Project
Gunns Mill is the site of the oldest surviving charcoal blast furnace in the country, situated 2.6km north of Littledean, Forest of Dean. It is a Scheduled Monument and a Grade II* Listed building, as well as being on Historic England’s Heritage at Risk Register. It was rescued from the threat of extreme dereliction by the Forest of Dean Building Preservation Trust. Our work at Gunns Mill completed a programme of drawn and photographic recording.
Our work consisted of three phases: Our Built Heritage specialists conducted a buildings analysis, while our Geomatics team undertook a measured survey, and our Fieldwork colleagues recorded the below-ground remains associated with the mill.
Historical background
The mill was built as a blast furnace in the 17th century, its remains representing one of the earliest and most complete blast furnaces still surviving, anywhere in the world. The mill is also a visible reminder of the importance of the Forest of Dean in the early development of ironworking, nationally and worldwide.
The furnace was built into the steep, south-east-facing slope of the valley of Westbury Brook, Flaxley. To the west was a dam for a pond that fed a head race. South-east of these structures was a broad, level yard in a sunken area enclosed by retaining walls.
In 1740 the furnace was converted into a paper mill and much altered. Between 1881 and 1982 the mill functioned as an ancillary building for a farm. Despite this, many features of the original blast furnace survive largely intact, including the charging house and blowing chamber.
Survey
The measured survey showcases some of the building survey techniques we can provide. The project presented challenges due to the poor state of repair of the buildings, obstructed access, presence of scaffolding and periodically inclement weather.
The survey of the retaining walls, boundary walls and blowing house was undertaken in 2015 during two days of torrential rain. Luckily the waterproofing of the Leica P20 laser scanner used to make the record was up to the job!
The P20 scanner is capable of scanning up to one million points per second with an inbuilt camera enabling the scans to be displayed in colour. Overlapping scans were made of the various structures and a network of tripods and targets was used to enable the scans to be joined.
The scan data was processed, otherwise known as registering, using Leica Cyclone software from which a 3d point-cloud was generated. A point-cloud is a set of three-dimensional data points defined by X, Y, and Z co-ordinates taken across the external surface of the structures on the site. This data can be used for producing building plans and elevations.
A stone-by-stone drawing of the retaining walls was executed in AutoCAD using the point cloud data. Analysis of this identified numerous phases of building, rebuilding and changes in the ground levels, associated with the various uses of the site over time.
A second phase of building survey and analysis, carried out in 2016, was concerned with the interior of the bridge house (a stone-and-timber structure above the blast furnace that once formed the charging house), the roof, the blowing chamber and the wheel pit. The interior of the bridge house was being held up with a network of scaffolding, which made the survey very difficult. However, utilising a new model of scanner, the Faro X130, the team were able to scan the building in a relatively short space of time. The lightweight and compact chassis of the scanner made it ideal for moving around in a confined space.
The Faro X130 allowed a different method for joining up the separate scans that were made of the structure. This time, instead of a network of tripods with targets, the scan results were registered together based on overlapping point data. To assist this ‘spheres’ were placed in the scans to act as tie-in points. The scan data from the Faro was processed in Faro Scene from which a three-dimensional point-cloud was generated. This was again used in AutoCAD to draft line drawings.
The compact scanner also enabled the team to scan the wheel pit, which would have dangerous to survey, given its constrained access. The scanner was positioned on each level on the scaffolding to enable scanning of the wheel pit. Phased line drawings were again produced.
Further reading
Hayman, R. 2016 Gunns Mill, Flaxley, Forest of Dean, Gloucestershire: Statement of Significance. CA Typescript report for FODBPT
Gunns Mills, Flaxley, Forest of Dean, Gloucestershire: Historic Building Recording. CA typescript report: 15106, 2015
Gunns Mills, Flaxley, Forest of Dean, Gloucestershire: Historic Building Recording. CA typescript report: 16097, 2016
