How to Choose Your Pottery Clay Body

What is clay?

Flat plates of clay particles

Before choosing your pottery clay body it is first important to understand a little about clay and why it is used as a component of a clay body. Clay is a mineral extracted from the ground which can be readily moulded like plasticine. At microscopic level, the particles of clay have a flat plate-like structure giving them this plastic like property. The 2 most commonly used pottery clays are Ball Clay and China Clay (Kaolin). Ball clay tends to be very plastic-mouldable but off white in colour. China clay is less plastic but has a whiter fired colour. Many other clays are available but they tend to be off white to red in colour and therefore less used in high volume white pottery manufacture.

What is a clay body?

The terms clay and body are used by potters almost interchangeably. However in most cases it is not technically correct! In simple terms a body is a formula containing clay and other minerals. Therefore clay is a raw material component of a body. In past times a body was simply a clay dug from the ground which was variable in both its composition and properties. However, modern commercial potters no longer rely on bodies manufactured from clay alone. To achieve specific properties they are scientifically formulated in a very precise way. A wide range of raw clays and minerals are used from all around the world. They vary in colour, strength, particle size as well as mineralogy and purity. No two clays from different parts of the world are precisely the same although for comparison purposes they can act in a similar way when used as part of a body formula. For studio pottery manufacture coarse minerals called grogs are often added to the clay body formulas to give additional texture appeal and improved properties.

How to choose the best clay for you

Firing range of your kiln

The maximum firing temperature of your kiln should be your first consideration when deciding which type of clay body to buy. It is no use choosing a stoneware clay firing 1250 C if your kiln will only fire to 1200 C. Lower temperature kilns are suitable for earthenware and terracotta bodies (less than 1200 C) whereas high temperature kilns are acceptable for most bodies including porcelain and stoneware.

Colour

The fired colour of the body will often dictate the cost. In general whiter clay bodies are more expensive than buff or cream coloured ones. The type of clay body you use is often prescribed by your colour selection. For example porcelain and bone china are always very white after firing.  To give you an idea of the fired colour produced by the different clay body types see the section entitled 'range of bodies'. Please also note that the firing temperature has a significant affect on the fired colour!

The making Process

The making process you intend to use to shape the body is also an important factor when considering the most appropriate clay body. Clay body sub-types allow you to select a clay suitable for all major forming operations including throwing, hand-building, sculpting, casting or machine making.

Size of your work and end use

The size of the pottery piece you plan to make is also important. Larger pieces often require a more heavily grogged (less plastic) clay whereas smaller pieces of work can require more plasticity. In addition, the end use of the ware, whether it is purely ornamental, for outside use, or designed as functional tableware will affect your choice of clay body.

Texture

For the more experienced potter the texture of the clay body after firing is often important.. The feel, look and strength of the ceramic piece is strongly affected by this sub-type of clay. For example the grogged subgroup of bodies generally add texture, strength and stability, whilst an ungrogged body will result in a smoother more polished finish.

Glaze compatibility

Whichever clay body you choose it is imperative that you select a glaze which is compatible with the body. This technical compatibility is critical to producing an intact piece without faults after firing. Be guided by your supplier who can supply compatible glazes for most body types and firing schedules. For peace of mind you should always test a sample of the glaze and clay body in your own kiln prior to any major production. The temperature and firing schedule of your kiln will influence whether your clay body and glaze are compatible after firing.

Range of Clay Bodies

The are numerous clay bodies produced commercially around the world available to the craft potter, studio potter and commercial pottery. Indeed there are so many that it is impossible to detail them all here. Bodies are often developed to make them suitable for the making process. Therefore suppliers often state  subtypes which define which clay bodies are more suitable for hand throwing, casting, hand building etc.

Clay bodies however can be classified into a relatively small number of categories according to their colour, firing range and texture. Below are a few major examples showing the fired colour of individual body types.

Porcelain

Bone China

Earthenware

Stoneware

Terracotta

Bone China

Bone China

This is a smooth textured extremely white firing body that is also translucent. It is unique in that it contains a high proportion of calcined bone ash . Biscuit firing at approx 1220 C gives it a high strength making it suitable for producing delicate highly decorative items as well as tableware. This type of ware after glazing is often decorated with onglaze colours or precious metal decoration to create stunning pieces of pottery. Electric firing kilns usually produce the best bone china quality.

Porcelain

This is a smooth textured translucent extra white firing body similar to bone china. However two types of porcelain are made, a 'hard porcelain' which requires a glaze firing in excess of 1400 C and a 'soft porcelain' which requires a glaze firing to approx 1250 C. Biscuit firing however is often around 1000 C which allows pieces to be glazed more successfully. High temperature gas kilns are often used to fire this type of body. Porcelain, like bone china, can also be decorated with onglaze enamels and precious metal to create delicate highly attractive giftware as well as tableware.

Stoneware

Stoneware

This class of clay body is commonly used by craft and studio potters. It has a relatively high biscuit firing temperature in excess of 1150C.  Many commercial bodies, available in a range of off white to buff colours, are fired in the range 1250-1300C to give maximum strength. By glazing with a reactive type coloured glaze a wide variety of effects and colours can be achieved. Because of this stoneware has found high popularity with craft potters looking to create unique coloured or or textured hand made pottery. This body type can be used to produce both decorative and functional pieces such as tableware.

Terracotta

Terracotta

This class of body is typified by its unique red terracotta colour. This clay body has a high iron content in its mineral components giving the unique red colour. Like stoneware the smooth texture can be modified, by addition of grog (coarse material), to give a much rougher finish. Terracotta bodies have a relatively low firing temperature of 1000-1050C and are therefore porous and have relatively low strength after firing. Commonly unglazed it is often used for sculptures, planters, tiles and garden ware where the technical properties are not so demanding. For more demanding environments such as tableware the body is often glazed to give a stronger more durable product.

Earthenware

This type of body is often used for hand painting by hobbyists.

For craft pottery this body is biscuit fired to approx 1000 C to allow easy brushing of underglaze colours or coloured glaze on to the porous surface. Following  glaze firing to 1050 C to 1150 C the body colour and underglaze colours show through the transparent glaze producing highly decorative ware.  After glaze firing the body remains porous with reduced strength compared to fully vitrified bodies like porcelain or bone china.

Body colour ranges from white to buff and some bodies are also grogged to provide texture.

For commercial tableware optimum body strength is achieved by firing the biscuit in the range 1180-1220 C but this peak is not often needed for lower strength or decorative only ware.

Special bodies

This range of bodies include, highly coloured bodies, low firing bodies, Raku bodies, and special highly textured bodies. In the case of Raku the body is modified to allow rapid heating and cooling without  cracking, usually by the addition of grog. Making raku pottery successfully requires more expertise than other pottery types and often depends on a trial and error approach. To learn more about Raku read my separate article on Raku making.

Summary

Clearly, choosing the right clay body for you poses a number of technical questions. It is more difficult than is immediately obvious. However I recommend you talk to your clay body suppliers in the first instance and not just search the internet. Their vast knowledge of their products will make the whole process of selection of suitable body and glaze just that much easier.

Good Luck
Happy potting!
The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

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How to choose your pottery kiln

K&F front loader

Making a kiln purchase is a big step for many potters. It is probably the biggest financial outlay you have as a potter and you would expect a new kiln to last 10 years or more depending on frequency of use. Most pottery beginners start by attending pottery classes and having their ware fired by the tutor. After developing your skills and with a long term commitment to pottery making you are in a better position to decide your needs:-

Set your Budget

Decide your budget. Make it realistic and include cost of delivery and installation. Also don't forget internal shelves and supports are not usually supplied as part of the kiln and need to be purchased separately. Include a good temperature controller. A kiln is only as good as the controller! Remember planning permission maybe required for some external installations? Decide whether you want to buy new or second hand. Although this will vary considerably as a rule of thumb a good quality second hand kiln  is often about 20% cost of new.

Select the energy source

Decide what type of kiln energy source you will use. Often the type of ware and pottery you intend to make can determine this. For example reduction firing of glaze will require gas or oil firing whilst decal firing is better done in electric firing. It is worth noting that, in general, electric is a cleaner and more controllable fuel than gas but more expensive. Other fuel sources such as Oil, LPG, and wood vary tremendously depending on location and availability.

Identify kiln size and site and type

Decide what size and where it is to be located, does it need special ventilation? How many pieces do I want to fire now? and the future? What size or weight will they be? Will I fire bisque and glost and decoration?

Whether to choose a front loading or top loading kiln often depends on the size and number of the pieces you wish to fire. Generally top loading kilns are smaller than front loading kilns which are easier on the back for placing and unloading.

Visit showrooms or research online

There is no substitute for seeing the kiln you want to buy in person. This will give you a real sense of size and ease of use. A good supplier of kilns will have a range of kilns for display purposes and talk you through many of the decisions you need to make. However as a general rule you need to know the following before you talk with your supplier

What size and shape of pieces you wish to make and how many?

What temperature you wish to fire glost or bisque or decal

Whether you have refractory shelving and supports for the pieces you want to make.

Whether you have 2 or 3 phase electric supply. Domestic supplies are predominantly 2 phase.

Whether you have mains gas supply or other

Calculate firing costs

It is possible to work out the firing costs using the KWH rating or gas usage as a guide. For smaller hobby kilns the difference in costs between gas and electric may not be high but for bigger kilns this needs to be factored in to your buying decision. Again a good kiln supplier will be able to advise you on this.

Choose both for now and the future

Make your selection based on your plans for the foreseeable future. Agree the price for everything including the set up of kiln in its final location not just to your doorstep. Unless it is a simple domestic plug in kiln, employ a professional electrician or gas fitter to ensure your kiln is installed correctly. This gives you peace of mind as well as meeting any legal requirements.

Test out the kiln 

Once you have your new kiln installed test it out first with known tried and tested body and glaze. New kilns can take a while (a few firings) to settle as the refractories are more porous than a used kiln and the gasses from firing soak into the brickwork.

Happy Potting!

Enjoy the new sense of freedom having your own kiln brings and good luck!

More information and other technical articles on kilns, pottery and ceramics can be found at my website The Potters Friend.

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Starting out in Pottery? Read this first!

Making pottery is just like baking  a cake but more complex!

I can tell you one thing about pottery that is in-disputable. It is diverse and more complex than you could ever imagine. Think of it as being like baking a cake. There are literally thousands of different recipes and baking instructions, including the one your grannie used to make! Many of the cake recipes make great cakes but they are all slightly different.

 Clay bodies and glazes are just the same but even more complicated! This is what many potters find fascinating. They can develop their own clay body, glaze and process in a way that makes them special and their pottery products unique. Please remember that!

 However for the novice all this complexity creates confusion. Listening to more experienced potters providing a snippet of knowledge is not always good for you. Beginners try to relate it to their needs and often it results in failure.

 So here is what the beginner really needs to know and not forget.

Start with a pottery class

Start with a pottery class. Here you can learn the basics with the support of a tutor. You can meet new friends and even take comfort in that you are not alone in your struggles to learn a new skill! A good tutor will not only guide you but provide material recommendations to allow you to reach the next stage of working alone.

Choose a Quality Clay and Glaze supplier

Clay Body

If you must go it alone there are many suppliers out there who can help you make the right choice of initial materials. They have the experience and knowledge of a wide range of clay bodies and glazes.
Tell them what you want to make and how. For example. "I want to make pottery by using a potters wheel. I want to glaze it with a white glaze by dipping and I have a pottery kiln that has a maximum temperature of 1250C. "

They can then provide a clay and glaze that are compatible which when processed correctly will give you the best possibility of success.

Process the clay and glaze as recommended by the supplier 

Orton Cones

The making and firing process used to make the pottery is as important as the clay and glaze materials. Therefore process the clay and apply the glaze as recommended by the supplier. Fire the clay and glaze in the kiln as recommended by the supplier.  Firing is often controlled by a firing cycle input into an electronic controller and/or Orton cone values. For more information on firing read my detailed article how to properly fire your pottery.

Once you are happy with the finished result repeat it several times. This proves the process you have  used will work consistently as well as giving you confidence in your skills.

Make a note of the clay body, the glaze, the firing schedules and take a photo of the fired appearance. In this way you will always have a record of how to reproduce your great work of art!

Only once skilled add your own artistic flair

Making pottery is a combination of art and science. The art to design something unique and the science to make it into a product. Only when you have the basic pottery skills and can repeatedly create the same basic product should you become focused on artistic flair.

Developing  your own style and delight in making something unique

Lori Duncan Raku

Potters are often most satisfied when they have developed their unique style or products. Using baking as analogy, top bakers often specialise in making bread, cakes or pastries and have their one stand out dish. Likewise potters specialise in stoneware, raku or wood firing or other depending on their influences.

However they develop this over many years of making and studying pottery. 

So do not rush to specialise too early. What all master potters have in common is a strong basic knowledge of clay, glaze and firing.

Keep on learning and enjoy

Pottery making is meant to be enjoyable so make sure this happens. One way to do this is to continue to learn new things. In pottery making this is relatively easy as the subject is so diverse and changing that you never become a master of all.

Once adept at basic skills look at all elements of what you do not just the making technique. Using materials such as colour creatively can open a whole new spectrum of ware. For example if you only made plain glossy white, look at making some coloured glazed pieces.

One way to do this successfully is to enrol on a pottery course specialising on one type of technique. For example studio potters often run courses to help others develop their knowledge of raku or wood firing or sculpting etc. For more information see my list of pottery courses/classes available in the UK

Above all enjoy!

Good Luck
Happy Potting
The Potters Friend

Self cleaning plates and lego style brick houses? Are these ceramics of the future?

Legoland By Copyright © 2003 Kaihsu Tai
via Wikimedia Commons

Self cleaning Glass

I recently read an article about the possible future of textiles. Designers were predicting the living shoe and the strawberry plant that grows lace. Whilst fanciful such ideas have a slim chance of becoming reality.

That made me think about ceramics. What will be the future of our beloved ceramics in 50 years from now? Perhaps it will be self cleaning plates and lego style brick houses? Again these may seem far fetched to most but they are already possible in a small way.

The next 50 years in ceramics - my vision

My vision  is based mostly on what is possible now but not necessarily in ceramics

100% Recycling

Without doubt ceramics will be recycled in a more comprehensive and systematic way. Recycling points will be readily available in every part of the country. Ceramics will be reprocessed by barcode or other tracer systems which will allow them to be separated  for more economic recycling or re-use.

Greater personalisation in Design

The changes in pottery fashion are more rapid now than at any other time. This trend is likely to continue. Changes to our dining habits, less formal dining and more casual dining, is reflected in the ceramics we buy. Ceramics are now mostly considered a disposable item and need to fit more closely with our lifestyles. This has led to a wider choice of colour and shape. For example a whole range of ware may be used for dining instead of a single dinner plate.

The future however is likely to bring us even more personalisation in design. Designs will be agreed at the point of sale or online and products will leave the factory 24 hours later. Scope for designing your own tableware is not far away now and will be common place in the future. Personalisation is already available in a small way but this will grow exponentially as technology progresses.

Lightweight strong ceramics

Abalone Armor Paper-thin armor courtesy of nature Andreas Walther

Ceramics will become lighter and stronger! Lightweight ceramics have already been developed for military purposes but ultimately these ceramics will become part of the pottery scene. The advantages of light weight and strength are significant not only to the carbon footprint but also to manufacturing and transport costs.

Egg shell porcelain courtesy
of Victoria and Albert museum

 A good example of ultra thin ceramics is Eggshell porcelain , a delicate porcelain often decorated with a watermark-like image dating back to the Ming dynasty (1402-1424). Nevertheless because of lack of strength it has not found widespread use for dinnerware. Future ultra thin ceramics will have sufficient strength to make them suitable for a wide range of ceramics.

Imagine drinking from a paper thin highly decorated mug that is  heat retaining but virtually unbreakable?

Warm self cleaning sanitaryware

Composite materials are mixtures of materials eg ceramic and plastic which when combined give superior properties when compared to the individual materials alone. Such type of materials are common place in the military and aerospace industry. However they are less well used in domestic applications. Nevertheless car windshields are often a composite of glass and plastic. Here a layer of plastic is embedded in between two layers of glass to give extra strength and toughness.

In time other composite materials will emerge in household products. I can imagine a bathroom where the ceramic composites used are warm to the touch, with antibacteral surfaces that are virtually self cleaning. Water is recycled at point of use via ceramic composite filtering systems.

3D Digital printing- or how to doodle your own cup and plate!

2d digital printing of colour

3d digital printing of a sugar bowl

Digital 2d colour printing is already well established for printing of coloured images on to paper. It is already making significant inroads for the decoration of tiles and tableware. In the case of ceramics the 4 colours used in ink jet photocopier type machines are replaced by up to 10 ceramic coloured inks. These are then used to decorate ceramics directly or via an intermediate stage such as paper transfers. The result is a high quality image in rapid time using an amazingly flexible system. But digital printing does not stop there! 3D printing of ceramics is in its infancy. Greater design possibilities as well as the personalisation already discussed above will become the norm.

 But what does this all mean? In essence a 3d computer design will create a 3 dimensional object via a robotic system by building a series of 2d layers. A good analogy would be building a wall brick by brick but at particle or even molecualar level.

 Already there is a 3d printing pen that transfers what you draw into a 3d object in plastic. If you had a pen that would allow you to doodle a 3d object of the future what would  you draw?

Eco Style Ceramic bricks and panels

Eco house from

Although the detail of design, decoration and processes used in brick making has changed greatly in recent years, the basic idea of shaping raw clay into rectangular blocks remains much the same. Ceramic bricks have changed little in hundreds of years. In many ways this is cost driven so will need radical changes in house construction for this concept to change. 

However this is happening albeit slowly as more eco houses are being constructed (in part) off site and delivered to site for final assembly. This trend will continue. The idea of coventional bricks will change as this type of housing grows. It is not too stretching an idea to believe that strong lightweight ceramics in the form of legotype bricks or panels could be used in all future house construction. 

Who knows the eco house of the future could be manufactured entirely off site and only services and fixings completed on site! 

What do you think?

So whilst far fetched I believe many of the above have a reasonable chance of becoming reality. What do you think of my ideas? Do you have some (better) ideas of your own? I look forward to hearing all about them.

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

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How to Choose a Pottery Wheel

Buying a pottery wheel can be quite an expensive issue. Whilst second hand wheels exist it is difficult to judge their state of repair and future reliability. Therefore make sure you make an informed choice. So what factors should you consider when buying your pottery wheel? Weight of clay When choosing a pottery wheel the weight of clay you intend to throw decides the power of the motor you will need for a power driven wheel. Typical weights are as follows: ¼ HP for small pottery items 20lbs (10kgs approx) max for beginners ½ HP for medium sized items up to 100lbs (25 kgs approx) max suitable for most commercial potters 1 HP for large items 200lbs (50kgs approx) for experienced potters making very large items. Quality and ease of operation Quality of wheel can be an issue as it comes at a price. However you should also consider at least: Noise-older type wheels can be quite noisy whilst some recent models are whisper quiet Smoothness of pedal operation Smoothness of wheel particularly at low speeds- a jerky transition of speed can cause difficulties -important for delicate shaping Reversability-can it be used by left and right handed people? Electric or kick-wheel operation? Foot pedal or hand lever operation? Whilst kick wheel models may be cheaper they can take some getting use to if you are already used to an electric powered Wheel head size and predrilled holes- The wheel head size dictates the max diameter of clay piece which can be thrown. Holes in the wheelhead allow a bat (special disc) to be used. Pins allow release of the bat, holding the thrown piece, from the wheelhead. The thrown piece can then be removed easily for drying while a new piece is thrown. Mobility Some wheels are very light in weight and designed to be portable whilst others are meant to be static. Consider if you want to move the wheel between different locations. Heavy powerful wheels can be more durable and long lasting. Examples of different wheel types are: A Tabletop Wheel The Shimpo Aspire can handle your most creative challenges. Its lightweight, compact design makes it easy to transport, and a 20 lb centering capacity makes it a practical addition to any studio or classroom. The Aspire features a 1/3 HP, 100W DC motor, a 7" alloy wheel head, and a removable one-piece splash pan. The wheels speed ranges from 0–230 rpm, controlled by a hand lever. Also included are two 9¾" bats. It measures 9" H × 14½" W × 20" L and weighs 25 lb . Amazing, affordable, accommodating … Aspire! Shimpo’s Aspire Pottery Wheel includes a two-year limited manufacturer’s warranty.   A Powerful model suitable for seated use Brent wheel Model B is designed to fill the needs of schools and studio potters. Competitively priced, this wheel is quiet and solid under 150 lb (68 kg) loads of clay. A 1/2 HP motor provides the power and weight necessary for school use. A splash pan and 12" Plasti-Bat are included. Shipping weight is 112 lb (51 kg). Truck shipment is required. 10-year limited warranty. CE-certified. Super quiet model for seated use-The Shimpo's Whisper Potter's Wheel is extremely responsive, with high torque at all speeds. It's powerful, and by far the quietest pottery wheel on the market. The ½ HP, brushless DC motor has a direct drive (no belt) system and a broad speed range for optimum control. A fixed foot pedal with hand lever operates the electronically controlled motor. The motor is reversible for lefthanded and righthanded users. The 12" wheelhead and two-piece splash pan are standard. Centers up to 100 lbs. The wheelhead also turns freely, so it can be used as a banding wheel at 0 rpm.

Warranty and Cost Warranty and cost often go hand in hand. A warranty can be anything between 1 and 10 years and cost will be upward of $350 dollars for a new wheel. It is worth asking if it can be repaired locally within the warranty if anything goes wrong..

Digital printing of ceramics

Digital printing of ceramics and decals

Digital printing of decals and ceramic tiles is already well established. Computer generated images can be printed directly onto ceramic tiles via special inkjet systems. Alternatively a photocopier type system can be used to print onto decal paper. Of course both these systems require special machines and ceramic inks or toners to produce optimum results. Nevertheless these systems are causing a revolution in the ceramic tile industry.

The detail of decal printing

Ricoh desktop ceramic printer

In the case of digital decal, you can either create a drawing or photograph and then convert it to a digital image suitable for printing on to decal paper. The image is produced as a decal by a photocopier type machine. After printing the image is sealed to the paper with a covercoat and allowed to dry. The decal can then be applied in the same way as conventional decals by a water slide method. Decorators have the choice of either outsourcing decals (small quantities) or for large quantities buying a machine and toners and printing themselves..

The detail of ink jet printing of tiles

Pigment	System	Temperature	Colour
Yellow	Zr-Si- Pr	1250 °C
Red Magenta	Zr-Cd-Si-Se	1250 °C
Cyan	Zr-V-Si	1250 °C
Black	Fe-Cr-Co-Ni	1250 °C
Primary 4 colours used in ink jet printing

In the case of ink jet printing, the digital image is converted to a ceramic image using a specially adapted ink jet printer and special inks. The ceramic tile passes beneath the printer head and the ink jet colour nozzles deposit the image on the tile.

Gamma Ink Jet Tile Printer

Typically up to 10 ceramic colours are used to produce high quality digital images, 4 similar to the primary colours shown above plus additional spot colours. Browns, coral reds and white are commonly used to fill gaps in the colour space. Special printer heads are used to cope with the highly abrasive ceramic inks. For maximum definition and colour intensity special glazes are sometimes used as the base for printing the coloured image. The printed item can be fired almost immediately after drying.

3d Printing of Ceramics

I am constantly amazed at the ceramic challenges I see each year. Whilst making pottery and ceramics is an ancient tradition, new industrial processes and products (or the re-introduction of many older techniques) continue to stretch the imagination.

3d printed sugar bowl

Both in the UK and USA, 3 dimensional digital printing is one such technique. Developed initially for rapid prototyping of plastic objects it is now being used for small special ceramic parts. The potential to build up a ceramic mug  layer by layer  from a computer image is certainly feasible.
 I have seen the initial prototypes which have been glazed and fired. Many technical challenges remain for this process to be viable commercially but I have no doubt that in the long term this will happen.
For more information about this technique see the link project below:-
 University of Bristol project

3d printing using liquid clay syringe

3d extruder printing

This type of printing is also in the early stages of development for ceramics. Many questions remain particularly control of rheology and removing air pockets and machine development.. However the picture on the right shows how the process works. An air powered syringe forces liquid clay out of the nozzle to build up the object in layers. The head could be computer controlled to map out a digital design.
Potentially the ceramic designer can be creative in designing complex forms as well as simple shapes.
Picture and link to more information kindly provided by Ceramic 3D Extruder (FabLabZuidLimburg) / CC BY-NC 3.0

So what does this mean to the hobby or studio potter?

The opportunity to make low cost efficient designs which can be personalised to meet your customer needs. Digital ceramic decals are already established and widely available. But keep following the progress of 3d printing as the potential is endless. Perhaps now is the time to look at ways these techniques can expand your opportunities?

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.

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.