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.

The Elusive Red Pottery Glaze

Symbolism of Red

Red is one of the most popular colours to be used worldwide. The color red is associated with lust, passion, love, beauty, and danger. In Asia it is a symbol of courage, loyalty, honor, success, fortune, fertility, happiness, passion, and summer.

Consequently many types of asian pottery including the beautiful chinese red glazes of the Sung Dynasty and the exquisite Japanese Imari porcelain feature red as the main colour.

However in more recent times the demand for mass produced red pottery of high intensity and consistent colour has created a real challenge for ceramic engineers involved in pottery manufacture.

Japanese Red
Imari porcelain is the name for Japanese porcelain wares made in the town of Arita, in the former Hizen Province, northwestern Kyūshū. They were exported to Europe extensively from the port of Imari, Saga between latter half of 17th century and former half of 18th century.

Photo of Imari by George Le Gars

Though there are many types of Imaris, Western Europes conception of Imari is associated with the type of Imari produced and exported in large quantity in mid-17th century. This type is called Kinrande and is coloured porcelain with underglaze cobalt blue and onglaze red and gold.

The blue is rich in colour and hue and is based on a cobalt containing pigment. The red colour, however, whilst remarkably bright and interesting is not such a clean hue and is produced using iron oxide or iron based ceramic pigments.

Iron ochres and natural red clays are some of the earliest pigments used by man and continue to be used today particularly in hand crafted pottery of rustic nature.

Rouge Flambe
Similarly many of the early ceramic red colours and glazes used on European pottery were not always pure in colour and were based on copper or gold in colloidal form. These colours were used to copy original Chinese patterns and glazes perfected in the 18th century.

Rouge Flambe by Royal Doulton Co

Interest in reproducing flambe glazes began in about 1855 in France but soon spread across Europe, particularly to England and Germany.

In Britain, the Staffordshire potter Bernard Moore used experimental and highly accomplished red flambe glazes on Chinese-inspired shapes to wonderful effect. Requiring a special glaze formula, firing cycle and condition they were expensive to manufacture and difficult to reproduce.

Shown here is a vase made by Royal Doulton decorated with the spectacular Rouge Flambe glaze.

The Challenge of Pillar Box Red

Red Pillar Box by Ian Britton 
FreeFoto.com

The development of a reproducible pillar box red glaze and decoration has been an elusive challenge for potters and ceramic engineers through the ages.

In particular the large scale development and manufacture of pillar box red colour and glaze for pottery is believed to have taken over eight thousand years to achieve. It was not until the twentieth century that true pillar box red glazes and onglaze colours became widely available for decoration of pottery.

Cadmium Sulpho-Selenide Red Glazes

Red glazes and colours based on cadmium have been available for about 50 years. The early glazes and colours of this type depended heavily on cadmium being added to the glaze as a pigment mixed crystal Cadmium sulphoselenide (CdSSe). However glazes of this type lacked temperature stability above approx 800C.

To overcome this the cadmium was added to the frit batch and subsequently melted into a glassy state. In many cases the frits also contained lead, sulphur and selenium. This type of glaze required firing in a specific way to achieve optimum results. The firing time and the kiln atmosphere were both critical. The best results were obtained using a short firing cycle to a peak temperature of approximately 1000C. Good air circulation in the kiln, and a good separation of pieces were also recommended. Ideally Cd/Se fritted glazes should be fired separately from other types of ware, because fumes from other coloured glazes can affect the colour achieved.

First generation Zircon/ Cadmium Red Glazes

Actual Red

Target Red

In the 1970s ceramic engineers in Europe developed a new generation of red colours and glazes. These were totally different in nature as they were based on a zircon encapsulated cadmium pigment. These were patented in 1973 by Broll et al of Degussa.

Briefly encapsulation  means that at microscopic level the cadmium is surrounded by the zircon crystal. This gives two major advantages. The pigment is more temperature stable up to 1200 celsius and more chemically stable thus giving lower cadmium release after firing and exposure to acids. These types of red glaze could be used on a variety of firing schedules and were infinitely more flexible in use.

However these pigments had a serious drawback which limited their widespread use. They lacked intensity and consequently needed high pigment loading to give any degree of colour saturation.


Second and third generation Zircon/ Cadmium Red Glazes

Red Vase by Newland Pottery BV

In the last 20 years or more the encapsulate type of pigment has been developed further to the stage where intense pillar box red coloured glazes (see target above) are now possible. These pigments have allowed more stable sanitaryware glaze colours to be produced where high temperatures in excess of 1250C are used. In addition they are suitable for use in a range of glaze and onglaze colours as they are mixable with other non cadmium based colours.

China is the major manufacturer of these types of pigment as they have proven to be a lower cost supplier of intense encapsulate pigments.

The challenge of the stable pillar box red glaze now appears to have been met by ceramic engineers. The  stable cadmium encapsulate pigment is being used by most pottery manufacturers and appears to meet current legislation in many parts of the world. However cadmium is still an emotive word for many being considered a toxic element in much the same way lead. So who knows? Will the journey to find a new pillar box red start all over again? What do you think?

This article was inspired by the recent publication of  a book entitled THE DEFIANT RED by my friend the highly respected ceramic expert Leen W Baaij.

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.