Blaenavon Ironworks began production in 1789 with 3 furnaces. Two more furnaces were built c1810. The original partnership, led by the Hill family, sold the works in 1836 to the Blaenavon Iron and Coal Co. Initial investment at the site included the construction of a water balance lift for conveyance of pig iron from the site, but further investment did not occur until the 1850s. A new blast engine was built, and a new furnace was blown in during 1861. Investment was also made on a new site kown as Forgeside. By 1900 only one furnace remained in blast, but this was blown out in 1904. Thereafter the site was used by small-scale engineering companies until it became a guardianship monument in 1974.

Furnaces 4 and 5 were built c1810 and modified c1880. Partly taken down in 1911 when some of the facing stones were given to build the church of St James.

A battered masonry furnace casing 5m high, with exposed core above it of 4m height and a later brick stack approximately 3m high. The front has a round brick casting arch and narrower segmental brick arch above the tap hole. The sides and rear have brick-arched tuyere recesses and a brick-vaulted passage is at the rear. Inside, part of the boshes remain visible above the furnace bear.

R Hayman, Hayman & Horton, 6/1/2003