Coade stone is an artificial material which is presumed to be the invention of Eleanor Coade and was manufactured by her around 1769-1821 and, after her death, by her works manager William Croggon (1821-1843). The firm went bankrupt in 1843. It is generally considered to be a Georgian product and was commonly used for decorative features on brick built houses, and also for spectacular garden ornaments. It was not the only synthetic stone produced around this time but it was probably the best in quality and certainly the best known. As far as I know it was no longer available by the time of the Victorian Gothic revival.
The intention of the company was to produce a synthetic stone that closely resembled natural limestone. Coade stone was not a low temperature product as was once believed, nor a stone, but was a true ceramic fired at 1150ºC to a cream colour. The reason for its production was purely commercial. The manufacturers believed that they could mould and cast any shape that could be carved in natural stone, and employed many artists and designers for this purpose. Probably the most famous piece is the lion, illustrated above which was once the symbol of the Lion Brewery and which is now on display at the Lambeth side of London’s Westminster Bridge. The brewery itself was demolished to make way for the Festival of Britain.
Coade stone was fired from ‘Coade Clay’. The formula for this was:
Crushed Flint 5-10%
Fine sand 5-10%
Silica lime glass 10%
Ball clay (Devon) to 100%
The kiln firing of pieces took four days. Large castings were broken up into units and subsequently linked with bronze ties. A later switch to corrodible iron pins was predictably disastrous for the long term survivability of the products, but the resistance of Coade stone itself to weather and pollution damage is as good as, if not better, than natural stone. Any further questions you may have on this fascinating material will be answered in a excellent Shire Book on the subject written by Hans van Lemmen.