1999:747 - GLASSHOUSE, Offaly

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.

This research excavation was grant-assisted by funding from Dúchas The Heritage Service, Department of Arts, Heritage, Gaeltacht and the Islands, on the recommendation of the National Committee for Archaeology of the Royal Irish Academy.

'Sonas', Curraghscarteen, Moyglass, Co. Tipperary and 'Illaun View', Glenbower, Coolbaun, Nenagh, Co. Tipperary