Slate Dressing Mill

This is a long twin-mill building, constructed in the late 19th century as part of the Maenofferen quarry, which was established in the early 19th century and later developed by J W Greaves in the 1850s. The complex is an exceptional survival of a largely unaltered and still used late 19th-century slate dressing mill, and has group value with other listed items at this remarkably well-preserved slate quarry.

The building itself is approximately 60 meters long and is built of slate block construction. It features a double slate roof, with the northern range half-hipped to the west and having a corrugated iron gable end to the east; the southern range is half-hipped at both ends. Continuous catslide outshuts extend along the long sides, forming a flat-roofed connecting corridor in the center, linking the two ranges. Plain skylights are positioned along each long side to illuminate the internal bays, and three plain tin louvres are present on the north range. Rail tracks enter through sliding boarded doors at gable ends and the central conjunction of the ranges, allowing for the delivery of quarried slate and the removal of finished products and waste. These tracks run the entire length of the building internally, emerging through similar openings at the opposite end. A locomotive shed extension is located at the western end of the southern range, incorporating a small slate chimney and an entrance on the south return. Various blocked openings are present along the long sides of the mill.

The interior of each range comprises 26 bays, with all original king-post trusses retained. The northern range is currently unused, but contains various late 19th-century Greaves patent rotary dressers (developed in the 1850s and patented in 1886), as well as a B6 slate planer by Turner Bros. of Newtown. The southern range retains its original overhead transmission shafting, which is in use, powering 12 belt-driven Greaves rotary dressers. Thirteen dressing alcoves remain in use on the south side, with most others still present, some featuring slate slab partitions and all with L-shaped wooden stacking benches. The central 13th bay, walled off from the rest, contains the former wheel pit, though the water wheels have long been removed. The mills were originally water-powered, but were converted to electric power around 1906.