This was the first ironworks to be built in the Rhymney valley and dates from 1800, established by partners in a Bristol concern. The works lay on the east side of the river, between it and its tributary Nant Melyn, and at a county boundary junction point of Brecon, Monmouth and Glamorgan, after which the works was named. One furnace was constructed, the blast provided by a 20 h.p. Boulton & Watt beam blowing engine. Remains of the furnace survive. The site is considered important as a possible survivor of the first generation of single coke fired water blown blast furnaces in South Wales, a link between the single eighteenth-century charcoal furnaces and the batteries of coke fired steam blown furnaces of the post-1790 period. Nearby is Bute Town Village where terraces of housing were built for ironworkers (NPRN 18180).
The remains of the furnace - essentially its base - lie in a field on the east side of the B5257, located at the foot of a steep west-facing bank. Made of fused grey brick, it stands (in 1985) to a height of 2.3m on the west side, sloping down to ground level on the east. The furnace is 3.5m in diameter, the inside filled with with brickwork, turf covered and with some large stones showing. It is surrounded by fallen rubble and to its west is a convex grass-covered scree area 2.5m wide and 1m high. The bank to the east and north of the furnace has ben cut vertically, and further south there is slag in the scree below the bank.
By 1803 new furnaces were being constructed further down the valley to where the focus of production now shifted (34140 & 421366).
Sources:
L.Ince, The South Wales Iron Industry 1750-1885 (1993), 137.
Cadw scheduling details (NMRW Archive).

RCAHMW, 21 January 2016