Shiny Pottery Without The Glaze!

Roman red gloss terra sigillata bowl with relief decoration
Photo from Museum Römerhalle Bad Kreuznach

Terra Sigillata

After reading about burnishing recently I researched the technique even more and came across Terra Sigillata which literally means 'clay with little pictures'. This definition makes sense if you look at many of the early pots made using this technique which feature figures in relief. However later pots similarly described do not contain figures. So Terra Sigillata has come to be known as pottery manufactured with surface slips which when fired produce a glossy surface ranging from a soft lustre to a brilliant glaze-like shine. Samian ware is also sometimes used to describe all varieties of terra sigillata including undecorated ware.

The history

Ancient Greek pottery neck amphora showing a swordsman.

Image Courtesy of Marie-Lan Nguyen

Most well known for its use in ancient Greek ware Terra Sigillata now describes specific types of plain and decorated tableware made in Italy and in Gaul (France and the Rhineland) during the Roman Empire. These vessels have a range of glossy surfaces and are often found in their characteristic colour range of pale orange to bright red.They were produced in standard shapes and sizes and were manufactured on an industrial scale and widely exported.
The sigillata industries grew up in areas where there were existing traditions of pottery manufacture, and where the clay deposits proved suitable. The products of the Italian workshops are also known as Arretine ware, and have been collected and admired since the Renaissance.

What is a Sigillata slip made of?

Refined Kaolin particles

Terra Sigillata, is made up of finely separated clay particles in water. The clay particles are mostly composed of kaolinite crystals which have a flat plate-like structure.  These particles therefore have a high width to height ratio, called the aspect ratio making it easy for them to fit together in a stack just like a pack of cards.The platelet however are tiny and at less than 2 microns wide can only be viewed using at high magnification microscope. To put this in perspective a human hair measures approx 70 microns in diameter-35 times the size!.

How Does it Work?

When the slip is applied to the surface of a clay article as several very thin layers, the flat plate-like particles align naturally to give a smooth reflective finish. The application process is critical as applying one thick layer is likely to cause the layer to crack as it dries and shrinks. The slip can be successfully applied to leatherhard or fully dry clay. However the best results are reported as being by applying to it to dry ware that has had the surface roughened using 120 grit sandpaper.

For a shinier appearance a quick polish of the surface with a piece of chamois or plastic film or even with a finger tip is effective. This increases the fraction of platelets laying flat on the surface thereby creating increased reflected light.

How does this differ from burnishing?

Polishing and burnishing can give a highly reflectively surface finish but it is a highly labour intensive process. It requires the potter to compress the clay with a polished rock or the back of a spoon in small circles for hours on end until the whole surface has been compressed. This process causes  the clay platelets on the surface to lie flat, while all the other coarse particles are pressed down into the surface. However it creates a very smooth, highly reflective surface.

In contrast, Terra Sigillata avoids the laborious polishing process by naturally laying down a thin layer of aligned clay platelets. Minimal polishing is required to achieve the same high shine as hours of burnishing.

Making the slip

You can make terra sigillata slip from almost any raw clay or clay body that contains kaolinite. However, only a portion of the starting material will contain sufficient fine clay particles to make a good Terra Sigillata. It is therefore important to use a separation process to split the 'good' clay from the 'bad'. This involves first mixing the clay then allowing it to sediment before decanting off the good material.The yield and performance of the slip will be influenced by the particle size of the clay. Therefore in general finer starting clays produce better slips and yield.

Details

The first step is to make a highly fluid de-flocculated suspension of the clay of SG 1.15. A mix of sodium silica/soda ash deflocculant and water is used. After sedimenting for approx 20 hours the ultra fines are decanted off to be used to produce the slip. The suspension at this stage is very watery and needs to be concentrated by evaporation or gentle heating of slip. Approximately 10% of the initial starting material suspension will remain to be used as slip.The slip needs to be approx SG 1.15 for best application.

Applying the slip

Application is made to the dry clay body by using a wide, thick soft brush heavily loaded with slip. Long flowing strokes should be used and drips should be avoided. if a drip appears remove it immediately with your brush. Work around the piece, building up thin coats until I the surface begins to conceal the sanded texture. If available a potters wheel can be used to give an even application. Overall, it needs a very fine coat of slip to avoid cracking and peeling of the layer.The success of this process is a matter of experience-as different slips require layer thickness for optimum effect.

Polishing of the applied Slip Surface

As soon as the the slip layer looks touch dry the surface is polished with a piece of grocery bag plastic or soft material. Amazingly the surface should develop a bright shine in just one polish.

Firing of the slip applied ware

Like burnishing, the terra sigillata surface needs to be low fired for best results but this depends on the clay slip and the underlying body used. Whilst a true burnished surface is recommended for firing at less than  cone 012 (875C), a terra sigillata-coated surface lightly polished can sometimes be fired as high as cone 02 (1125C).

Red terra sigillata also changes colour as well as gloss as the temperature is increased. Starting with orange red then brick red, it will finally turn red-brown before fusing due to the high iron and flux content.

Other clays like stoneware, ball clay and china clays will give white to a light tan depending on their source and chemistry.

Colouring of the Terra Sigilatta

Fine oxides or finely ground ceramic stains can be mixed with the slip to create a wider range of fired colours. However in general the greater the addition of stain the lower will be the gloss on the finished product. Best practice requires the stains to be thoroughly dispersed using a ball mill process. However very finely divided oxides like copper carbonate or cobalt carbonate can be added in small quantities without significant loss of shine.

Summary

Clearly the making of Terra Sigilatta is a skilful but labour intensive process. The decanting process is critical to obtaining the highly decorative glossy finish. However the quality of finish with the soft buttery feel is worth the effort. Indeed many alternative effects have been  obtained using the sigilatta slip in underglaze or combination with other potting methods. It still amazes how the ancient potters developed such sophisticated pottery effects over 2000 years ago. Modern pottery continues to learn from the past by application of our  greater scientific understanding! Long may it continue!

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend. Go now to sign up for my free newsletter.