In the 1700s- 1800s, stone walls were capped with decorative slag coping stones, and blocks were used like ordinary stone blocks in masonry. Examples of these can be seen along the walls by the side entrance to Blaise Museum, by Church Lane.

Slag is a by-product of copper and brass smelting, a formerly common industry in Bristol and the surrounding area. Brass goods in particular were mass-produced locally and traded widely. The growth of the Transatlantic traffic in enslaved Africans and the British brass industry were linked. Brass and copper trade goods were a very important part of the slave ship cargoes bound for Africa, because copper was highly valued in Africa.