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Collyweston slate tradition and technology

COLLYWESTON SLATERS

Collyweston slates have been made for hundreds of years by mining the fissile rock, known as log, and in the winter, when a there was the promise of a cold spell, laying it out in the fields for the frost to split it. During the late 20th century there were less cold winters. Consequently the traditional craft declined and many historic roofs were replaced with fake slates. But there were still plenty of skilled slaters so what was needed was a method of frosting the log without having to rely on a cold winter. This would also make continuous rather than seasonal production possible.

LOG WAS MINED BY HAND FOR HUNDREDS OF YEARS

The slate log lies on a bed of hard sand which was removed with a pick and the log supported on pillars of waste stone. After undermining the log to a depth of twelve or fifteen feet the pillars were knocked out and the log allowed to fall.

In the Winter the log was laid out in the fields and kept wet throughout the days and nights. Once the frost had opened the fissile layers the process was completed with a cliving hammer.

FROSTING AND CLIVING

That was then. This is now.

CLIVED SLATE

To try to solve the loss of historic roofs caused by the lack of new Collyweston slates English Heritage's Building Conservation and Research Team commissioned research into an artificial frosting process at Sheffield Hallam University's Natural and Built Environment Division in 1998. This method was successfully trialed during the re-roofing of Apethorpe Hall and the process made available to anyone who wanted to use it.

APETHORPE HALL

Claude N Smith, Collyweston Slaters had been mining slates since 1965. In 2014 they decided to modernise the mine and adopt the new frosting process.

Mining at Claude Smith

The first step was to dig an adit to replace the old shaft and ladder access. Once complete machines could be driven in.

The log is excavated with a Brokk 100 remotely controlled robot. This cuts away the sand bed and then drops it in pieces for removal with a front end loader.

As soon as it's removed from the mine the log is kept under water until it's frosted.

INTO THE WATER - 1 DAY - WATER OUT - INTO THE FREEZER - 1 DAY - OUT OF THE FREEZER - INTO THE WATER - REPEAT ONCE OR TWICE

Once the frosting has opened the fissile layers it can be clived by hand and dressed to shape.

The full story of the frosting trials is here.

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