Raw Materials - The Importance of Clay and its Uses
China Clay
In Cornwall China Clay. or Kaolinite, was formed many millions of years ago in fissures on the granite moors. The main minerals in these fissures are felspar, quartz and mica. The china clay is actually a form of decomposed felspar which has formed a soft white paste. Because it is formed in fissures it has to mined vertically.
In the earliest times of its production clay was removed from the ground with hand tools and taken to an area close to a diverted stream where it was washed so that any coarse particles of sand sank to the bottom of the water. The clay was then placed in settling pits and left to dry in the open air.
Later production became more efficient, speeding up the drying process time. Long buildings with heated floors were built to dry the clay.
Since the 19th century clay has been removed with high pressure hoses from the sides of the pits. Production today is highly mechanised and computer controlled.
It was first discovered by a chemist/potter named William Cookworthy at Tregonning Hill in 1746. Prior to this date only earthenware and stoneware pottery was produced.
The business he founded flourished and other pits were opened.
In 1919 the three major producers amalgamated and became 'English China Clays', this company was acquired by Imetal of France for £756million in 1999.
Clay has created a most important factor in modern industry, post the Industrial Revolution of the early Nineteen Century.
Early building materials
This is far above and beyond the early uses of clay in the building industry, with its uses in producing clay bricks and the mixing of various clays with horse-hair and other fibrous materials to produce wall panel infills of some strength, used in frame house construction from the Middle Ages. These forms of clay were combined with sand and other aggregate materials to form the modern brick-making industry, still fully operational to this day, where moulds are used to form the brick components, which are then fired in kilns to harden them, using middle age technologies, now developed into mass production for the building of larger buildings, where standardisation of the basic brick unit could be reliably reproduced in one of our earliest forms of mass-production.
China Clay again
Coming back to the other prime development of clay which, for my part of the world was probably, aside from early mining of tin and other ores, the start of heavy industry in Cornwall, with the development of what has become known as China Clay. Clays have been used the world over to form utensils and containers for kitchen and cookery purposes in the form of pottery cups, plates and other kitchenware from jugs and bowls to the decorative forms of kitchenware such as condiment sets and other production items. Depending upon the location, different clays would generate different results and qualities of production which individual potters and potteries would develop as their house styles and patterns. Clays were also used as a basis for decoration of pottery and different mixes of clays and other compounds, together with the use of colouring materials introduced an element of design in the form of 'slip'. Coupled with fired glazing techniques, this was the start of a world-wide industry.
With the extraction of fine clays from the granite-laden hills of Cornwall in particular, fine grades of clay, which were filtered and sieved to reduce the particle size within the clays, China clay became a major part of Cornish exports, initially by sea from the ports on the South Coast of the county, and shipped up via the Wirral peninsula, where, by the beginning of the nineteenth century, would then ship by canal lighters to the Staffordshire potteries, where basic pottery was very much their main production.
Special barges were designed to transport the clay around the coast. They, and the men that manned them, were called 'muddies'.
With the introduction of china clay to this sector of the potteries, fine china was introduced by the major potters, such as Royal Worcester, Doultons and Josiah Wedgewood, all becoming household names for quality. Development of the basic china clay with the addition of such items as ground bone offered strength and lightness to their finished product and as a result the reputation of English China became a world standard.
Clay from Cornish pits today is in competition with other major producers around the globe, in particular China, which has a much reduced labour cost when compared even with Cornish wages!
The fine clays now have established firm markets within the world’s paper manufacturing industries, with considerable exports to traditional timber-laden markets, such as Scandinavia and Finland who use fine clays to coat paper under pressure rollers and heat in the coating process to produce high gloss and smooth paper finishes so desired by the world’s printing industries.
Medical uses Kaolin
A further variant of clay has been used in medicine for many years, with the development of kaolin, a very fine clay which is treated and applied in a heated bandage, to respond to heat, for the traditional poulticing of inflammatory infection, such as boils and absesses, where heat completed the treatment cycle of nature's inflammatory processes to dry up the infected tissue and promote healing. Another medical use of kaolin was in the preparation of anti-diarrhoeal preparations, with the the clays being in suspension with morphine to produce the doctors' favourite for such problems, Kaolin et Morph. Commercially this led to international brands, such as Kaomycin, where the product also includes Neomycin antibiotic to support a quick fix for the Mexican Two-Step or Montezuma's Revenge! Other uses for fine clays is in the development of fireclay, with its almost universal adoption for production of sanitary ware for the bathroom trades in various finishes and and qualities, using detailed casting and forming processes, coupled with highly controlled firing procedures
Technological uses Insulation and components
Further uses of fine clays are found in parts of the electronics industries, to create insulation membranes and other structural formers for coil winding and chokes,together with further variants foritems such as types of capacitors, including the chains of ceramic china insulators, used on the national grid pylons for many years, before they were substituted for glass insulators as the high tension voltages carried increased, .
Terracotta
Terracotta is another variety of appropriately refined clay, partially dried and cast, moulded, or hand worked into the desired shape. After further thorough drying it is placed in a kiln, or atop combustible material in a pit, and then fired. After pit firing the hot ware is covered with sand to cool, and after kiln firing the kiln is slowly cooled. When unglazed, the material will not be waterproof, but it is suitable for in-ground use to carry pressurized water (an archaic use), for garden ware, and sculpture or building decoration in tropical environments, and for oil containers, oil lamps, or ovens. Most other uses such as for table ware, sanitary piping, or building decoration in freezing environments require that the material be glazed. Terracotta, if un-cracked, will ring if lightly struck, but not as brightly as will ware fired at higher temperature, forming Stoneware, This fired Terracotta material is weak when compared to stoneware. Some types of terracotta are created from clay that includes recycled fragments of terracotta known as ‘grog’.
The unglazed colour after firing can vary widely, but most common clays contain enough iron to cause an orange, or brownish orange colour. Other colours can include yellow, gray, and pink.
Terracotta has been used throughout history for both sculpture and pottery, as well as for bricks and of course roof tiles or shingles. In ancient times, the first clay sculptures were dried (baked) in the sun after being formed. Later, they were placed in the ashes of open hearths to harden, and finally kilnswere used, similar to those used for pottery today. However only after firing to high temperature would it be classed as ceramic material.
Crude terra-cotta female figurines were originally uncovered by archaeologists in excavations in the Indus Valley region, now forming a part of Pakistan. Significant uses of terracotta have included the Terracotta Army of China. These figures were formed and fired in sections, as may be apparent from the warrior who has 'lost his head' in this close up detail above. The lower picture offers some idea of scale of this major find in 1974.
Pencils
A further variation of of clay used every day by thousands of people (and I include myself) is a pencil. We all know that pencils no longer contain lead but graphite.
Pencils are graded from H to 9B
Graphite was discovered in the 1500s below a tree that been blown down in a storm. It was first used by shepherds to mark their sheep.
Pencils were first successfully developed by a Frenchman, Nicholas-Jasque Conte in 1795 after Faber, in Germany, had failed.
He was the first to mix graphite with clay to produce varying grades of 'lead'. The mixture was formed into sticks and fired, like any other clay product, and sandwiched between wood. His methods are still used today.
The grades range from 7H to 9 or 10B and maybe beyond.
H is very hard and leaves a faint impression because it is mainly graphite. Grade H pencils are used mainly for such things as technical drawing where you need a very fine line. The most commonly used grade is HB a medium blend.
The softer B grades are maninly used for drawing. (Submitted by Duncan Tribute and Kate Western - Please edit or add any saliant information to this entry)
Bricks.
The first recorded bricks were found in Mesopotamia 6000yrs ago. These were formed by hand, sun dried and varied, very much, in size. They were the forerunner of a wide range of structual clay products used today, indeed, the majority of us spend our lives surrounded and sheltered by clay.
Bricks were first introduced to Britain by the Romans although clay had been used as a daub long before their arrival. They were re-introduced by Flemish craftsmen in the Middle Ages. Their use became more widespread in the 13th and 14th centuries due to the increasing shortage of timber. (see 'The use of Wood in Elizabethan Times'.)
By the Tudor period the brickmakers and bricklayers had emerged as seperate craftsmen well able to rival the stone masons.
At that time bricks were generally made on site in wood, heather or turf fired clamps by intinerate workers.Originally they were moulded by hand using wooden moulds and a portable table. This resulted in verying sizes and qualities. Later on pug mills were used, the first being horse-powered. Pug mills meant that the lengthy periods of washing and drying were no longer necessary. Drying chambers were introduced which meant that the bricks could be dried and fired more quickly speeding up the whole process of brick making.
Until the early nineteenth century bricks were still being made by numerous small brickyards supplying local needs but eventually increasing demand led to the invention of brickmaking machines and industrial kilns.
One of these small brickyards was situated in Hoo, Where I live, and not only bricks were made but paving tiles, roofing tiles, chimney pots, flower pots and drainage pipes.
The chemical structure of clay alters when fierce heat is appled and the water which is combined with the clay crystals is driven off at 400c, then at 1000c+ it reaches its melting point. Between these two temperatures there is the sinter point. This is the point where the edges of the clay particles melt and fuse together. Following this is the maturing point, the temperature which produces the densest structure in the clay before melting and causing distortion
If other minerals are mixed with the clay (either naturally or artificially) they will act like cathylysts and lower the temperature at which these reactions take place. Natural clays, therefore, have a wide range of maturing points depending on the minerals they contain and where they are found. This also determines the colour of the finished product. Part of brickmaking was to know the clay and to be able to determine what temperatures to aim at.
Before the National Curriculum, when teachers were allowed to act spontaneously and sieze the interest of their classes, I took a group of thirteen year old pupils down to the River Medway where we gathered some clay. We took it back to school and washed it to remove any decaying debris, dried it, washed and dried it again and so on. We were then able to form a few pots from it that survived the biscuit firing but please see the photo below to see what happened when we tried to add a glaze to them at a much higher temperature! Conclusion, the clay from the Medway was suitable for producing terracotta ware only. The chlildren were amazed that what we fired as grey clay produced red!
Kate Western.
Joan reminded me of the clay pipes we bought as children for blowing bubbles. Above is my small collection of clay pipes. The two long stemmed ones at the top are called 'Churchwardens'. The broken ones have all been picked up by myself or by my children along the river bank here. Rumour has it that lots of ashes from The Great Fire of London were brought down here to reclaim some of the marshland. Some of them do, indeed, look burnt. I think it more likely that they have been thrown overboard by seamen when they have broken. The two at the centre bottom both bear the initials RAOB which I think means Royal Ancient Order of Buffaloes. The shapes of the bowls are many and varied as you can see and all are made from china clay.
Kate Western 2011
Ian Kimber 2011