A rare coincidence of geological conditions was required to form and preserve the ball clay deposits:
The Wareham Basin
Subsequently the great Alpine mountain building forces lifted up the underlying chalk, ending the formation of the Tertiary deposits and creating the Purbeck Hills that now hems in the Wareham deposit along its southern boundary.
and Petrockstowe Basins
In some situations in the Bovey Basin the swamp vegetation was so abundant that it led to the creation of thick beds of carbonaceous lignite. This 'Bovey Coal' has at times been used as a brown coal fuel and in horticulture.
In the case of the Petrockstowe Basin, (left), a river flowed northward through the basin, depositing mainly sandy and silty sediments in the central trough, while the ball clay seams were laid down as overbank deposits in the quieter shelf areas on the northeastern and southern flanks. Lignites are less common, but not unknown.
As the Sticklepath fault developed over millions
of years, the earliest beds of sediment were buried as further movement
on the fault allowed sediments to form above them, and they gradually
consolidated and formed beds which dipped towards the centre of the
basins. The Bovey Basin is over 4,000 feet (1,300 metres) deep and
about 7 miles long by 5 miles wide at its widest point (11kms by 8kms).
The Petrockstowe Basin is much smaller: about 2,300 feet (700 metres)
deep, only 4 ½ miles (7kms) long and less than one mile (1.5kms)
wide. The full depth of the Petrockstowe Basin has been core drilled,
but only the top 40% of the deepest part of the Bovey Basin has been
explored to date.
Pit near Chudleigh Knighton in 2001. The pit shows a sequence of seams
of light-coloured clays or sands and dark lignitic clays or lignites
deposited in the Bovey Basin.
Sands, Lignites - and
The relatively shallow Wareham deposits are less complex than the others and are largely free of lignite. As a result of erosion, the original deposits, up to about 500 feet (150 metres) deep, are now mostly confined to separate 'lenses' several acres in extent in which the clay is about 15 to 50 feet (5 to 15 metres) thick in, at most, four or five rather thick seams under between 10 and 200 feet (3 to 60 metres) of overburden. From the whole deposit about 26 different clay types are produced. The source rocks have given rise to clay minerals with a very fine particle size - giving high green strength - and a pale ivory to buff colour. These properties are ideal for floor and wall tiles, but the clays are also used as components in most other types of ceramic.
In the Petrockstowe Basin the clay minerals were
derived from a single source, the Culm Shale uplands of mid Devon.
In their resultant high green strength and ivory to buff fired colour
these clays typically resemble Wareham clays - and therefore are ideal
for the production of tiles and large electrical insulators. However,
the Petrockstowe Basin is complex with a large number of clay types,
many of which also include carbon. This gives them particular properties
making them especially valuable as components in other fine ceramic
These important variations in clay mineral, combined with the presence of carbon and quartz in certain seams and the very large number of relatively thin beds, have resulted in the Bovey Basin in a huge number of permutations and more than 250 clay types - the largest range in any ball clay deposit.
Traditionally, Bovey Basin ball clays have been divided into two broad groups. Firstly, whiter firing 'potters'' clays, often containing a significant amount of carbon that enhance their plasticity. These were called 'black', 'dark blue' or 'light blue' clays, depending on the colour when dug: they are sometimes regarded as the 'true' ball clays. Secondly, less white firing and less plastic 'stoneware' clays that contain little or no carbon but an appreciable amount of fine quartz sand (these clays are often referred to as 'brokes'- because the non-sticky quartz causes them to break up readily).
Today, the different types of clay are blended to obtain the properties ideal for each ceramic application. The technical expertise of the ball clay producer is to identify the properties of each clay seam as they change across the deposit, to understand how the clay will behave when formed and fired as part of a ceramic body, and to blend together different seams to produce a large number of consistent products suitable for each individual customer's particular manufacturing process.