Terra Sigillata involves adding more water than would normally be required for a regular slip. This is because of the plate-like structure of each clay particle. The more surface area, the more water you need to coat the particles.

The Role of Deflocculants

Within this solution, a deflocculant is added. A deflocculant is a substance that forces clay particles to separate and stay suspended in water. Common deflocculants include Epsom salt, Darvan, or sodium silicate. Each of these contains sodium ions, which modify the electrical behavior of clay particles.

How Deflocculation Works

Deflocculation forces clay particles to further separate and flatten by changing their electrical charge. Naturally, clay particles want to clump and sink, but deflocculating allows the finest particles to float freely in suspension. The heavier particles will sink to the bottom, forming three distinct layers: water, finest clay particles in the middle, and heavier particles at the bottom.

After allowing the suspension to settle for 12-24 hours, it should resemble the image above. There should be very little water on top. Siphon down to the heavy sediment, which you can discard or use as regular slip.

Choosing the Right Clay

Most potters prefer using highly plastic clay, such as ball clay (Kentucky OM 4), XX Saggar, or EPK kaolin. These clays have finer particles compared to others, which means more usable Terra Sigillata can be produced from the same amount of raw clay. This high suspension of finer particles will yield a smoother and glossier finish.

Applying Terra Sigillata

After applying Terra Sigillata to unfired clay surfaces, it can be polished with a soft cloth or brush. This polishing can achieve a shine that ranges from a smooth, silky luster to a high gloss.

It's important to remember to apply Terra Sigillata only to leather-hard to bone-dry (greenware) pottery. If applied to bisqued pottery, the different shrinkage rates can lead to cracking, causing it to detach from the surface of your work.

Conclusion

Exploring the world of clay and Terra Sigillata opens up a realm of creativity and connection to nature. As you work with these materials, you not only engage in a craft but also immerse yourself in a tradition that celebrates the beauty of handmade goods. Embrace the journey of creating with clay, and let it guide you toward a deeper understanding of art and self-expression.

Whether you're molding, throwing, or painting, remember that each piece you create carries a piece of your spirit. So, let your hands dance with the clay and enjoy the process of bringing your visions to life!