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Clay (Composed primarily of kaolin, ball clay and feldspar)

Stacked clay at 2017 British Ceramics Biennale | Photo and copyright: Steph Liddle

Clay is versatile, hardwearing, and commonly used material. It is a type of fine-grained natural soil containing clay minerals, which has plasticity when wet but becomes hard after being exposed to very high temperatures. It is used across the world, and has been used for millennia to create everything from function objects for the home to decorative objects and sculptures. When fired in a kiln, clay turns to “ceramic” – a material that is hard, durable, and watertight when fully vitrified and glazed. 

The characteristics of clay and ceramics vary depending on the type of clay used – from deep red, porous terracotta to bright white, smooth porcelain. The way that clay looks, and acts, can be altered by adding different materials and minerals to the clay body and by using different glazes and finishes.

Ceramics have been used for centuries, and fragments of ceramic vessels and objects can outlast the maker significantly. These fragments of ceramics, dug up from the earth or washed up on the beach, play an important role in forming our view and understanding of the social history of the world and give us an understanding of how people lived. From the fragments of Roman pottery found buried deep in the dirt, to the shards of stoneware jars washed up on Scottish beaches, each one has a history to tell.

Location and Sources

Clay is found globally, with different regions having distinct varieties of clay with different qualities. In the UK, the majority of clay is found and processed in the south of England. The soil in this area is rich in kaolin and ball clay, and towns such as Stoke-on-Trent have historically been the centre of the UK ceramic industry.

Clay is less common in Scotland, and what can be found is less likely to be suitable for pottery use than that found in the south of England. The type of clays most commonly found in Scotland are “brick clay” and “fire clay”.

Brick clay was used to create bricks and tiles for architectural purposes, while fire clay used to make bricks, tiles, and pipes for items where extremely high temperatures would be present like kilns, chimneys, and steam boilers. Fire clay is found deep underground, often close to coal mines and contains high percentages of alumina and silica, which gives it a very high melting point and allows it to withstand extremely high temperatures once fired.

Application and product output

Clay is widely used both in industry and on a small scale by individual makers and artists. It’s a material that most of us use in multiple forms every day and many of our common household objects are made from clay – from the toilets and sinks in our bathrooms, to the plates and mugs in our kitchens!

Clay from different locations has different qualities, making it suitable for different uses. The majority of the clay in the UK is found in the south of England, where there are rich deposits of kaolin (china clay) and ball clay. Kaolin is found only in Cornwall and Devon, and ball clay is found only in Devon. Both of these clays are fundamental to the composition of clays that are widely used to produce pottery and ceramics. Kaolin clay is also used in the production of paper – which accounts for around 70% of the total sales of kaolin. It is used in paper production as a filler between the paper fibres to improve printing quality, and as a coating to enhance the surface properties of paper.

There are 3 main classes of clay used to produce pottery – earthenware, stoneware, and porcelain. Each of these classes have different varieties of clay within them which have their own characteristics and uses, but can be classified into one of these based on their firing range and composition.

Earthenware

Firing range: 950℃ to 1100℃ 

Composition: high percentage of ball clay, with smaller proportions of kaolin and feldspar

Characteristics before firing: Highly plastic and typically doesn’t shrink, sag or warp excessively.

Characteristics after firing: Earthenware is softer than other bodies which means it rarely becomes fully vitrified. This results in ceramics that are porous and absorb liquids, and that are less durable than stoneware or porcelain.

Stoneware

Firing range: 1100 to 1300℃,

Composition: roughly equal percentages of kaolin, ball clay and feldspar

Characteristics before firing: Fairly plastic and easily worked – other clays may be added to modify it such as ball clay for added plasticity

Characteristics after firing: Stoneware is hard, non-absorbent, and extremely durable when fully vitrified. This makes it ideal for domestic pottery and ceramics. 

Porcelain

Firing range: most porcelain has a firing range of 1300℃ to 1346℃, but some porcelains including Parian porcelain are fired significantly lower at around 1240℃

Composition: high percentage of kaolin, with smaller proportions of feldspar (does not contain ball clay)

Characteristics before firing: Often considered the most difficult clay to work with as they have low levels of plasticity, and work is prone to warping during drying and firing.

Characteristics after firing: Porcelain is hard, non-absorbent, and extremely durable when fully vitrified. It’s most distinctive characteristic, which sets it apart from other clay varieties, is the very white colour and a delicate translucency when thin walled.

The British ceramic industry has historically been centred around Stoke-on-Trent in England for centuries, where there were large reserves of earthenware clay and good links by canal to Cornwall and Devon, and many large-scale industrial ceramic manufacturers still operate factories there. This includes world-renowned potteries like Wedgewood and Spode. There are also a large number of individuals and small potteries, ceramic artists, and makers across the UK who use clay which is predominantly produced in Stoke-on-Trent. In Dundee this includes ceramicists Lauren Swan, Steph Liddle, Jen Collins, and Jill Skulina – with many more in the wider Tayside region.

(From left to right) Photo and copyright: Lauren Swan | www.laurenswanceramics.com;and  Steph Liddle | www.stephliddle.com
(From left to right) Photo and copyright: Jen Collins | http://www.jen-collins.com ; and Jill Skulina | jillskulina.wixsite.com/jillskulina

There are also a small number of ceramic artists across Scotland who are interested in sourcing and working with locally dug clays, which they process themselves in small batches. This includes Butter Wynd Pottery in St Andrews – a husband and wife team who dig clay from a farm near St Andrews. They process this clay themselves in small batches and produce a range of functional illustrated homewares. In 2012 they undertook a project with Fife Contemporary Art & Craft called ‘Made From Fife’ which saw them dig and process local clay from a number of sites across Fife and produce a small run of one-off plates using the clay they found.

“Made From Fife” – finished plates | Photo and copyright: Butter Wynd Pottery | http://madefromfife.blogspot.com

Production and sustainable consumption

The UK is a world leading producer and exporter of kaolin and ball clay. Both minerals have a very limited geographical occurrence – kaolin is found only in Cornwall and Devon, and ball clay is found only in Devon – and their importance has been recognised by the establishment of Mineral Consultation Areas to ensure that they are not unnecessarily sterilised by other forms of development.

Clay, including kaolin and ball clay, is predominantly extracted from the ground using open cast mining methods, although some clays require underground mining to access them. These clay quarries are known as “clay pits”.

Ball clay is extracted from the ground using an open pit method. Underground mining was also previously used, but ceased in the UK in 1999. Excavators are used to selectively remove individual grades of clay from the ground that are then moved to storage and blending facilities using dump trucks. Ball clay undergoes limited processing once it has been extracted – around 70% is size reduced and sold in this form, while some is dried and milled. Once it leaves the site of the clay pit, it will be sold to clay manufacturers who blend the ball clay with other minerals to create workable earthenware and stoneware clay bodies to be sold for ceramic production. It can also be bought in its powdered form from many pottery suppliers to be used in blending clay to your own recipe. An estimated 850,000 tonnes of ball clay was produced in the UK in 2017.

Kaolin clay requires significantly more processing before it is ready for use. It is also mined using an open pit method, but has to be released from the rocks that it is within. This is done using powerful water jets which creates a slurry that contains kaolin mixed with other minerals including quartz, mica, and feldspar. The kaolin is filtered out through a series of steps where the water and other minerals are removed until only the clay is left. This is then dried and sold to be used in the production of clay earthenware, stoneware, and porcelain clay bodies for use in the pottery and ceramic industry, as well as being used in the production of paper. An estimated 970,000 tonnes of kaolin was produced in the UK in 2017, 90% of which was exported.

Brick clay is usually extracted in a similar way to ball clay, with the exception of fire clay which is found much deeper underground, usually close to coal mines. These clay are then processed into a workable material through a variety of steps including grinding and mixing with water to create a plastic clay which can then be used to produce bricks. Approximately 5 million tonnes of clay were extracted in the UK for brick manufacture in 2014.

Clay is a significant resource in the UK and it is often considered to be a sustainable material due to its longevity and durability, especially in comparison to single use plastics, but it’s important to remember that it’s not a renewable material and our resources are finite. 

Although clay isn’t renewable, recycling clay can reduce waste and make sure that the most is made of this valuable resource. Before firing, clay can be recycled to be reused both in small-scale pottery and ceramic studios, and on an industrial scale. Once clay has dried it can be returned to its original, workable state by drying, crushing, and wetting to produce both clay slip and clay bodies that can be used to create new ceramic wares. 

If looked after well, ceramic objects have the ability to last and remain in use for a very significant length of time. Ceramics which develop hairline cracks can be repurposed for other uses (decorative, or used as plant pots instead of mugs for example). In Japan, kintsugi is the art of repairing broken pottery by mending areas of breakage with lacquer dusted with powdered gold, silver, or platinum. Rather than hiding the breakage, it embraces it as part of the history of the object and allows it to go on to live another life. In Britain, it wasn’t unusual for ceramic items to be repaired using metal “staples” before the invention of epoxy resins. Though this method of repairing with metal staples, or rivets, may look crude to modern eyes, it ensured that mended crockery could withstand continuous use.

Supported by the Connections Through Culture programme of the British Council, our Materials Library Expansion Project is the first collaboration between UNESCO Creative Cities of Design #Cebu and #Dundee.  #MATIC #CreativeDundee #BritishCouncilPh #BritishCouncilCTC

Works Cited

“Guide to Kiln Temperature Ranges for Pottery”. Soul Ceramics, https://www.soulceramics.com/pages/guide-to-kiln-temperature-ranges-for-pottery. Accessed 10th February 2021.

Mather, John. “Working With Clay”. The Geological Society, https://www.geolsoc.org.uk/Geoscientist/Archive/June-2015/Working-with-clay. Accessed 10th February 2021.

Cranston, Mark. “North East Scotland – Brick Clay Deposits”. Scottish Brick History, https://www.scottishbrickhistory.co.uk/north-east-scotland-brick-clay-deposits/. Accessed 10th February 2021.

“Clay Getting”. Butter Wynd Pottery, https://butterwyndpottery.co.uk/clay-getting/. Accessed 11th February 2021.

“Made From Fife”. Made From Fife, http://madefromfife.blogspot.com. Accessed 11th February 2021.

“Birkhill Fire Clay Mine: A Brief History. Bo’ness and Kinneil Railway, https://www.srpsbirkhillgroup.org.uk/?page_id=202. Accessed 10th February 2021.

“What is Ball Clay?”. Kaolin and Ball Clay Association, https://kabca.org/what-is-ball-clay.php. Accessed 11th February 2021.

“What is Kaolin?”. Kaolin and Ball Clay Association, https://kabca.org/what-is-kaolin.php. Accessed 11th February 2021.

“Mining and Quarrying in the UK”. Mining and Quarrying in the UK, Department for Business, Energy, and Industrial Strategy, 20th December 2019. https://www.gov.uk/government/publications/extractive-industries-transparency-initiative-payments-report-2018/mining-and-quarrying-in-the-uk. Accessed 11th February 2021.

Pilcher, Mark. “A whiter smile, a greener environment: Cleaning up with China clay”. Environment Agency Blog, https://environmentagency.blog.gov.uk/2014/04/25/a-whiter-smile-a-greener-environment-cleaning-up-with-china-clay/. Accessed 11th February 2021.

“The Riveting Art of Repairing Broken China”. National Trust, https://www.nationaltrust.org.uk/features/saving-the-family-china. Accessed 11th February 2021.

“The UK Clay Brick Making Process”. Brick Development Association, https://www.brick.org.uk/admin/resources/g-the-uk-clay-brickmaking-process.pdf. Accessed 11th February 2021.

“The Clay Life Cycle”. Tile and Bricks of Europe, https://maticph.files.wordpress.com/2021/03/8575b-full-claylifecycle.pdf. Accessed 11th February 2021.

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