070309.Limpley Stoke, Base Bed, Bath Stone and BeyondMarch 9, 2007 at 12:04 AM | Posted in Chisel Marks, Conservation, Limpley Stoke, Wiltshire | 10 Comments
Once, there were fifty mines around eighteenth-century Bath but today there are only three. The mine operated for a time, however, during the Second World War, the Ministry of War took it over and installed the supporting brick piers, which are unnecessary since the mine leaves 20% of the quarry stone as interspersed piers.
(below left: near the entrance of the mine is the only clamp. It looks rusted and unsafe, and is certainly useless today. below right: brick pier, c. 1940s)
Bath buildings from the medieval period until today (with the exception of the 1970s, which primarily used a reconstituted stone block, ie: concrete,) have used this Bath oolitic limestone. Apart from Bath, at least eight colleges in Oxford were built from stone quarried here.The mine was reopened in 1982 and hoped to have 30% of the market but now has 85-90% of the Bath stone market. This is because the quarry stone is highly consistent, thus good for general marketing and architect’s specification.
Recently the stone has been supplied to all over the country and the United States, including work on the cathedral in Dallas, Texas, the restoration of St. Patrick’s R.C. Cathedral in New York City, Buckingham Palace, Hampton Court, and Windsor Castle. The queen’s millennium fountain at one of her estates was constructed from the base bed, and the Southgate Developers are seeking to use this quarry for the new section/development in Bath that I haven’t yet posted on but is now thoroughly razed and into phase one.
The stone used to be hauled down the hill and taken onto canal boats along the Kennet and Avon, and now it is taken on the A4 highway that runs parallel to the canal.
There are two beds: the top, which contains a gray-shale content, and the base bed, which was formed with shell bits. Prior to 1782, all Bath stone quarried was from this top bed, which makes matching stone for historical building conservation quite easy since it’s all from one source, which is still plentiful and consistent. Further, this stone has stood the test of time locally, proving it’s good for conservation and greenfield development.
Forest Marble is the finest stone. It’s somewhat rare and the best (or most common) forest marble in the area typically came from Claverton Down hill (where the University is now). This “marble” is very very strong and holds its own against the weather so it’s used for cornice areas or parts on buildings that generally are very susceptible to the weather. Cornice corners have to be natural bedded but this becomes vulnerable at the corners (if people don’t understand stone natural bedding, I’ll post on it when I get a good photo). The top bed is weaker but is generally used for ashlars and areas in buildings not generally expected to be weathered. The base bed is stronger than the top bed.
(above:) Old rusty “Raza” Saw with top bed indicator. The rusty hooks hanging from the ceiling are the old 18th-19th century hooks that were used to hall stone out. (below left-right: modern saw (connected to first picture’s machine) and modern sawed stones.)
The Layers (don’t quote me on anything because my notes are scattered since I wrote them in the shivering in the dark):
-DRY CLAY CAP- (keeps mine dry, except for where it’s broken)
-FOREST MARBLE CEILING-
-1 metre BRASH(?) LAYER- (top bed) [GREEN ARROWS]
-2.5 metre OOLITIC LIMESTONE- (base bed) [YELLOW AND BLUE Xs]
-THIN CLAY VEIN-
-[indecipherable layer] more oolitic limestone- (base bed)
When the mine was reopened in 1982, the stone that had been extracted along the existing tunnels was the top and 2.5 metre base bed. I don’t know the exact height but it’s very high for a quarry and very consistent. The stone would be sawed to clear the top, sides and eventually back face (with an earlier version of a raza (sp?) saw). The base of the block would be chisel pegged out (although the clay vein made this easy. (And by “easy” I mean I have no idea what I’m talking about. It was “tough” for me to go into the mine up to my knees in freezing water to take photos…so take “easy” with a grain of salt.)
The first steps in 1982 to get the stone on the market again to prove the quarry quality was excavating below the then base level of the thin clay vein along the existing tunnels. They did this by ‘stitch drilling,’ which as the quarryman explained to us was not easy at all. You have to drill perfectly straight holes in a line along the base stone, which is harder than the top bed, to create a natural fault line. After each drilled hole you have to begin to pump the stone and quickly drill the next hole on the line. There’s no room for error and plenty of opportunities.
This requires a lot of work and is not a great idea financially but (apart from the drill bits that will likely get destroyed if you don’t drill a perfectly straight hole) it requires little significant investment in machinery. This initiative got the stone on the market, which raised interest and capital for the quarry project, which in turn was then invested in new modern technology for the mine.
The quarry mine was the first one in Europe to employ a cable method of supporting the quarry ceiling. Basically, steel cables drilled into the top bed suspend the quarry ceiling allowing more area to saw out stone. Each cable has devices next to it that monitor movement, so its…safe. (below: cables, etc. above “crocodile lake,” which is permanently flooded due to a fault line that splits the dry clay cap.)
(above:) My sneakers have yet to dry and “[high-pitched cough] … I think I’m getting the Black Lung, Pop. It’s not very well ventilated down there.”