The present day lack of supplies of reclaimed Collyweston roof slate or tiles encouraged Claude N Smith Ltd to apply for permission to open an old mine and the lack of reliably hard frosts each winter spurred the development of a method of artificial frosting using an old 40ft truck insulated refrigeration container. The company puts ten pallets of unsplit freshly mined 'logs' into it, takes the temperature down to around -10˚C and the resulting cliving produces around five or six squares (50sqm).
 
A generation or so after it closed, Claude N Smith Ltd (now owned by Nigel and Viv Smith) has re-opened a slate mine in the Northamptonshire village of Collyweston. Abandoned in the 1960s as frosty mornings became sporadic and unreliable, resulting in difficulties in splitting the stone, the mine is now viable once again due to advances made by Sheffield Hallam University and Historic England. Working together they have developed a method of using modern freezer technology as a reliable method for splitting to create the thin Collyweston slate sheets. In recent years the lack of 'new' material has resulted in many historic buildings having to resort to using reclaimed Collyweston slate for their roof repairs. It is hoped that this new mine will produce a steady source of Collyweston slate to meet the demands, with Kings College, Cambridge hoping to be the first to benefit. To reach the new slate source miners had to remove over 100m of rock but are now ready to start producing Collyweston slate once again. Apart from being a good news story for the roofing industry, this new venture will breathe new life into a 600 year old industry.
 
Messenger Construction of Stamford (established in 2011) writes that it was approached by English Heritage (now Historic England) to carry out trials to replicate the natural process of splitting or "cracking" the stone blocks or "log", which would naturally occur when the log was exposed to frost during the winter months. The trials have proved successful and new Collyweston Stone slates are regularly being produced.
 
Gabriella Misuriello, Conservation Projects Manager at Churches Conservation Trust said, "I'm delighted that we are working with Messenger Construction on this innovative project at St Andrews Church Ufford, near Stamford, supporting and promoting the continued use of traditional Collyweston roofing slates. The new method of production of Collyweston slates has created a great deal of interest in the heritage sector as the stone hasn't been quarried and slates produced in any great quantity since the 1970s and so it is a much prized roofing material."
 
A Historic England source said that the process of the stone being prepared for roofing has been reduced from three years to a few weeks. To get the stone prepared it traditionally used to be left outside for three winters until the frost revealed layers that could be broken into perfectly flat slates. But by dousing the rock in water and then putting it in a large freezer, tests have shown the natural freeze-thaw process can be sped up significantly. Collyweston mines were all closed down by the 1970s and the slate is needed for repairs to hundreds of buildings around Northamptonshire, Lincolnshire, Cambridgeshire and even in London and east Yorkshire that still have the slate on them.
 
"Reclaimed slate has become harder to find and so more expensive, so we thought 'we're sitting on top of tons of the stuff, why not use it?'," Nigel Smith told the Daily Telegraph. ""That sealed the decision to re-open really," said Mr Smith, 50. "Now we've also been approached by other building owners, including Clare College, Cambridge, with a view to supplying and fitting a new slate roof for them." At the same time he was approached by King's College to supply them with £350,000 worth of slate for a new roof to replace the existing one on Bodley Court, whose uninsulated tiles have become thin and degraded from being exposed to condensation caused by its central heating.
 
Collyweston slates which come from beds which geologists describe as calcareous sandstone or sandy limestone - similar to the highly reputed Stonesfield slates in Oxfordshire which are also unable to decide whether they are limestone or sandstone. Interestingly both were adopted 400 years ago by their respective famous nearby university colleges for some very fine and very large roofs - Stonesfield at Oxford University and Colleyweston at Cambridge University. Alec Clifton-Taylor describes them as having 'aesthetically inferior to Cotswold (stone tiles) … less richness of texture and less variation in colour … fawn and light grey when freshly dug with a darker hue on weathering … but appreciably thinner than Cotswold … with a Collyweston roof weighing half as much as a Cotswold' Pattern of English Building.
 
The Collyweston Slaters Trust website has a comprehensive guide. 'Roman slates were almost certainly made from stone which was found on the surface, known as 'gifts', as it was probably not until the 16th century when the frosting process for making slates was widely used. In 1375 and 1390 it is recorded that 14,000 slates were supplied to Rockingham Castle; so it can be inferred that in the mediaeval period there must have been an organised industry of some kind and a system of production, possibly as a by-product of quarrying activity. It is known that by 1633 there were both open pits and mines in the fields around the village of Collyweston. There are calculated to be 1,500 buildings in north Northamptonshire alone with Collyweston stone slate. Over half of these are listed buildings. Collyweston stone slates last for hundreds of years and are capable of almost continuous reuse. When the roof of an historic building in Pytchley was stripped for repair recently, it was found that more than half the slates, which were laid in the 17th century, were still sound and reusable.' It also has diagrams showing the diminishing courses and their names (see link below)
 
The fissile - splitable on bedding planes - nature of Collyweston is enhanced by frosting - leaving the rocks lying in the open for a year or two and constantly wetting them which freezes the quarry sap and opens the cleaving - or cliving as it is know locally - planes (see more below). The beds overlie Northampton Sands and the surface of the slates exhibits the proximity of the shore with ripple markings, worm tracks and burrows as well as plant remains. Overlying the Collyweston beds are oolitic limestones such as Stamford Marble, Ancaster, Barnack Rag used by the Romans and Ketton freestone from which Peterborough and Ely cathedrals were built.
 
Extract from Memoirs of the Geological Survey by John Judd, published 1875 - available free on Google books.

The Collyweston Slates have been dug over a considerable area in Sheet 64, old pits being traceable from Wothorp near Stamford to the western side of Collyweston, a distance of more than three miles. At the first of these places they are said, by tradition, to have been met with much nearer the surface than in the present workings, and this statement is confirmed by the geological relations of the beds in this neighbourhood. The valuable fissile character of the beds is merely a local accident; and in some directions the bed of stone has been followed and found to become non-fissile and in consequence worthless for roofing purposes. There is only a single bed of stone (the lowest limestone of the series) which is used for making roofing slates. This varies greatly in thickness, being often not more than 6 inches thick, but sometimes swelling out to 18 inches, and in rare cases to 3 feet; while, not unfrequently, the bed is altogether absent and its place represented by sand. Rounded mammillated surfaces, like the "pot-lids" of Stonesfield, abound in these beds.
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° W. of N. (magnetic), while another set of joints, less pronounced, intersect the beds nearly at right angles.
During the spring of the year the water in the pits rises so rapidly that it is impossible to get the slates out
The slates are usually dug during about six or eight weeks in December and January. The. blocks of stone are laid out on the grass, preferably in a horizontal position. It is necessary that the water of the quarry shall not evaporate before the blocks are frosted, and they are constantly kept watered, if necessary, until as late as March. The weather most favourable to the production of the slates is a rapid succession of sharp frosts and thaws. If the blocks are once allowed to become dry they lose their fissile qualities, and are said to bo "stocked." Such blocks are broken up for road-metal, for which they afford a very good material. The limestone beds above the slate rock are burnt for lime.
The slates are cleaved at any time after they are frosted. Three kinds of tools are used by the Collyweston slaters. The "cliving hammer," a heavy hammer with broad chisel-edge for splitting up the frosted blocks. The "batting hammer" or "dressinghammer," a lighter tool for trimming the surfaces of the slates and chipping them to the required form and size. The "bill and helve," the former consisting of an old file sharpened and inserted into the latter in a very primitive manner. This tool is used for making the holes in the slates for the passage of the wooden pegs, by means of which the slates are fastened to the rafters of the roof. These holes are made by resting the slate on tho batting hammer and cutting the hole with the bill.
The slates are sold by the "thousand," which is a stack usually containing about 700 slates of various sizes, the larger ones being usually placed on the outside of the stack. The slates when sold on tho spot fetch from 23s. to 45s. per thousand. Many of the Collyweston slaters accept contracts for slating, and go to various parts of England for the purpose of executing their contracts.
The land at Collyweston is generally held by slaters by copyhold, the slaters paying 6s. 8d. per " pit" to tho lord of the manor (a "pit "is 16 square yards) with an extra charge of Is. 6d. per pit to the measurer. A few workings aro rented of the lord of the manor, the slaters paying 30s. per pit with an additional Is. 6d. for the measurer. These payments are made every year at the annual "slaters' feast" held in January.
Tho manner in which the slates are placed on the roof is as follows:- -The largest are laid on nearest the Avail plate, and the size of tho slates is made gradually to diminish in approaching the ridge. The ridge itself is covered by tiles of a yellowish white tint, made at Whittlesea, and harmonising well in colour with the slates themselves. The larger slates are, in the ordinary way, fixed to the rafters of the roof by means of wooden pegs driven through a hole in the upper part of each slate. But roofs are often covered with small slates which are fixed by mortar.

 
Each of the diminishing courses of Collyweston slates was historically given a name, running from Long Tens at 24ins to Even Mopes at 6ins. (See the pic from Elliottt Roofing of all names recorded by Mr Harrod)