Developing a future repairs strategy for a sandstone city: A petrographic investigation of building stone in Glasgow, Scotland
Introduction
Glasgow is recognised as having some of the finest Victorian architecture in the United Kingdom. As the industrial and commercial capital of Scotland it saw a rapid expansion from the late 18th century, through the 19th century and into the early 20th century, initially from the shipping and merchant trade and latterly through industrial production and manufacturing. Stone was the building material of choice, and the rapid urban growth resulted in a wide range of building types, from utilitarian high density residential blocks (tenements) to elegant terrace and villa developments, schools, churches, public and commercial buildings. By the 1890s Glasgow was known as the ‘Sixth City of Europe’ with some of the finest examples of architecture of its time.
Glasgow is a city characterised by sandstone, dominated by the locally-available pale- or cream-coloured ‘blonde’ sandstones of Carboniferous age. Following the arrival of the railways, ‘red’ sandstone of Permian/Triassic age was used from the mid-1880s, imported from quarries further to the south in Ayrshire and Dumfriesshire. By the early 20th century, red sandstone had become favoured for many public buildings, schools, churches and tenement fronts, and became important in the latter stages of stone construction in the city.
Today, many of Glasgow's buildings are of an age and condition where the masonry is showing signs of decay and requires repair. A legacy of former air pollution from industrial sources and domestic coal burning has accelerated stone decay in many parts of the city. Further significant deterioration of sandstone has been caused by inappropriate intervention and poor quality repairs. Stone cleaning, much of which took place during large-scale public-funded urban regeneration schemes in the second part of the 20th century, has resulted in widespread damage [1], [2]. The proliferation of so-called ‘plastic’ repairs using impervious cement materials applied to the softer porous sandstones, often to stone damaged by stonecleaning, has also resulted in considerable damage. The extensive use of these techniques in the second half of the 20th century has created a situation where many stone buildings in the city are now in need of urgent attention and stone replacement.
This paper describes results of a petrographic investigation of sandstone samples from buildings in Glasgow, undertaken as part of a recent large-scale project which was carried out to assess the future stone masonry skills and the stone materials needed to conserve and repair the stone-built heritage of Glasgow over the next 20 years. It is recognised that a shortage in the availability of stonemasonry skills and training, and a lack of supply of appropriate ‘matching’ stone for repairs is having a detrimental impact on the stone-built heritage of Glasgow, and Scotland in general. To address this issue the project ‘Safeguarding Glasgow's Stone Built Heritage: Skills and Materials Requirements’ was established by the Scottish Stone Liaison Group (SSLG), commissioned by Scottish Enterprise Glasgow. The British Geological Survey (BGS) was appointed to undertake stone condition surveys and petrographic analysis of stone samples, and it is these latter studies that are described in this paper. The full results of the project are published in a detailed report by the SSLG [3].
Section snippets
Materials and Methods
The project involved undertaking condition surveys of a representative sample of sandstone facades, selected in order to represent the range of stone buildings and stone types throughout the city [3]. The volumes of stone requiring replacement and the stonemason time and skills levels required for the repairs were calculated from these data, and extrapolated for the whole city. Characterisation of the types of sandstone required was undertaken by obtaining small samples of stone from selected
Sources of Stone Used for the Construction of Glasgow's Stone-Built Heritage
The blonde sandstone used for the majority of stone buildings in Glasgow was obtained from a large number of quarries within and around the city, and varies widely in character and quality. Approximately 75 known (i.e. named) sandstone quarries of significant size and output were present in the Glasgow district, although there were probably many more smaller scale excavations [6], [7], [8]. As in many other cities, the earliest quarries were located close to the city centre and subsequently
Results: Identification of Sandstone Types from Petrographic Analysis of Samples from Buildings
A total of 126 samples of building stone were collected from 112 surveyed buildings. The samples were selected in order to obtain fresh and unweathered stone where possible. It is not the purpose of the study to investigate the detailed mechanisms of decay occurring in the stone, although some general observations have been made from the petrographic analysis. The objective of the analyses is to characterise the variety of original sandstone types used throughout Glasgow, in order to provide a
Discussion: Supply of Blonde and Red Sandstone for Repairs and the Need to Reopen Quarries
It is apparent that blonde sandstones with a wide range of petrographic characteristics were used for the construction of buildings in Glasgow. This is not surprising given the long time period (several centuries) over which the city developed, and the large number of documented quarries which supplied building stone to the city [7]. There is less petrographic variation in the type of red sandstones which are known to have been supplied from a smaller number of quarries, and over a shorter time
Conclusions
The different types of sandstone used for the construction of Glasgow have been characterised by petrographic analysis of samples from a representative set of buildings throughout the city. The samples have been categorised into ten different types of sandstone (six ‘blonde’ and four ‘red’ sandstone types) in order to create a framework to inform the selection of stone for future repairs. Whilst all of the original sandstone quarries in the Glasgow district and the Scottish central belt are
Acknowledgments
This research forms part of the Scottish Stone Liaison Group ‘Glasgow Project’, funded by Scottish Enterprise Glasgow (Construction Skills Action Plan), with Glasgow City Council, National Heritage Training Group and CITB-Construction Skills. The authors thank Dr Graham Lott (BGS) for reviewing the manuscript, and publish with permission of the Director, British Geological Survey NERC.
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