Glass works in Glasshouse (clonlisk By.)

Present remains consist of a 17th century barrel-vaulted wood-fired glass furnace belonging to Huguenot glassmakers. Site was excavated by Jean Farrelly and Caimin O'Brien under licence no. 99E0191 (Farrelly, O'Brien, Paynter and Willmott 2014, 45-89). A 3D model of the glassmaking furnace can be downloaded from Offaly County Council website at www.offaly.ie/eng/Services/Heritage/Archaeology/3D_Surveying_of_Archaeological_Monuments/ An upstanding forest glass furnace near Shinrone became the focal point for investigating 17th-century glass manufacturing. Several families of French Huguenot glassmakers from Lorraine had been manufacturing glass in England in the late 16th and early 17th centuries. Members of two of these families, Bigos and Henseys (de Hennezels), are known to have held land in Offaly during this period. The Henseys are the most likely to have run the Shinrone glasshouse as they owned the townlands surrounding the upstanding furnace. In 1615 the manufacture of glass in wood-fired forest-glass furnaces was banned in England, though not in Ireland. However, in 1638-9 the exportation and manufacture of glass in Ireland was prohibited, and in 1641 another bill prohibited the felling of trees as a fuel supply for glass furnaces. Before the excavation a magnetic gradiometry survey was conducted by Joe Fenwick (Centre for Archaeological Survey, Department of Archaeology, NUI Galway) over an area 40m x 40m centring on the upstanding furnace. It was hoped that magnetic anomalies, which can be caused by intensive heat, would give an indication of the position of subsidiary furnaces. Several anomalies were recorded, the highest reading from the upstanding furnace. The second-highest reading came from an area 17m south-west of the main furnace. In order to investigate the high anomaly south-west of the furnace, a cutting 2m x 4m was opened. Upon excavation an iron band of relatively recent date was recovered that would have caused the high magnetic anomaly. A test-trench 1m x 4m was then excavated along the length of the cutting. Although no archaeological features were present, a thick layer of fine sand lay 0.5m below the sod. This may be the source for the sand used in the glass-making at this site. A second cutting, 6m x 8m, was opened to the north, west and south of the upstanding furnace. Immediately beneath the sod was a large scatter of debris from the furnace, which included furnace fragments, crucible fragments and bricks. Below this were areas of internal flooring consisting of small stones in a mortared floor. In the north-west of the cutting an area of sand/ash was uncovered. Immediately north of the furnace's stoking trench was an area of fire-reddened clay, the result of intense heat. It was initially thought to be the result of direct, in situ burning; however, the results of archaeomagnetic dating of the deposit (by Gould and McCann, Clark Laboratory, Museum of London) revealed that it was not physically related to the primary heat source for the glasshouse. It was suggested that magnetisation of the deposit may have occurred via an intensely heated surface overlying this deposit or may have come directly from a lesser heat source. This fire-reddened clay produced a date range of AD 1620-50 at 68% confidence level and AD 1610-60 at 95% confidence level. This represents the last firing at the site. These dates verify that forest glass was produced in Ireland for several decades after this had ceased in England. South-west of the furnace's southern stoking hole were two circular, shallow, flat-bottomed pits within a portion of the mortared floor. The pits, one with a diameter of 0.6m and maximum depth of 0.09m, the other measuring 0.93m x 1.3m with a maximum depth of 0.1m, have been interpreted as receptacles for vessels associated with the glass-making process. Immediately south of this a mortared trench for a stone wall was revealed. It was 0.7-0.9m wide and 0.15m deep and ran the width of the cutting in an east-west direction. This wall, from which most of the stones were robbed out, may be the southern boundary wall of the glasshouse itself. The remains of a compact clay floor were uncovered immediately south of the foundation trench. Finds included sherds of light green window glass, typical of broad (Lorraine) glass, and waste glass including droplets and dribbles from within the stoking trench of the furnace, which are direct evidence of the glass-making process. Other glass found on the site may have been brought to the site to provide cullet, broken glass that was recycled to make a fresh batch of glass. Large sherds of used crucible were recovered, though only two rimsherds and no bases. From the rims recovered it can be estimated that these vessels were 0.5m in diameter and 25mm thick. Crucibles are usually bucket-shaped with straight sides. The imprint of the base of one of the crucibles (diameter 0.4m) is still apparent on the seige platform within the upstanding furnace. Other finds included pottery sherds, slates, bricks and waste material. During this second season of excavation of the 17th-century glasshouse it was hoped to establish the limits of the glasshouse itself. Situated on an east-facing slope, the upstanding furnace would have been enclosed by a building, possibly wooden, though the wall foundation uncovered in the first season (Excavations 1999, No. 747) indicates that the southern side at least had a stone foundation. In order to follow the line of this wall two cuttings, 2m by 4m, were opened east and west of the line of the wall. In the eastern cutting the wall extended 1m into the cutting before terminating. There was no post-hole at the terminal to suggest that this had been an entrance and the wall trench did not reappear in the remaining 3m of the cutting. In fact the stratigraphy in the rest of this cutting was very shallow, with a thin layer of mortar appearing between the sod and the natural subsoil. The wall trench continued upslope through the western cutting, where it remained a consistent 0.9m wide and 0.15m deep. Further cuttings north and east of the upstanding furnace were opened in the hope of intercepting the limits of the wall trench in these areas. In the northern cutting, 2m by 6m, an irregular linear feature running east-west was uncovered. A cutting to the east, 2m by 3m, and one to the west, 2m by 4m, were opened to follow this feature. However, it terminated in both cuttings. Though it is 3m from the area of intensive burning at the northern mouth of the furnace, it has been suggested that this was the foundation (0.2m deep) for some form of fire-screen wall (3.7m long), possibly wooden. This linear feature was cut into the natural sandy subsoil which appears to have been the internal floor of the glasshouse. Two trenches, 1m wide, were extended from these cuttings, 11m northward and 8m eastward, but no wall foundation was intercepted. However, the stratigraphy in these trenches was very shallow, with a layer of stones and debris beneath the sod and a layer of mortar beneath. Most of the mortar has a flat surface on at least one side, and in some cases on both sides. As there was no trace of a collapsed wall, it seems likely that this may have formed an area of hard internal flooring. In the northern extension, this mortar layer terminates 9.5m from the upstanding furnace. Immediately north of this termination point there appears to be a contemporary cobblestone surface. This mortar layer could also be followed in the eastern extension, beneath which is natural subsoil. Another trench, 2m by 4m, was opened 14m north-east of the furnace. There are several undulations in the area surrounding the glasshouse and there was speculation that these may be mounds of glass debris or cullet, waste glass deliberately brought to glass-making sites to speed up the melting process. However, this cutting proved to be archaeologically sterile. Unlike the downslope portion of the site east an