The History of Ball Clay Production

 

Over the centuries there has been an evolution in the methods used to extract the valuable seams of ball clay. The methods varied slightly between the three areas of production. They are summarised below and then described in more detail.

  • Shallow trenches – from the 1600s
  • Small open pits – the natural development of shallow trenches, which grew in size when pumping techniques improved in the 19th century.
  • Square pits – a development of small open pits in South Devon, enabling clays to be worked at a greater depth
  • Shaft mining (underground) – widely adopted from the second half of the 19th century
  • Inclined shafts (underground) – a variant of shaft mining adopted in North Devon from the end of the 19th century until the 1960s
  • Adit or Inclined tunnel mining (underground) – came into use in the1930s and adopted in place of most shaft mining from the 1950s to the 1990s
  • Large scale opencast working – progressively replaced all underground mining during the second half of the 20th century: the only method after 1999.

The tenant farmers who first found clay under their fields dug it with whatever farm implements came to hand. As time went by special techniques and tools were developed to work the clay. Despite some local variations, they were broadly similar in each of the three production areas.

The basic system was to dig a shallow trench. After removing unwanted overlying material called ‘overburden’ (or ‘head’ or ‘ridding’), the ‘claycutters’ cut the exposed floor of clean plastic clay into a criss-cross pattern of 9 inch (23 cm) squares using heavy iron spades with 4 inch (10 cm) wide blades known as ‘thirting’ (or ‘thwirting’) irons. (These specialised tools are pictured right). Following this, another claycutter used a weighty, ash-handled tool like a wide-bladed pick or mattock called a ‘lumper’ to undercut each square to a depth of 9 inches (23 cm) and lever out the resultant cube of clay weighing about 36lbs (16.3 kilos): 70 balls made a ‘tally’ of 221/2 hundredweight (1.14 tonnes). The claycutters dipped their tools into a bucket of water to lubricate the cutting. A tool called a ‘poge’ – a curved iron spike set into a stout pole – was then used to pitch the cubes up the stepped sides of the pit to the surface and onto a packhorse or cart. A lighter version of the lumper known as a ‘tubil’ or ‘tubal’ was used to trim the working. In this way the whole floor area was removed to reveal the next layer for extraction.

Cross section of clay seams
The tenant farmers who first found clay under their fields dug it with whatever farm implements came to hand. As time went by special techniques and tools were developed to work the clay. Despite some local variations, they were broadly similar in each of the three production areas.

The basic system was to dig a shallow trench. After removing unwanted overlying material called ‘overburden’ (or ‘head’ or ‘ridding’), the ‘claycutters’ cut the exposed floor of clean plastic clay into a criss-cross pattern of 9 inch (23 cm) squares using heavy iron spades with 4 inch (10 cm) wide blades known as ‘thirting’ (or ‘thwirting’) irons. (These specialised tools are pictured below). Following this, another claycutter used a weighty, ash-handled tool like a wide-bladed pick or mattock called a ‘lumper’ to undercut each square to a depth of 9 inches (23 cm) and lever out the resultant cube of clay weighing about 36lbs (16.3 kilos): 70 balls made a ‘tally’ of 221/2 hundredweight (1.14 tonnes). The claycutters dipped their tools into a bucket of water to lubricate the cutting. A tool called a ‘poge’ – a curved iron spike set into a stout pole – was then used to pitch the cubes up the stepped sides of the pit to the surface and onto a packhorse or cart. A lighter version of the lumper known as a ‘tubil’ or ‘tubal’ was used to trim the working. In this way the whole floor area was removed to reveal the next layer for extraction.

Specialised hand tools for ball clay cutting

Small Open Pits

Open pit in the 1930sAs 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.

The Square Pit System

square pit systemThe 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.

The History of Ball Clay Production – continued >