Chris Scott – Solstice Heritage, formerly of Archaeological Research Services Ltd and Mags Waughman – NPA Monument Management Scheme Officer. From ‘An Industry on the Edge: the Alum Industry of the North York Moors coast’
Alum production was one of England’s earliest large-scale chemical industries and the North York Moors and Cleveland Hills was the main production area for the first 200 years of the industry, which took off in England when political and religious rivalries in Europe led to a market for home produced alum. Many of the surviving remains of the industry are of national importance and the National Park has eight alum production centres which have been designated as Scheduled Monuments because of this.
The alum industry operated in North Yorkshire from c.1604 until 1871. It was a major component of the wool/cloth trade because the chemical produced acted as a fixing agent for colour dyes. The alum refining process began with the quarrying of shale bedrock which was heaped with fuel into large mounds, or calcining clamps, and then burned for a long period (up to 8 or 9 months). The burnt shale was then steeped in water in a series of pits to release aluminium sulphate into a solution known as alum liquor, which was siphoned between the pits and settling cisterns to concentrate and clarify it. The liquor eventually ended up in the ‘Alum House’ where it was boiled over coal fires to concentrate it further and then an alkaline sulphate was added (human urine or seaweed) to create the required product – alum crystals.
Many of the production sites in the North York Moors were on the coast where alum shale was exposed in the cliffs and could therefore be easily extracted. The sea also allowed for good access to the imported ingredients of urine/seaweed and coal, and for transporting away the end product. However, the soft sediment coastline of the east coast of Britain is vulnerable to erosion and now as a result, these coastal alum sites continue to be worn away and will be inevitably lost over the next 100 years. Six of the scheduled alum production sites in the North York Moors are in such vulnerable locations on the edge of the coast. Since 2009, we have been working with Historic England to improve the condition of Scheduled Monuments at risk in the National Park through our Monument Management Scheme but in the case of these scheduled coastal alum sites, long term conservation is difficult because we cannot remove the threat.
As an alternative to protecting the coastal alum working sites from inevitable erosion, we have been working to preserve the information they hold through archaeological investigation and recording. This may allow us to answer some of the significant questions which still remain about the regional alum industry, the development of the industrial processes and about each of the individual sites. So back in 2014 the National Park Authority commissioned Archaeological Research Services Ltd to carry out initial assessments to establish the current knowledge and condition of each site and to develop mitigation strategies for the most significant remains; using excavation, aerial photography and Lidar transcription, drawn and photographic recording and walkover survey. In all, four sites were investigated, with the aim of researching outstanding questions and where possible, to facilitate the lowering of the ‘Risk’ status attached to the monuments.
Investigation work was particularly difficult and dangerous because of the locations of the sites – on the precarious and eroding cliff edges. Specialist safety, rescue and access contractors Spartan Rescue Ltd were brought in to create site-specific risk assessments, method statements, access plans and emergency rescue procedures. Each member of the excavation team had to be strapped into a restraint harness for most of the time on site. Happily, investigation of all four sites was completed without mishap and the team was able to put together a full report, the highlights of which are summarised below.
Boulby Alum Quarries and Works – this is one of the best examples nationally of a technically advanced 19th century alum-quarrying complex.
The archaeological works at Boulby comprised the excavation of three trenches across a putative pump house, two liquor cisterns and an associated potential building platform, and followed on from previous excavations in the 1960s/70s. The cisterns were well preserved – these circular masonry structures were built within a cut into the shale bedrock, presumably to create the correct finished height to allow alum liquor to be run into them by gravity alone, and sealed with a fine yellow clay as waterproofing. The cisterns had eventually been abandoned, most probably intentionally backfilled, and a later trough for the movement of liquor had been cut through the most easterly tank. Excavation of the easterly cistern showed that the floor and wall had been affected by subsidence at some point, and so one possible reason for its partial demolition may have been that the structure no longer held liquid effectively. The soft geology regularly seems to have been a problem for the alum industry, with oft-reported landslides and subsidence impacting on, and partly caused by, the quarrying operations.
The building platform lay close to the cisterns and was shown to have been created by the mass dumping of quarry waste into a previously quarried area, filling it to a depth of at least a metre, and probably much more. This serves to illustrate both the changing landscape of the ‘New Boulby Works’ during its operational life-span and the limited space within which the quarry functioned. The building on top of the platform had a stone slab floor, made from the local sandstone that had to be removed to access the shale, and timber walls supported on horizontal beams in slots. Evidence for a pantile roof gave a good idea of the overall character of this small industrial building. The other building investigated, presumed to have been a ‘pump house’, proved to be a much grander and well-made structure, with finely built masonry walls and a fantastically preserved flagstone floor. Finely tooled sockets in the walls and chamfered sockets within the floor suggested that machinery had once been fitted into this building. However no conclusive evidence of the building’s function was uncovered.
Saltwick Nab Alum Quarry
Two trenches were excavated at Saltwick and produced archaeological evidence that supplemented the previous survey data compiled by York Archaeological Trust detailing the location of cisterns, warehouses, burnt heaps and tanks on the quarry floor. Work focused on the excavation of an existing erosion scar at the seaward lip of the quarry. Previous visits had noted in-situ eroding deposits, which were investigated in order to try and understand the nature of the deposits being lost to the sea. The trench exposed huge deposits of quarry waste, spent burnt shale and piles of clay and silt which had been dumped by barrow. These overlay surfaces of hard trampled shale and waste: the working floors of the quarry on which were preserved flat wooden-plank barrow boards used to make barrow routes for the movement of materials within the quarry. These boards, worn through by use, spoke of the colossal human effort needed to shift such vast tonnages of stone by hand. Also within the trench, cut into the quarry floor, was a drain heading towards the sea, presumably for draining away the waste from the steeping process.
The evidence at Saltwick showed that, although much of what is being lost appears at first glance to be just dumps of quarrying and processing waste of limited archaeological potential, this material can often bury and preserve the fine detail and artefacts of everyday life in an alum quarry, and it is this which is being slowly washed away and lost to the sea.
The archaeological work at Saltwick included the production of a detailed photographic and drawn record of the foreshore slipway/ramp feature (used for sea transportation), in order to mitigate the uncontrollable and imminent loss of this feature through the ongoing erosion.
Kettleness Alum Works
The Kettleness site had previously benefited from detailed earthwork survey and documentary research carried out by English Heritage (now Historic England), so the new project aimed to examine specific areas of interest identified by this earlier work. The excavation trenches were targeted across a putative calcining place and putative calcining clamp.
A prominent mound at the landward side of the quarry, had previously been interpreted as a spoil heap or, possibly, a rare survival of an in-situ calcining clamp. The excavations demonstrated that it was indeed a calcining clamp, but rather than for processing alum shale it was an equally rare survival of an unburnt clamp for the calcining of ironstone. The North York Moors and then the Cleveland Hills and Middlesbrough, saw large-scale growth of the ironstone industry during the middle and later part of the 19th century. This site may have represented a diversification of production during the later 19th century, moving into the product in most demand (see This Exploited Land Landscape Partnership).
Kettleness is one of the better understood sites and the targeted excavations have answered specific research questions in relation to the site, as well as demonstrating significant evidence of the likely re-use of this alum site for the extraction and refinement of ironstone. The information gleaned from the excavation, together with the detailed earthwork survey, can now be better used as a model from which to interpret earthwork evidence at other sites, particularly potential “clamp” or “spoil heap” features.
Sandsend Alum Quarries and Works
At Sandsend, a complex of three successive alum quarries (Gaytress Quarry, Ness End Quarry and Deep Grove Quarry) extend for nearly a kilometre along the coastal margin. The investigations carried out comprised LiDAR and aerial photographic transcription, followed by a detailed walkover survey to examine remains on the ground. This initial survey work was necessary because there had been much less previous investigation at Sandsend compared to other sites so it was required first to establish a comprehensive baseline of evidence.
The survey at Sandsend identified features relating to the quarrying of sandstone, alum shale and possibly cement stone, as well as barrow-ways, route-ways, spoil heaps, evidence of water management, banks of steeping pits and liquor channels covering the entire scheduled area, and in places extending beyond it. This work has added significantly to the understanding of the breadth of remains within the quarries, as well as the position of the remains in relation to the coastal edge, and therefore the relative risk of erosion. These can now be much more successfully compared and contrasted with other well understood or surveyed sites, such as Kettleness, allowing the production of more targeted research across this group of important sites.
Now that the project has finished, we have a good base of evidence which we will use to inform and support both future management and targeted works to mitigate the inevitable loss of these monuments. We will continue to address the issues facing these sites and look for opportunities to investigate and record the remains under threat in order to achieve ‘preservation by record’ where they cannot be physically saved. The coastal alum sites contain the remains of one of England’s earliest large-scale chemical industries and illustrate the important place the North York Moors had in this industry.
If you want to have a look yourself at a coastal alum working site – the National Trust owned Peak Alum Works at Ravenscar is open to the public and is safe to access without emergency rescue procedures.