The previous Building London blog post, https://buildinglondon.blog/2022/05/24/39-collyweston-slate-part-1-the-city-of-london-guildhall-roof/ looked at the use of Collyweston Slate on the roof of the City of London’s Guildhall and even though while the Building London blog only concerns building materials used historically in London, and it is not at all certain that Collyweston was used on Guildhall before 1953, because Collyweston slate is an historic building material generally, it is definitely worth looking at!
And I suspect that Collyweston was used in London at some point in the Mediaeval period, as other Lincolnshire limestones were, with fairly simply transportation to London through the Fens and Wash and up the Thames, but there is simply no evidence I have yet found/seen.
Collyweston ‘slate’ is very different to the classic blue-black Welsh Slate seen on millions of roofs across Great Britain and Ireland, made from metamorphosed mudstone  being a type of limestone of the ‘Lincolnshire Limestone Formation, Inferior Oolite Group’,  found in a relatively small area in the east Midlands in a band from Corby to Stamford, that uniquely, when left out in the frost splits into ‘slates’! 
Terry Hughes in ‘Vernacular Slating in the East Midlands’ states
“Strictly, [ the term slate ] should only be applied to stones derived from sediments which have undergone low grade metamorphism: the process of compression and heating under which the sedimentary minerals are recrystalised and re-orientated perpendicular to the compression. They can be split into thin strong and flexible sheets which are not parallel to the original sedimentary bedding layers. In practice, where the specific type of rock is not relevant or is obvious, the term slate can be used to mean other roofing stones such as shales, limestones or sandstones.” 
Ruth Siddal of London Pavement Geology states “Collyweston Slate [is] Part of the Jurassic Lincolnshire Limestone Formation (Inferior Oolite Group), the ‘slates’ are actually cross‐bedded, sandy limestones which split along the cross‐bedding planes. Although named after the village of Collyweston in Northamptonshire, the slates are extracted from quarries along the outcrop of the beds in north Northamptonshire, south Lincolnshire, Rutland, and northwest Cambridgeshire” 
This ability to split was first noticed many centuries ago and the first use of the ‘slates’ was of ones found naturally split on exposed stone, but gradually production moved to mining in pits and drift mines. The British Geological Society Building Stone Atlas for Northants states “Collyweston Slate is a cross-bedded, sandy limestone that ranges from a few centimetres to nearly a metre in thickness. … the Collyweston Slates are worked by splitting along the laminae of the cross-bedded foresets that are enriched in mica-flakes and shell fragments. Frost is a key factor in achieving the splitting, and the earlier sources of slate were close to the surface where frosts had already split the rock. From the 16th century onwards, the technique of frosting appears to have been used and the slates were mined via shafts which extended down into the soft sands of the underlying Grantham Formation; these sands were then mined outwards from the base of the shafts and the overlying stone supported with pillars of stone blocks. Once the overlying bed could be heard to be moving, the miner withdrew and removed the pillars allowing the Collyweston Slate bed to give way. The released blocks (termed ‘logs’) were then collected, kept damp and then exposed on the ground surface to frost which caused them to split into thin layers. These layers were then shaped to a variety of different sized slates, which when used on the roof are seen in diminishing courses, grading in size from the smallest at the ridge to the largest at the eaves.” 
The Collyweston Historical Society writes “The 19th century method of extraction was to mine out the Collyweston slate seam. Miners used to lie on their sides, picking away at the sand under the stone slate seam, using tools known as foxing picks. Permanent columns of waste stone were built to support the ceiling, and temporary ones for the undermined seam. The miner would tap the seam above his head to check that it was safe. When the seam was about to fall, a series of clicks, known as talking, could be heard. Ideally, it would be ready to fall at the end of a day’s foxing, and the miner would retreat pulling the temporary supports out as he went.
The undermined part of the seam would then fall to the floor, hopefully breaking into easily managed pieces known as log. If the seam did not fall, steel wedges would be driven into it and a bar known as a lion’s tail would be used to lever the seam down. The log was taken by a barrow, known as a shim, to the surface.” 
Terry Hughes writes “John Morton’s Natural History of Northamptonshire (1712) gives one of the earliest descriptions of the method of obtaining the slates “which being sprinkled with Water and exposed to Frosts, do readily cleave into thin and eaven Plates as are fit for covering the Roofs of Houses”. 
And see also these 19thC descriptions here “The slates are worked either in open quarries or by drifts (locally called ‘fox-holes’) carried for a great distance under ground, in which the men work by the light of candles. The upper beds of rock are removed by means of blasting, but the slate-rock itself cannot be thus worked, for though the blocks of slate-rock when so removed appear to be quite uninjured, yet, when weathered, they are found to be completely shivered and consequently rapidly fall into fragments. The slate-rock is therefore entirely quarried by means of wedges and picks, which, on account of the confined spaces in which they have to be used, are made single sided. The quarrying of the rock is facilitated by the very marked jointing of the beds, a set of master-joints traversing the rocks with a strike 40 degrees W. of N. (magnetic), while another set of joints, less pronounced, intersect the beds nearly at right angles.” 
The Collyweston slates were much used in the local area and as far afield as Cambridge , relatively easily accessible by the River Welland from the quarries, firstly in the now long gone Cambridge Castle in 1286  and then many colleges buildings and churches including the Round Church  and where today many old building are still covered with them, though re-covered,  but it’s use declined drastically with the cheaper and lighter Welsh slates of the 19thC.
With the decline in use the mining also contracted then stopped and after the 1970s there were no ‘new’ slates being dug and the only trade was in re-cycled, re-dressed, slates that came from demolished barns or houses stripping old Collyweston slate roofs and re-roofing in cheaper Welsh slates or brick tiles. Historic buildings in Cambridge and elsewhere still needed sources for renovation work though and in the 1990s David Ellis, who then helped re-roof Guildhall in 2007, was able to source Collyweston stone from the Cuckoo Lodge Quarry, where limestone was being quarried for aggregate, for that purpose. 
Climate change was also a factor in the decline of the production and use of Collyweston, as the hard frequent winter frosts needed for splitting the stone could no longer be relied on – “During the 20th century, production declined – partly because of a series of mild winters when frosting was unsuccessful – and by the century’s end had effectively ceased.”  so in the 21st century the now leading Collyweston Slate producers Claude Smith and his son Nigel pioneered splitting the ’logs’ in freezer units and have since created a new drift mine to increase production as demand for the slate has increased for restoration work.   
Visit to the Claude Smith Collyweston Slate mine and works
Last summer Building London visited the Claude Smith site and had a tour of the re-opened mine, the freezing sheds and the slate dressing workshops all of which were totally fascinating. It’s clear how massively important small quarrying/mining concerns like theirs are for building stone heritage and craft and it was a privilege to visit and massive thanks to Nigel Smith for this.
The old Claude Smith mine which closed in 1970 was accessed by a shaft and in 2014 a local geology group visited the mine via the old now blocked off shaft! 
The new entrance to the mine is via a slope making it a ‘drift mine’  and indeed the road the mine is on, and where there used to be many drift mines, is called Slate Drift. This method of access allows small diggers to drive in and out rather than have to winch the stone out. The mine is only 8m under ground! The planning application notes that surface quarrying in the area started approx. 400 years ago and this switched to mining in the 1800s and that the site of the Claude Smith mine was first mined in the 1850s when the road was called Slaters Drift. 
When in the mine the various horizontal beds are obvious in the walls, and what helps the mining is that the limestone bed immediately above the Collyweston Slate bed is a hard stable limestone that can be relied on as a roof and below the Collyweston is a soft sandy stone that can be easily dug away. I’m not entirely clear what bed of the Lower Lincolnshire Limestone it is as apparently “ there are no detailed sections for the Lower Lincolnshire Limestone in Peterborough around the Thornaugh, Collyweston and Cross Leys Quarries.”  though the Collyweston is the very bottom bed of the Lincolnshire Limestone Formation, as illustrated in the sedimentary logs from the nearby Ketton Limestone Quarry, and so the hard bed above would be maybe something like the Ketton or Barnack freestone found nearby. 
The Collyweston stone is removed by first digging out the soft sandstone and the letting the Coolyweston bed drop in lumps called ‘logs’, which are then taken above ground. Historically these ‘logs’ would have then been wetted and left out to over-winter so the frost would split them. Now they are immersed in plastic tubs to soak them then transferred into freezer units to split. 
After splitting they are taken to the workshop where they are dressed, i.e. the edges are tidied and drilled for the fixing holes they will need to be secured to batons. Because the stone is so variable many different sizes are made, and Collyweston roofs inevitably are made with the slates going in horizontal courses from the eaves to the ridge.
Note in the photographs the colour of the slates varies between blue and yellow. The blue is un-oxidised stone and with time and exposure to oxygen, this will also turn yellowy and then brown.
While nowadays Collyweston is now only mined at Claude Smith’s, historically it was mined or quarried across a much wider area from in the west, Medbourne near Market Harborough, through Kirby near Corby, Kings Cliffe and to Wothorpe and Burghley Park near Stamford and the re-opening of this Collyweston Mine is a great success for historic, vernacular building stone use and heritage. 
Massive thanks to Nigel Smith for showing me around the Claude Smith mine and of course to his father Claude Smith. And to Sandra of the Collyweston Historical Society for her correspondence and helping me to research this post.
While the actual working mines and quarries are all private property and of course dangerous, with permission needed to visit them, Collyweston has a public Nature Reserve with evidence of previous historic workings and spectacular limestone grassland flora and fragments of the slate can be seen underfoot on the paths. 
and there is a similar site of calcareous grassland on an old Collyweston quarries at Kings Cliffe. 
The villages in the area including Collyweston and Duddington are also very beautiful too and if you are in Collyweston, visit the Slaters Arms where the roofing can be clearly observed!
There’s lot of other quarrying with historic importance in the area too, and Building London will be covering Ketton, Clipsham and Barnack in future posts. And there is a similar lovely nature reserve at Barnack where stone was quarried from the surface in mediaeval times and is now an important calcareous grassland. 
How to get there
Public Transport: Collyweston and Kings Cliffe are the best places to visit. Google states it’s a 2 hour journey from London to Collyweston by public transport. Take a train from Kings Cross to Peterborough then the 47 bus to Uppingham.
Cycling: Again, start from Peterborough and maybe do a big loop incorporating Barnack, and with the massive Ketton cement works, where some stone is produced, always somewhere in view, or alternatively take the fast train to Corby from St Pancras and take in both Rockingham Castle and Rockingham village where Collyweston slates were much used and are still much in evidence. Or cycle from Peterborough to Corby!
Driving: From London, go straight up much up the A1 and take the A47 turnoff near Peterborough.
 ‘Vernacular slate and stone roofs in England’ by Terry Hughes in England’s Heritage in Stone Proceedings of a Conference Tempest Anderson Hall, York https://englishstone.org.uk/York_files/ESF%20-%20Terry%20Hughes-1.pdf
 “An assessment of the aggregate properties of the Lower Lincolnshire Limestone in south Lincolnshire and surrounding areas” http://planning.southkesteven.gov.uk/SKDC/S15-1611/1339403.pdf
 “Revitalising Collyweston limestone slate production by artificial freeze/thaw splitting” Laycock et al [ £ ] https://www.sciencedirect.com/science/article/abs/pii/S0950061817320780