Small Open Pits
As trenches widened they developed into open pits, which, although
of some size considering the manual labour involved, were small by
the standards of today. These remained the most economical way of
extracting seams of clay that were close to the surface and not overlain
by too much overburden. Neatly terraced slopes were a feature of the
best of these pits. However percolating groundwater and rainwater
tended to cause open pits to flood, and their depth and area were
therefore limited by the capacity of pumps to dispose of the water.
Hand operated elm-barrelled pumps with a maximum lift of 15 feet (4.6
metres) were used originally, and although it was possible to have
a series of pumps each with such a 'lift', it was not until the introduction
of Cornish plunger pumps towards the end of the 19th century that
open pits were developed to any great depth.
An open pit showing cubes of ball clay being
cut vertically using a thirting iron (central figure), undercut using
a lumper (right figure, partly concealed) and loaded into a wagon
using a poge (left figure), c. 1930
The Square Pit System
ground water and rain that tended to cause open pits to flood also
caused their soft sides to subside. To control subsidence timber began
to be used. This developed in South Devon into a system of excavating
a sequence of 'square pits' that were timber lined and braced. (See
cutaway diagram, right, showing timber supports for a square pit.)
After trial and error the optimum size was found to be 18 to 24 feet
(5.5 to 7.3 metres) square. These square pits could then be dug to
a depth of 50 feet (15.2 metres) with a series of pumping 'lifts'
and ladders, but for a long time only the clay within the pit area
was worked. About 12 feet (3.7 metres) of solid ground was left unworked
between them and the waste from one pit was used to backfill another.
To relieve the strenuous labour of manually lifting
the clay and waste to the surface, a wooden crane of a type unique
to the ball clay industry called a 'crab' would be erected beside
the square pit to hoist the clay and waste to the surface in an elm
bucket. The crab was a pivoting 'gallows' type crane held in place
by two legs called 'tie backs'. Hand winches or horse drawn winches
(known as 'whims') were used to raise and lower the buckets.
Each square pit produced a few hundred tonnes
of the several types of clay through which the pit was sunk. Most
were worked for just a few months until incoming water became too
much for the hand pumps. Whilst shallow open pits remained the principal
means of extracting stoneware clays, the square pit system was used
to win the more valuable potters' clays that were typically found
at a greater depth.