Collyweston mine supplies slate to Cambridge roofing project
Slate taken from a Northhamptonshire mine that only reopened a year ago is being used on a massive roofing project in Cambridge.
Several tons of slate have already been removed from the Collyweston mine since production started in April last year.
Nigel Smith, a director of Claude N Smith, said they had a contract to supply slate to be used on the new roof at Cambridge College.
Although mining has been going on at the site since the 1500s - and possibly even further back to Roman times - Nigel said mining had ceased at the site in the early 1970s because it had stopped being economical to extract the slate.
“Health and safety issues started coming to the fore, there were wage increases and it was inefficient to continue mining,” he said.
A lack of reliability in “frosty mornings”, which became more sporadic also played a role, resulting in difficulties in splitting the stone.
But, more recently, advances made by Sheffield Hallam University and Natural England into developing a method of using modern freezer technology for splitting the slates have kept the industry alive.
The Collyweston mine was given a fresh lease on life after Nigel decided to investigate the viability of opening the site again.
His family has owned the land since the early 1990s and Nigel said a rise in the price of slate and more cost effective mining techniques had made it viable again.
Their minerals extraction application to re-open the mine was approved in 2015 and they later secured a contract to supply slate for repairs currently underway at King’s College.
Slate extracted from the mine is being used for 1,500 square meters of roof at Bodley’s Court, a building used for accommodation at the college.
The building stands on what was once a garden belonging to the Carmelite Friars, which the college bought in about 1542.
Most of Bodley’s Court was completed in 1893 followed by extensions in 1927 and in 1955.
“We’re currently taking about 20 tons of raw slate out a month of which we are able to use about 15 tons of that,” said Nigel.
“It’s all going to King’s College and things have been going really well since we opened a year ago.”
He said a new entrance to the mine and a shaft measuring 80 metres long was first constructed and opened in January last year.
“It took about a year to get it ready and to reach the seam,” said Nigel.
“We have permission to mine for 10 years which should give us around 2,000 tons in total.”
The slate seam, which lies approximately 10 meters underground, varies in thickness from about a meter wide to 700mm in places.
A remote controlled Brokk 100 electric breaker controlled by two operators is used to undermine the slate seam, causing it to drop.
“The work now undertaken by two people and the breaker used to involve 12 people, even more during the winter months,” said Nigel.
The raw slate is then transported out of the mine using a front end loader to the surface where it goes through a freeze/thaw process.
The blocks of slate are placed into water filled containers where they are left for a period of time to saturate.
The water is then removed and the saturated blocks of slate are placed into giant freezers.
Once frozen, the containers are removed and the slate allowed to thaw again before being placed in the freezer again.
This repeated freezing and thawing process splits the slate into sheets.
When ready, the sheets are taken to a workshop where they are “clived” - a process which separates them further into more manageable sheets of varying sizes.
The slates are then sawn square on three sides and finished with a dressing hammer to leave a chipped and more natural looking lower edge.
Finally a single nail hole is made using a power drill.
Slates of the same size are then sorted into containers and made ready for transport.
“The bigger pieces are used on the bottom of the roof and the smaller pieces nearer the top,” said Nigel.
“Everything is worked out between the contractor and us over sizes and quantity that we need to supply.”
Collyweston stone slate is of considerable historical importance as a building material in England.
During the 19th century it was used extensively in the local area and on more prominent buildings around the country. However, with the growth in rail transportation in the late 19th century, the industry fell into decline.
Fortunately, the resilience and long life of the product ensured that it was still in demand by some building owners and the craft has survived over the years.