The stonework
The Palace of Westminster was built using a sand-coloured limestone from the Anston Quarry in South Yorkshire. In 1839, a committee including the architect Charles Barry, two leading geologists and a stone carver toured Great Britain looking at quarries and buildings. Stone from Bolsover Moor in Derbyshire was selected as first choice from 102 stones, and it was used for the lower 4 or 5 meters of stone walls on the river front. The stone was not used beyond this height because it was considered not up to standard, and stone from nearby quarries around Mansfield Woodhouse in Nottinghamshire was used instead. This stone was mainly available in thinly bedded courses and not enough was available for such a large building, so Anston was used instead.
Decay
However, the Anston soon began to decay as a result of atmospheric pollution from coal burning in London, a lack of weathering at the quarry, and sometimes from the laying of the blocks without maintaining their natural bedding plane. Although defects in the stonework were visible as early as 1849, very little was done to prevent its decline during the 19th century. Barry himself experimented with various compositions on the stone and believed that the decay had been halted.
Restoration
During the 1920s, it was clear that something had to be done, especially when a large fragment fell off the Victoria Tower and members on the Terrace were advised to sit near the river rather than underneath the main wall of the building. In 1928, it was deemed necessary to use Clipsham stone, a honey-coloured limestone from the Medwells Quarry in Rutland, to replace the decayed Anston. A restoration project began in the 1930s, but it was brought to a halt during the Second World War and was completed only in 1960.
The effects of these repair works and the addition of new stone nevertheless began to make the Palace appear like a patchwork quilt. By the 1960s, questions about it were being asked in the House of Commons.
Cleaning
By the 1970s, the effects of pollution were again visible, and a new programme of stone-cleaning and restoration was started in 1981: the north, west, and south fronts, the river front and the Clock Tower were completed by 1986. The Victoria Tower, whose cleaning was completed in 1994, was the last part of the exterior to be dealt with. Of the inner courts, the Speaker's Court was the first to be tackled, with the work beginning in January 1994.