Terra Sigillata + Slips

What is Clay?

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

Soil is mostly made up of the mineral elements weathered and broken down into smaller particles. Clay is the smallest particle size these minerals that originated from nearby mountain ranges.

What Makes Clay Unique?

Clay minerals are built like stacked pages. Most clays are phyllosilicates, meaning their crystals are formed from: Silica Tetrahedra and Alumina/ Magnesia Octahedra.

Because of this structure, when clay breaks down through weather it doesn't shatter into chunks, it cleaves and bonds into flat layers or plates.

When water comes in contact with these clay particles, it slips between the layers/plates, it weakens rigid electrostatic bonds, and allows 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. These plates now can slide, align and are attracted to each other.

This creates plasticity: particles can move but they do not fall apart. This is why clay becomes plastic or (malleable) compared to dirt and sand--where water just fills the gaps between the particles without attraction and alignment.

Now complexed shapes can be formed(throwing, molding) and intricate designs can be created. If you were to keep adding water to clay until it reaches a creamy or paint-like consistency, now you have Slip.

What is Terra Sigillata?

Terra Sigillata is one of the oldest forms of surface decorations in ceramics. Latin for "Sealed Earth" The oldest use for the term terra sigillata was for a medicinal clay from the island of Lemnos.

 It was also called "sealed" because cakes were harvseted and pressed together and stamped with the head of Artemis. The clay's particular mineral content was seen as a proof against poisoning, and highly prized as a medicine and an antidote. This clay was dug and formed into cakes, that was then distributed and prescribed by doctors to be dissolved in liquids and wine.

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

The people noticed that the same properties that made the clay 'medicinal' made vessels dense, less absorbent, and more hygienic. As a result, coating vessels with Terra Sigillata reduced seepage, improved food storage, and limited odor and residue.

What is Terra Sigillata?

Terra Sigillata is a very ultra-fine mixture of clay and water (Slip) that is painted on the surface of clay. Terra Sigillata 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. The reason for this is due to the plate or sheet like structure of each clay particle. The more surface area, the more water you need to coat the particles.

Within this solution a deflocculant is added, a deflocculant is a substance that forces clay particles to separate and stay suspended in water. The most common deflocculants are: epsom salt, darvan, or sodium sillicate. Each of these contain sodium ions, which are perfect for modifying the electrical behavior of clay particles.

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

After allowing the suspension to settle for 12-24 hours- it should resemble the image above. There is very little water on top. Siphon down to the heavy sediment which you will throw away (or use as a regular slip).

What Clay To Use?

Most potters use a highly plastic clay such as ball clay (Kentucky OM 4), XX Saggar, or EPK kaolin. These clays' particles are finer compared to other clays which means there is more usable terra sigillata that is produced from the same amount of raw clay. This high suspension of finer particles will produce a more smooth and glossy finish.

After Terra Sigillata is applied to unfired clay surfaces, it can then be polished with a soft cloth or brush to achieve a shine ranging from a smooth silky luster to a high gloss. The surface doesn't need to be glaze, and in fact will impede the glazes ability to be absorbed into the piece.

Also its important to remember to only apply Terra Sigillata to leather hard to bone dry (greenware) pottery. If it is applied to bisqued pottery, due to the separate shrinkage rate ( because it is a clay) there is a high risk that it will crack off of the surface of your work.