File:Copper slag as a building material on Goose Green - geograph.org.uk - 1551966.jpg

English: Copper slag as a building material on Goose Green. Copper slag was a by-product of the brass industry, a major industry in the Bristol area in the 18th and 19th centuries. There were brass mills along the Avon between Bristol and Bath. Slag was normally cast in blocks as seen here but was also cast as coping stones as in 706939

"Before the establishment of a British brass industry calamine ore (Zinc) was exported to the Low Countries from mines in the Mendip Hills. By 1720 this ore was being supplied to the Brass Works at Baptist Mills. The Swedish Mine Official, Henric Kahlmeter, (Sweden was worried about the metallurgical advances being made in Britain - this was threatening their export trade) who successfully discovered the industrial processes in use at Baptist Mills reported that in the early 1720s there were 36 furnaces for making brass at the site. He reported that the copper works of Bristol and Redbrook (over the Severn) were the 'most considerable' in England. Ten years later Swedenborg described Baptist Mills as 'the principal place where English brass is made'. During this period around 300 tons of brass was being produce each year from furnaces in 6 Brasshouses. Each of these Brasshouses will have had about 6 furnaces. The Brasshouses were topped with large cones similar to those used in the glass industry."

The above was copied from: http://www.csm.uwe.ac.uk/~rstephen/livingeaston/local_history/brass2.html