Restoration of the Curvilinear Range of the National Botanic Gardens, Glasnevin, Dublin
Volume 2 Number 6 - June 1996
The Curvilinear Range is the most significant wrought and cast iron building in Ireland and is one of the most important nineteenth century glasshouses surviving in Europe today.
In 1990, the Range was in a state of advanced decay and structural instability which required it to be closed to the public. The combination or basement vaults, insufficient foundations and inherent structural problems arising from the enlargement of the range in 1869 led to the deformation of the structure and glazing bars. The interior tropical climate caused a rate of metal corrosion four times that which would normally be experienced in external Irish conditions. Prior to restoration some 300 glass panes a year were breaking due to the corrosion and instability of the original structure.
The Curvilinear Range of today developed in a piece-meal fashion over a period from 1843 to 1869. The involvement of Richard Turner gave the design a coherence which such ad hoc approaches seldom achieve. Turner designed the first 1843 section of the glasshouse but lost the tender to build it. A contractor named Clancy was the lowest tenderer but he underestimated the cost and went out of business after taking twice the tender period to complete the work. The Clancy name is today visible over the south door of the East Wing. Thereafter the Royal Dublin Society, who then managed the Botanic Gardens employed Turner to complete the work with their architect Darley for the remaining stages.
Turner's glasshouses reflect the industrial age in its most advanced form with standardised components and prefabricated elements manufactured off-site for later site assembly. This use of wrought and cast iron was at the leading edge of building technology in the mid-nineteenth century. The repeal of punitive taxes on glass in the 1840s, coupled with French innovation in glass production gave Turner the option of using curved glass in long lengths.
Up until that time, the shape of glasshouses was rudimentary and rectilinear as timber was the predominant construction material. With the intrinsic architectural and engineering properties of wrought and cast iron, Turner gave us a sophisticated and curvilinear range of glasshouses which were avant garde in their design and innovative in their technology.
Richard Turner was born in 1798 and was 83 years old when he died on 31st October 1881. He descended from a long line of Dublin ironmongers to become the greatest ironmaster of his day. In 1834, Turner built his new Hammersmith Ironworks in the Dublin suburb of Ballsbridge. Turner is principally associated with his spectacular glasshouses in the Belfast Botanic Garden, Northern Ireland (1839), Glasnevin (1843-1869) and at the Royal Botanic Gardens, Kew, U.K. (1848) but he also designed and manufactured some mundane things like boundary railings for Trinity College, Dublin, boilers, cisterns and even bedsteads.
Cast iron was the dominant building iron from 1730 to 1870 but from 1830 the importance of wrought iron increased steadily with the building of the railways. Cast iron susceptibility to fracture under tension gave rise to the use of wrought iron for long lengths of railway track. The superior compressive tensile ratio of wrought iron also lent itself to small structural sections and the extra design and technological potential it offered.
Unlike stone or brick with their own history of research and restoration techniques no such precedent existed for wrought and cast iron restoration. Wrought iron was replaced by mild steel in building at the beginning of the 20th century and is no longer available. This led the Royal Botanic Gardens, Kew to replace their original cast and wrought iron Palm House (designed by Decimus Burton, with modifications by Richard Turner) with a replica stainless steel building in the 1980s.
The problem of producing 20 different complex shaped glazing bars of good quality wrought iron to fine tolerances in small batch sizes at an economic rate for repair purposes was solved by means of a new forging process, named drop forging. Recycled wrought iron was heated, then placed on a profiled anvil and beaten into shape by a pneumatic hammer. Approximately 4,500 hammer blows per linear metre were required to match Turner's profiles. This was a key breakthrough.
The use of blacksmiths and new mechanized forging combined to make scrap wrought iron available for reuse. New wrought iron welding technology combined with exceptionally skilled welders enabled the grafting of new inserts where corrosion damage had occurred. Over 10.6 km of wrought iron is used in the glazing bars. Ductile cast iron was used for broken cast iron columns and gutters with the damaged item used as a mould framemwork.
A special system of metal work quality control testing including microstructure, spectrographic chemical analysis, tensile and bend testing was instigated to identify old metal fatigue fractures, and ensure that new restoration work was of a consistent quality.
Due to the doubling in width of the east and west wing in 1868 the structure had become inherently unstable. This problem was solved by inserting a new structural brace in ductile cast iron which integrates the structural support with Turner's original pilasters and the new planting troughs and heating ducts.
Great care was taken to match the imperfections of mid-nineteenth century glass. The glass was specially curved to suit the nine different radii of the roof. All edges were rounded to reduce stress cracking. The end of each pane was lapped and scalloped to assist rain run-off. A special system of structural glazing was used to meet modern safety requirements. There are 8,427 panes of glass; 18.129 m of honed glass edge; 5,973 glass scallop ends and 15.5 km of structural glazing mastic.
A sample cross section of all the original layers of paint showed that it was equivalent in thickness to 22 sheets of photocopying paper. By using advanced modern paint technology, rather than reproduction lead based paints, it was possible to be more environmentally friendly, while at the same time applying a paint system of very low permeability which is vital to the long term protection of metal work in high humidity glasshouses. Six coats of specialist paint were applied in the carefully controlled environment of the off-site paint workshop with only touching up and the application of a final decorative coat on-site.
The performance of the services to the glasshouse were improved and automated without infringing our conservation and restoration criteria. The new elevated fogging and ventilation system was carefully designed to avoid a clash with the original structure. Different plant environments are catered for by the use of computer controlled temperature and humidity sensors.
The completed works include:
- The faithful restoration the Curvilinear Range
- The provision of modern heating, ventilation and humidification services to international horticultural standards
- The provision of a new structure to the rear of the north wall, including a new underground plant room and a new single storey building to house new staff facilities
- Refurbishemnt of the vaults under the east and west wings
- Restoration of the Fern House to the rear of the East Passage
- The relocation of potting sheds, fuel tanks and horticultural bedding material away from the rear of the glasshouses thereby allowing public access around the building and opening up panoramic views from this area over the pond and river Tolka for the first time
The restored Curvilinear Range can once again play its full part in the Irish National Botanic Garden's role as a vital national institution involved with the scientific research, conservation, education and display of plants.