Collyweston stone slate is a fissile limestone from the Jurassic period (140-190 million years ago) and is not a true slate. It can be split along natural cleavage lines, however, which is the similarity between Collyweston and true slates. It is named after the village of Collyweston, in Northampton, which lies in the centre of the area in which the slate is quarried, and has been used as a roofing material since Roman times. From the middle ages until the 19th century it was used on almost all buildings within ten miles of the quarries and on prestigious buildings further afield. The advent of the railways meant that imported Welsh slate became cheap, and its impermeability meant that it was suitable for use on the fashionably lower pitched roofs. From the late 19th century Collyweston stone slating fell into decline - by the 1970's the craft was dying out. Fortunately, the durability of the slate and its attractive appearance, coupled with the protection of buildings with Collyweston roofs, has meant that building owners continue to demand the material and the craft has therefore survived.
Today, there are three main threats facing Collyweston stone slate. Firstly, it is under threat from the variety of alternative roofing materials available, many of which are cheaper to lay in the short term but not as durable in the longer term. Secondly, some roofing companies offer a free roof in a cheaper material in return for the Collyweston stone slate in order to obtain a second-hand supply of the material, and thirdly, the roofs of many unprotected buildings are stripped to provide salvage slate for use on roofs elsewhere. These latter practices are particularly damaging to the Collyweston trade because they reduce the demand for newly quarried stone slate, making it economically unviable and not competitively priced, and once lost, it is unlikely that a Collyweston roof would ever be replaced. If the demand for the material reduces, the skilled slaters will no longer be able train apprentices and the craft will die out.
The process of producing stone slate is a time-consuming activity, dependent on the weather and requiring great skill. From Roman times to the 17th century, most of the slate used was obtained from outcropping rock or from the by-product of other quarrying activity. An increasing demand led to the excavating of slate using open-cast methods and by 1633 there were open Fits at Collyweston. This was not a wholly satisfactory method, however, as the slate needs to retain sap in order to be split by frost action, so quarrying only takes place during six weeks in December and January.
A second method of extraction was to quarry the slate below ground via tunnels, shafts and adits known as 'fox holes'. Working conditions were cramped,, with the workmen having to lie on their sides due to poor headroom. They would pick away at the rock and sand which lay under the stone slate - working the face in this way was called 'foxing'. Columns of ragstone were left to support the ceiling, with pit props being used in later quarries. Occasionally the workman would tap the rock close to his head to check it was safe to progress along the face. When the ceiling was about to collapse, a series of clicks, known as 'talking', would be heard. Ideally, it would be ready to collapse at the end of a days foxing, and the workmen would retreat knocking the supports down as they went. The mass would then fall to the floor, hopefully breaking into easily managed pieces. If the ceiling did not collapse, steel wedges would be driven into it with a 'lions tail' (an iron bar) which would then be used to lever the ceiling away.
The rocks, known as 'logs', were loaded into barrows, or 'shims' and hauled out. It is vital that the stone remains damp so that the freeze-thaw action of the frost will initiate splitting. At the surface, the logs were laid out on a bed of shale, called the 'patch', to allow air underneath. The log would be watered until the initial frost split was visible. In mild winters, when the log had not split, it was pied', that is laid in a pit and covered in wet earth to prevent drying, and brought back up to the surface again for splitting the following winter. Even today, slaters are still reliant on the frost to spilt the log.