Bath Stone is a golden coloured, granular limestone deposited during the Jurassic era as part of the Great Oolitic period (about 195,000,000 to 135,000,000 years ago).
The grains are fragments of calcium carbonate which became coated with lime as they rolled about on the sea bed. When the grains are magnified small shell or rock fragments can be found at their centres.
At the time it was formed the great continent of Pangea was breaking up and Britain was situated roughly where the Sahara Desert is today, but as it moved northwards the Jurassic seas produced a variety of limestones, sandstones and clays.
Over a period of time the Jurassic rocks in Britain have been tilted into a gentle southeastward slope. These rocks make really good building stone and they have been worked at a number of places across Britain (yellow area on map)- ranging from Purbeck and Portland in the south through Bath and Box, to Northamptonshire, Lincolnshire, Collyweston and the North Yorkshire Moors.
As Bath stone is a sedimentary rock it has a natural layering or bedding and due to the gentle nature of its tilt very little vertical breaks or joints.
Bath Stone is classed as a freestone - this means that it can be sawn or squared up in any direction, independently of its joints. However for it to wear well when used for building work the stone must be laid on its bed - the same way up as it was underground.
When the rock is removed from underground it is important to stack it on the surface the correct way up. Newly mined Green blocks have to weather on the surface to allow the natural moisture in the rock (known as Quarry Sap) to dry out and the blocks to harden.
The Romans were prolific users of Bath stone in their buildings, evidence for its use has been found in several villas plus the main Roman baths in Bath itself. It is thought that the main Roman quarrys were to the south of Bath and around Box in Wiltshire.
Prior to World War II the mines were largely hand worked, machine working only coming in due to the labour shortage after the war. The basic hand working technique involved:
- A breach (cut) would be made at
the top of the stone, under the roof, to a depth of approximately 5 feet
- using a series of longer hand picks.
- Sawyers, using saws upto 6 feet long, then cut out the first stone
known as the wrist stone
- After the wrist stone was removed the sawyer would then cut down
the back of the stone - the back cut
- The blocks would be 'broken' from the bottom bed by wedges
Underground workings were mined on the pillar and stall system. When exploring abandoned mines today evidence of hand working can often be seen on the pillars which tend to taper inwards towards their base - due to the miners removing larger blocks of stone from the lower beds.
The Mines Today
Many of the mines are now abandoned, some are still in use by the military as stores (numerous legends abound of secret underground installations!), the major mine complex at Combe Down is now almost completely filled in, but a couple of mines are still working, although today modern machinery is used.
Instead of making the cuts by hand, the modern stone miner uses a special chain saw (see picture, left - the cutting arm is near the floor).
The machine has a hyraulic arm which can swivel to allow both vertical and horizontal cuts to be made.
The saw's chain carries tungston tipped teeth (right) the angle of which is designed to throw rock dust away from the cut. One of the biggest problems encountered with mechanical cutters is the clogging of the teeth.
Left: Stone miner at the face in Stoke Hill Mine.
Right: View of a typical modern mine stall with the breach visible at the top of the face and the vertical cuts made. The next step would be to break out the top centre stones.
The Mines Today
Report & pictures: Kelvin Lake