Article on Clipsham stone – Natural Stone Specialist, May 2015

I was very surprised to find the rather melodramatic headline, “The stone that has rescued the Houses of Parliament”, to Barry Hunt’s article on Clipsham stone in the May edition of the Natural Stone Specialist, especially as exactly the opposite is true. The introduction of a hard crystalline detrital limestone with low permeability into a much more permeable secondary dolomitic limestone of lower compressive strength is potentially a recipe for disaster.

Fortunately, much of the Clipsham stone introduced into the Palace of Westminster has been for the construction of extensions to the building in some of the courtyards, in order to provide increased accommodation. Only a relatively small quantity of the Lincolnshire limestone has actually been used for repairs to the Anston, and other, magnesian limestones in the original fabric.

Unfortunately, over the years a number of stories regarding the suitability of the stone used by Barry to build the ‘New’ Houses of Parliament, after the disastrous fire of 1834, have become to be regarded as fact. The original stone, selected by the Commissioners appointed by Parliament, was magnesian limestone from Bolsover Moor. Unfortunately the rate of extraction of acceptable stone of suitable bed height could not be maintained and production was switched to Mansfield Woodhouse. Once again the bed height was inadequate. By the time the walls of the new building were about four to five metres in height in 1843, only three years after the foundation stone had been laid, all the external stone was being supplied from the Anston area, being transported from the site using the Chesterfield canal.

This source finally fulfilled one of the main criteria for the supply of stone: that was a source of stone of acceptable bed height that could be supplied at a rate which matched the planned speed of construction. A record from August 1843 indicates that there were 300 workers in the quarries, with stone being supplied from Anston and Caen. It is not clear whether or not the quarry workmen in Caen are included in this figure.

Any problems which were encountered with the magnesian limestone were almost certainly a result of the high demand for the stone, at a time when extraction was largely manual and labour intensive. The geology of the Anston area is also an important factor. Rather than a series of regular beds of magnesian limestone, the strata can consist of a number of patch reefs, or bioherms, separated by areas of variably bedded stone of varying bed heights. These strata, formed in a shallow near-shore environment were, before dolomitisation, petrographically variable. In addition, thin beds of marl are not uncommon, this clayey material also being present as very thin films within some of the limestone.

As a result, although perfectly acceptable building stone is available in a range of bed heights, its quality can be variable. Careful selection of the stone is therefore essential. An appraisal of the stonework of the Palace suggests that this selection was not always achieved, almost certainly due to the pressures placed on those employed in constructing the new building to finish the work as speedily as possible.

Despite often repeated comments that the magnesian limestone was not suitable for use in the polluted atmosphere of London, the majority of the stonework has survived extremely well. It appears likely that the most damaged stonework involving the Anston stone is where softer material has been used for carved work, where poor stone selection has allowed flawed stone to be inserted in the building and where the stone has been incorrectly laid. The quality of properly selected Anston stone was demonstrated by Sir Henry de la Beche, the first Director of the Geological Survey, who was involved in selecting the stone for the Palace of Westminster, when he used it for the new Museum of Practical Geology on Jermyn Street.

Magnesian limestones have been used perfectly successfully in polluted atmospheres in London and elsewhere (Mansfield and Doncaster, for example). Petrographic and electron microscope studies have shown that one reason for this durability may be the thin layer of crystalline gypsum which can develop on the surface of the stone, protecting it from further reaction.

Where an historic building is constructed from magnesian limestone there is no reason at all why it should not be repaired and conserved with a similar stone. In fact, to attempt to repair such a building with a pure limestone, even one of the highest quality such as Clipsham, is completely unacceptable. Potential chemical reactions between magnesian and non-magnesian limestones are well known, including in Westminster, where Pearson’s use of Ketton stone in Westminster Hall, beneath the cladding of Huddleston magnesian limestone used by Smirke in the mid-19^th century, caused degradation of the former well into the 20^th century.

As well as the chemistry and mineralogy of the stone used for repairs to historic buildings, factors such as porosity, permeability and mechanical strength have also to be taken into account. The various factors to be considered when repairing such buildings are discussed in the Technical Advice Note published by what was then English Heritage (now Historic England) in 2006 entitled Identifying and Sourcing Stone for Historic Building Repair – An approach to determining and obtaining compatible replacement stone.

Any repairs and conservation work to the the original 19^th century fabric of the Palace of Westminster should, therefore, be using carefully selected magnesian limestone which is strictly compatible with, and preferably related to, the original stone from South Yorkshire. Where such repairs are in the immediate vicinity of a more recent non-magnesian limestone repair, consideration should be given to removing that incorrect stone and replacing it with magnesian limestone. However, where repairs are required to more recent additions to the building, which have been built with Clipsham stone, carefully selected Clipsham stone should obviously be used if an acceptable quality and bed height is available.

Yours sincerely

Dr David Jefferson
B.Sc.(Hons), Ph.D., C.Eng., F.G.S., F.I.Q., M.I.M.M.M.
Jefferson Consulting
Leicestershire