Portland stone, Bath stone, Sandstone: Used in building.

Monday, February 7, 2011

Portland stone.
Portland stone is quarried in Portland Islands on the coast of Dorset. There were extensive beds of this stone which is creamy white in colour, weathers well and used to be particularly popular for walling for larger huildings in towns. Many large buildings have been built in Portland stone because an adequate supply of large stone was available, the stone is fine grained and delicate mouldings can be cut on it and it weathers well even in industrial atmospheres.

Among the buildings constructed with ihis stone are the great banqueting hall in Whitehall (1639), St Paul’s Cathedral (1676), the British Museum (1753) and Somerset House (1776). More recently, many large buildings have been faced with this stone.

In the Portland stone quarries are three distinct beds of the stone, the base bed, the whit bed and the roach. The base bed is a fine, even grained stone which is used for both external and internal work to be finished with delicate mouldings and enrichment. The whit bed is a hard, fairly fine grained stone which weathers particularly well, even in towns whose atmosphere is heavily polluted with soot and it was extensively used as a facing material for large buildings.

The roach is a tough, coarse grained stone which has principally been used for marine construction such as piers and Iighthouses.

The stones from the different beds of Portland limestone look alike to the layman. It is sometimes difficult for even the trained stonemason to distinguish base bed from whit bed. Roach can be distinguished by its coarse grain and by the remains of fossil shells embedded in it. When taken from the quarry the stone is moist and comparatively soft, but gradually hardens as moisture (quarry sap) dries out.

Bath stone.
Many of the buildings in the town of Bath were built with a limestone quarried around the town. This limestone is one of the great oolites and a similar stone was also quarried in Oxfordshire. Bath stone from the Tayton (Oxfordshire) quarry was extensively used in the construction of the early colleges in Oxford (St Johns, for example) during the twelfth, thirteenth and fourteenth centuries. Many of the permanent buildings in Wíltshire and Oxfordshire were built of this stone, which vanes from fine grained to coarse grained in texture and light cream to buff in colour. Most of the original quarries are no longer being worked.

The durability of Bath stone vanes considerably. Sorne early buildings constructed with this stone are well preserved to this day, but others have so decayed over the years and been so extensively repaired that little of the original stone remains. Extensive repair of the Bath stone fabric of several of the colleges in Oxford has been carried out and continuing repair is necessary.

Sandstone was formed from particles of rock broken down over thousands of years by the action of wind and ram. The particles were washed into and settled to the beds of lakes and seas in combination with clay, lime and magnesia and gradually compressed into strata of sandstone rock. The particles of sandstone are practically indestructible and the hardness and resistance to the weather of this stone depends on the composition of the minerais binding the particles of sand. If the sand particles are bound with lime the stone often does not weather weIl as the soluble lime dissolves and the stone disintegrates. The material binding the sand particles should be insoluble and crystalline. Sandstones are generally coarse grained and cannot be worked to fine mouldings.

The stratification of most sandstones is visible as fairly close spaced divisions in the sandy mass of the stone. It is essential that this type of stone be laid on its natural bed in walls. 

Most sandstones have been quarried in the northern counties of England where for centuries this stone has been the material commonly used for the walls of buildings. Sorne of the sandstones that have been used are:

Crosland Hill (Yorkshire). A light brown sandstone of great strength which weathers well and is used for masonry walls as a facing material and for engineering works. It is one of the stones known as hard York stone, a general term used to embrace any hard sandstone not necessarily quarried in Yorkshire.
Blaxter stone (Northumberland). A hard, creamy coloured stone used for wall and as a facing.
Doddington (Northumberland). A hard, pink stone used for walling.

Darley Dale (Derbyshire). A hard, durable stone of great strength much used for erigineering works and as walling. It is hard to work and generally used in plain, unornamented wall. Buff and white varieties of this stone were quarried.

Forest of Dean (Gloucestershire). A hard, durable, grey or blue grey stone which is hard to work but weathers welI as masonry walling.

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