clay occurs in the deposits in the form of china clay rock, a mixture
of up to 15 per cent china clay and up to 10 per cent mica, and the
remainder being quartz.
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of the purest of the clays, composed chiefly of the mineral kaolinite
usually formed when granite is changed by hydrothermal metamorphism.
Usage of the terms china clay and kaolin is not well defined;
sometimes they are used synonymously for a group of similar clays, and
sometimes kaolin refers to those obtained in the United States and
china clay to those that are imported. Some authorities term as china
clays the more plastic of the kaolins. China clays have long been used
in the ceramic industry, especially in fine porcelains, because they
can be easily molded, have a fine texture, and are white when fired.
France's clays are made into the famous Sèvres (see A.
Brongniart ) and Limoges potteries. These clays are also used as a
filler in making paper. In the United States, deposits are found
primarily in Georgia, North Carolina, and Pennsylvania; china clay is
also mined in England (Cornwall) and France.
The formation of granite
At one time, most of what is now the British Isles was covered by
sea, upon the bed of which vast deposits of mud and silt collected.
In time, the accumulated weight of the sediments caused them to
become compressed and hardened, converting them from loose sediments
into highly cleavable forms of rock - the slates and shales which
still abound on the costs of Cornwall and Devon to this day.
At the time when the sediments were being laid down, great landmasses
were drifting towards each other on a collision course. The pressures
generated by this movement caused the seabed to fracture and fold. The
folding process was accompanied by intense subterranean activity, the
masses of molten rock forced their way upwards.
When the rock cooled, it became granite, a rock made of a mixture of
quartz, feldspar and mica. The formation of the granite took place
between 290 and 270 million years ago and today it forms the rocky
backbone of the south-west of England, being exposed in the Scilly
Isles, Lands End, Carnmenelis, Hensbarrow near St Austell, Bodmin Moor
The formation of china clay
Granite is one of the commonest igneous rocks, but varies
considerably in its composition from place to place. While the quartz
is never anything but quartz, the feldspar can be a silicate of
alumina with potash, soda or lime and the mica the potash-rich
muscovite or the iron-rich biotite.
In some parts of the South West, the feldspar in the granite is
higher in its soda content than its potash content and these places
are where china clay is found today. It came into being through a
complex sequence of events. While the molten rock was still cooling,
it was attacked successively by steam, boron, fluorine and tin vapour,
these acting on the alkali content of the feldspar and converting it
into china clay.
The South Western granite has been converted into china clay only in
those areas where the feldspar contained the necessary high soda
content. Though no china clay is to be found in the Scilly Isles, it
occurs in some places in the Lands End peninsula, at Tregonning Hill,
near Helston, at Carnmenellis, on the St Austell moors around
Hensbarrow Beacon, on Bodmin Moor and on parts of Dartmoor. The
greatest amount of china clay is found on the St Austell moors.The
china clay deposits are roughly funnel-shaped, with the widest part
uppermost, and the base of these funnels can be as much as 300 metres
deep. In certain locations, the deposits occur in clusters, and here
one finds the largest of the china clay workings, including Blackpool,
Goonbarrow, Littlejohns and Melbur in the St Austell area, Stannon on
Bodmin Moor and Lee Moor on Dartmoor. Some of the deposits cover many
hectares at the surface, while the deepest of the clay pits currently
being worked is some 130 metres deep. To work any pit beyond this
depth would mean that the pit would have to be enlarged laterally, as
well as in depth.
China clay occurs in the deposits in the form of china clay rock, a
mixture of up to 15 per cent china clay and up to 10 per cent mica,
and the remainder being quartz. The clay itself varies considerably
from pit to pit. In some deposits, such as those at Blackpool,
Littlejohns and Goonbarrow, the clay is ideally suited for the filling
and coating of paper, while in other deposits, such as Treviscoe and
Wheal Remfry, the clay is best suited for the making of ceramics.
The china clay deposits of Cornwall and Devon are known as primary
deposits, because the clay is found at the site where it was formed.
Deposits of primary clay are found in Czechoslovakia, France, Germany,
Spain, parts of the USA and in the USSR, but in all these the
feldspars in the granite have been broken down by weathering, and the
deposits of clay are strictly limited to the depth to which the
weathering has penetrated: because of this, the clay reserves are much
smaller than in Cornwall and Devon.
In Czechoslovakia in particular, where the china clay industry has
been in existence for some two centuries, many of the largest deposits
have become completely worked out and only the discovery of fresh
deposits has enabled the Czech clay industry to continue.
In some countries, including Greece, Hungary, Iceland, Italy, Mexico
and Turkey, there are somewhat different deposits of clay, which are
also primary in nature. Here, however, the rocks that have undergone
alteration were of volcanic origin, and the alternative media have
been sulphurous steam and fluids.
Consequently, the clay in these deposits contains minerals, which
make it unacceptable for use in either paper or ceramics.
Some china clay deposits are found a long way from the site of their
formation, having been formed originally by weathering, and then
having been washed away and transported by rivers to the places where
they have eventually become deposited. These deposits are known as
secondary deposits, and the largest of these in the world are found in
the USA, in Georgia, South Carolina and Alabama. The clay occurs in
beds up to 20 metres thick and is suitable for use after processing in
paper, but not in ceramics, because of its high titanium content.
Imerys is a major producer of china clay in Georgia.
Development of the China Clay Industry
The early history of the industry is, as would be expected, very much
concerned with the discovery and production of china clays for use in
ceramics. The story, though, starts thousands of years ago and
thousands of miles away.
China, the pure white porcelain used by the Chinese, was discovered
many thousands of years ago and has always been a much-prized
material. Despite many attempts to find sources elsewhere, it remained
elusive until a few deposits were found in some parts of Europe and in
America early in the eighteenth century. The search to find deposits
in England was increased.
When china clay, or kaolin, was discovered in England, it was
realised that it was of a much finer quality than found elsewhere in
Europe. A Quaker apothecary-cum-potter, William Cookworthy made the
discovery in Cornwall in 1746. He experimented with various samples
and in 1768 he took out a patent to use the material, soon producing
items at his Plymouth Porcelain Factory. Until that time English
pottery had consisted of coarse earthenware and stoneware ceramics and
had suffered considerable competition from elsewhere.
As more potteries made use of porcelain, so the demand grew and by
the early nineteenth century the kaolin industry had become highly
successful, with many of the Potters owning rights to mine the
material for themselves. In addition, by the middle of the nineteenth
century, china clay was increasingly being used as a raw material by
the developing paper industry.
Early in the twentieth century, the industry was made up of some
seventy or so individual producers, each competing on price with
little regard for marketing or standards. There was almost no capital
investment or product development and over-production was great, wages
were low and working conditions were poor.
Despite this, by 1910, production was approaching a million tons a
year and paper had completely overtaken ceramics as the prime user of
china clay. Over 75% of output was exported, with North America and
Europe being major markets. The china clay industry in Cornwall and
Devon held a virtual monopoly on the supply of that mineral to the
world market. Just after the First World War, the three leading
producers joined forces - forming English China Clays Limited in 1919,
placing almost 50% of the industry's capacity under one banner.
China Clay Production
China clay mining is a complicated process which can be split into
three distinct sections - opencast mining, refining and drying
Known as pit operations, this process firstly requires the removal of
ground overlying the clay. This is known as overburden, which can vary
in depth between one and fifteen metres. Once the clay is exposed, the
method of mining is best described as a hydraulic mining process. This
means, quite simply, that a jet of water under enormous pressure is
fired at the pit face from a water cannon known in the industry as a
monitor. This liberates from the quarry face the china clay, together
with sand and mica.
The material runs in slurry form to the lowest part of the pit, known
as the sink, where it is lifted by centrifugal pumps to mechanical
sand classifiers, where the more coarse sand elements are removed. The
sand is disposed of on belt conveyors, which deposit it on low profile
tips that can then be landscaped and seeded with grass.
Having removed the coarsest of the sand, the clay suspension is then
transported by underground pipeline to the second process: refining.
Refining consists of a series of mineral processing techniques -
predominantly sedimentation - designed to remove the smaller sized
waste particles, mainly minerals such as very fine quartz, mica and
feldspar, leaving only the required china clay behind.
Value can also be added to the clay at this stage by a variety of
processes designed to engineer size and shape of the product, together
with the use of a chemical bleaching process that improves its
Having refined the clay, it is moved on to the final process: drying.
Drying consists of firstly converting the liquid clay into a solid
material by a process known as filtration. The products of filtration
normally have a moisture content of about 25%.
Passing the clay through a thermal drier further reduces this
moisture content. The driers are fired by natural gas and see a
product emerging at around 10% moisture. The product is normally sold
in pelletised form - particle size ranges from 6-12mm.
Despite its extensive china clay deposits in Cornwall and Devon,
Imerys continually researches new methods of extending the life of its
reserves. For example, new refining techniques have been developed to
obtain more china clay per matrix and sophisticated technical
innovations provide the ability to adjust particle size. Consequently,
the company is confident that its reserves are sufficient to last for
China Clay Industry Statistics
Clay bearing ground in mid-Cornwall covers approximately 25 square
Nine tonnes of waste rock and debris are produced for every one tonne
of china clay.
Imerys operates 17 pits in the UK. 14 of these are situated in
Cornwall; 12 concentrated in the St Austell area. The three pits
outside of Cornwall are situated on the south-western side of
Dartmoor, an area known as Lee Moor.
As well as Imerys, there are two independent producers: Goonvean in
Cornwall and Watts Blake Bearne in Devon. Goonvean operates five pits
and WBB two pits.
87% of the china clay produced in Cornwall is exported and some 60
grades are produced. The major markets are Western Europe, with the
Nordic countries being the most important.
Clay for export is normally despatched through the local port of Par,
taking 30% of output, and the deep-water port of Fowey, taking 70%.
Clay into the UK market is distributed equally between road and rail.
Imerys employs over 2,500 people directly involved in the mining and
distribution of china clay to the ports. The value to the local
economy is immense, with over £120 million going directly into
the Cornish economy. About half this figure is paid in wages, with the
remainder being used to buy goods and services.
Uses of China Clay
List of uses in "The Hensbarrow Granite District", the
first detailed account of the china clay industry, written by the
distinguished Cornish geologist J. F. Collins, F.G.S. in 1878:
The first use to which china clay was applied in the West of England
was, as already stated, the manufacture of porcelain and this is still
popularly believed to be its sole use. This is, however, by no means
the case, probably not more than one-third of the clay now being
produced being so used.
Large quantities are used for "bleaching", i.e filling for
the spaces between the threads of calicoes and other cotton goods and
also as a material for giving weight to cotton yarn; and still larger
quantities are used by papermakers to give "body" to their
papers, especially to printing papers.
The manufacture of alum, sulphate of alumina and ultramarine uses up
large quantities annually, besides which small quantities are used by
manufacturing and colour makers, photographers and others. It has also
been used, not avowedly however, as a "body" in the
composition of some artificial manures, the utilisation of sewage and
even in the adulteration of flour.
Should the present low prices be maintained its use will no doubt be
still more largely extended in directions as yet unsuspected.
Early 20th Century
J. Allen Howe's "Handbook of kaolin, china clay and china stone"
was published in 1914 and listed the following uses:
China clay in ceramic manufacture, porcelain; bone china;
earthenware; white tile body; graniteware; white stoneware; glazes.
Paper manufacturing (note: some 60% of output was being used by the
paper industry by the early 20th Century)
Miscellaneous, paint; plaster; alum; ultramarine; whitewash; drugs;
soap and face powder; wine clarification; polishing powders; lead
Mid 20th century
List of uses published in the "Technical Salesman's Handbook"
issued by E.C.L.P. & Co. Ltd in 1954:
Paper and board, coating clays; fillers; brush coating clays; machine
Ceramics (excluding refractories), bone china; hard porcelain
(including tiles, tableware, sanitary and electrical porcelain); fine
earthenware; earthenware (lower grade); earthenware (sanitary); porous
wall tiles; electrical porcelain (England); semi-vitreous china
(American); semi-vitreous porcelain (American); hotel china
(American); household china (American); belleek (American); glazes
(mill additions and frit additions); porcelain enamels; radiants.
Paint and distemper
Cosmetics, insecticides, dusting and medicinal
Miscellaneous, soap; cleaners; inks; fuses; pencils; grinding wheels;
welding rods; boiler coverings; asbestos; dyes; accumulator cases;
Paper, Kaolin performs two quite separate functions in papermaking.
As a filler or loading, it is incorporated within the body of the
paper, both reducing its overall cost and improving its printing
properties. It is also a coating pigment, enhancing the surface
properties of the paper, such as brightness, smoothness and gloss,
thus allowing the accurate reproduction of colour printing.
Type of paper
Board Printing and writing paper
Newsprint Typical kaolin content (%)
up to 10
up to 30
up to 8
Ceramics, Kaolin was originally used in the manufacture of whiteware
ceramics. The amount of kaolin used for this purpose is now greatly
exceeded by that used in the paper industry. The ceramic and
refractories industries remain a major market, accounting for nearly
one million tonnes of kaolin in Western Europe.
The major markets for English kaolins in the whiteware ceramic
industry are tableware, vitreous-china sanitaryware, wall tiles (in
the UK), electrical porcelain and glazes. In addition, kaolin and
calcined clay are used for refractory applications.
Paint applications: Water based decorative paints: gloss; semi-gloss
and silk; exterior matt - smooth and textured; interior matt; primers.
Solvent based decorative paints: gloss; semi-gloss and eggshell;
matt; undercoats; primers. Protective and OEM coatings: metal primers;
furniture lacquers; domestic appliance finishes; electrophoresis
coatings; coil coatings; traffic markings; printing inks.
Rubber applications: Kaolin is incorporated into both natural and
synthetic rubber compounds and is the rubber industry's most widely
used non-black filler with reinforcing properties:
Cable insulations: cable sheath; hose; extrusions; belting; footware;
pharmaceutical; seals; general mouldings; plant lining; domestic
mouldings; latex; tyres/inner tubes.
Plastics applications: Kaolin has many uses as a filler in plastics:
Speciality films: PE master batch; PE cables; PE film; pPVC cables;
pPVC extrusions; pPVC plastisols; PP mouldings; polyamide mouldings;
PBT/PET mouldings; unsaturated polyester; urea formaldehyde; phenol
Metakaolin for the building and construction industry. A high quality
pozzolanic material which is blended with Portland cement in order to
improve the durability of concrete and mortars.
White cement: In the production of white cement, iron is a
deleterious component which can be avoided by using kaolin, an alumino
silicate with a very low iron content.
Glass fibre: The preferred raw material for introducing alumina to
glass compositions for glass fibre manufacture is kaolin.
Agricultural industries: The caking or setting of granular
fertilisers (prills), is a serious problem since the caked mass must
be broken down into its individual particles before it can once again
become a free-flowing product. Kaolin, some of which is amine-coated,
acts a non-stick coating to the fertiliser prill. As colour is of no
significance, cheaper grades can be used.
Other industries: Pharmaceutical applications; quality leather;
textiles; inks, dyes, adhesives, crayons and pencils; toothpastes and
cosmetic applications; chemicals industry.
Into the future
Some of the uses listed below are already commonplace while others
are innovative and in the development stage.
Plastic film, Video and audio tapes, where clays are used as
Laundry products, Washing powders and detergents.
Decorative concrete, Mortars and renders.
Mark-resistant polypropylene for automotive use Thermoset mouldings
for baths, shower trays, etc.
Lightweight concrete Water treatment systems
Biotechnology, Ability of lightweight high-strength ceramic materials
to support micro-organisms.
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