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.

Of uncertain date as structural evidence suggests it is integral with the adjacent cast house but is not shown on the view of the works dated 1798 and is on the site of the original boiler house. It was altered for use as a foundry after smelting ceased in 1904.

Single-storey foundry of rubble stone with a corrugated iron roof. The front gable end has a central pointed arch with rock-faced voussoirs that obscures former round-headed doorways R and L, which have tooled voissoirs. A similar but surviving doorway is on the L side and retains an impost band. On the R side the wall has been rebuilt in white brick above a wide iron-lintelled doorway opening to a cupola furnace (nprn 67619). The gable is given simple pediment treatment, has a central round-headed opening with bullseye openings R and L.

In the L side wall are 2 blocked wide segmental doorways and a blocked pointed opeing to the centre, all with brick arches. Above are 3 lunettes with brick arches. The rear wall, now facing the charging platform retaining wall but earlier facing a hot-blast stove, has an elliptical-headed doorway with brick arch. It opens to a brick-walled enclosure narrower than the elliptical arch, of white brick up to 2.5m high.

Inside are 3 lunettes in the dividing wall with the cast house. The 6-bay roof has light steel trusses and 2 large cast iron brackets to support cross beams.

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