Date: Thu, 9 Dec 2010
From: Michael Kiernan
Re: [CORNISH] Chimney construction
An interesting subject. By chimneys I assume you mean 'stacks' associated with engine houses. (never called chimneys in Cornwall). No definite rules seem to apply as each of the thousands built over the years was individual. The vast majority were round - but the very occassional one was square. I've only seen round ones in Wales which were associated with Cornish mines (particulary in mid-Wales and the north such as Parys Mountain on Anglesey), but I bet someone will know of a square one.
No difference between the type of mineral being mined. But always remember that coal mining is rather different from hard rock mining and the former not often associated (if atall) with the Cornish method of mining.
In the Linares district of Spain will be found what is perhaps the largest grouping of surviving Cornish engine houses with stacks (outside of Cornwall) - all these are round except one which is square but that was associated with producing lead shot. In a similar way the one surviving stack at Pontgibaud in France is square but that is really not an engine stack but rather a chimney to take away fumes from a lead smelter. On the other hand that firms lead shot tower (located miles away at Coueron on the Loire) is round- 70 metres high and a national monument of France.
The stack is most often attached to a corner of the engine house, but can sometimes be a little distance away, depending on the location of the house. Apart from the need to get rid of the obnoxious fumes the stack had another more important function. The coal used in the boilers to produce the steam often utilised poor quality steaming coal (it was not often a mine was lucky enough to be situated near a source of fuel - there was no coal in Cornwall). There was therefore a need to produce a good draft to ensure the coal burnt at a good temperature.
Another point about the construction. Everyone notices that usually about two thirds of the height is constructed from granite and the top bit of brick. Granite was cheap and 'andy and so an obvious candidate, The chimneys taper so at the bottom are pretty thick and thinner at the top. Granite is very difficult to dress to a small size and so the miner was forced to use expensive but thinner brick towards the top. I'm not sure about some being short & squat - perhaps the bricks have fallen away?
The classic configuration of engine houses and their stacks is three in a group. One for each of the main purposes of the engine - Pumping, Hauling and Stamping. Wheal Peevor near Redruth and a field away from where I live has such a surviving configuration and is now a World Heritage site.
Sometimes, one stack could serve a group of boilers (see the stack at Pool between Redruth and Camborne and walk through the tunnel glancing up the inside of the stack - careful of pigeons that may be roosting inside - this, I think, served five boilers)
A very few calcinators survive. These were not really engine houses as there was no engine inside. Instead it contained a great metal table which slowly revolved. Tin stuff was shovelled on this and the fire underneath heated up the ore; the trapped arsenic turned to gas and was passed through a labyrinth and precipitated back to pure arsenic powder. The remaining fumes were exhausted through a stack, often a little distance away on a hill - the fumes were obnoxious and could kill cattle etc. in nearby fields.
I suppose technically the engine houses are not really houses but rather giant engine frames. Whatever, this iconic structure really represents Cornwall. Great pride was taken in their construction which explains why some call them 'Cornish Cathedrals'.
South Polgooth Mine, Engine House Wall & Stack - once part of the largest tin mine in the world
Example of a 'square' stack, taken from "The Book of Carbis",
kindly donated by John Carbis,Dec 11, 2010
(variation in colours are from the original)