Historically, Portland stone has been the most widely used of the Jurassic limestones of Britain. It is arguably the principal building stone of London, at least since the post-medieval period, and became popular in the 17th century following the demise of timber as the predominant construction material, particularly after the Great Fire of Many of the most significant public buildings in London have been constructed or faced in Portland stone, from churches to government buildings.
A list of such buildings in the capital would encompass important streetscapes, notably in Whitehall, Trafalgar Square, and the City, as well as a number of national museums and galleries. Fully understanding the properties of Portland stone and following best practice in its repair and maintenance is clearly important to the long term care and protection of a large number of significant historic buildings.
Public buildings typically feature some decorative stonework and sculpture in their design and conservation planning needs to take account of these elements, which might require specialist input. The source of this stone, the Isle of Portland, is a key part of the Dorset and East Devon Jurassic Coast, which has been accorded the status of a world heritage site in recognition of the significance of its landscape and geology. Evidence of past quarrying activity is ubiquitous on Portland.
The site benefitted not only from the quality of its stone, but also from ease of transport due to its proximity to the sea. Portland is a relatively pure limestone containing, on average, 95 per cent calcium carbonate. It is creamy white in colour, weathering to a duller greyish brown tone and often to darker brown in localised areas where water penetration occurs through neglect.
It is an oolitic limestone but the ooliths spheres are not readily visible. Fossils are prominent in most Portland stone and, as natural weathering occurs, the harder fossils tend to stand proud of the surrounding matrix. The bed with the most obvious fossils is Portland Roach, which has large shell fossils and bears the concave imprints of those which have been lost.
