Forge Slope Colliery

Overview

One of the slopes driven into the hillside at Blaenavon to feed the ironworks, opened in 1878 and worked by the Blaenavon company until June 1931. The returns then stop counting it separately: from 1933 its people are included in Big Pit.

History

Coal was worked at Blaenavon to supply the ironworks from about 1800, at first by levels and drifts driven into the hillsides along the outcropping seams. Forge Level was driven in 1812, Mine Slope sunk about 1840, and there was a Dodd's Slope as well. Forge Slope, opened in 1878, belongs to the later part of that sequence. It was never a large colliery. The returns give 124 employed in 1894, 123 in 1896, 136 in 1900 and 149 in 1910, working the Horn seam and later the Rock. Gas coal was the product until 1910, when coking, gas, household and steam coal are all returned. The 1923 figure of 741 is much larger and reflects a reorganised undertaking rather than a sudden expansion of the slope itself. The colliery closed in June 1931, and from 1933 the official returns show it with nobody employed and the note 'inc. in Big Pit'; by 1937 the note reads 'inc. in Blaenavon'. That is the pattern of this coalfield: the separate levels and slopes were gradually absorbed into the big shaft. Big Pit itself had been developed from 1860 on the site of the earlier Kearsley Pit, and took in Mine Slope, Forge Level and the shafts of the neighbouring Coity Pit as it grew. Forgeside, the works these pits fed, has a claim on the history of steel. In December 1877, the year before Forge Slope opened, the site was used for the larger-scale experiments by Percy Carlyle Gilchrist and Sydney Gilchrist Thomas that succeeded in using a basic lining to convert iron made from phosphoric ores into steel — something the Bessemer process could not do.

Geology

The outcropping seams of the eastern South Wales coalfield, worked into the hillside rather than down to. The returns name the Horn seam to 1900 and the Rock by 1910.

Workings

A slope driven into the hillside, in the manner of the Blaenavon levels and drifts before it. Almost the whole workforce was underground — 117 of 124 in 1894 and 142 of 149 in 1910 — which is what a drift without winding gear looks like in the returns.

Closure

Closed in June 1931. The 1933 return records nobody employed and the working included in Big Pit; by 1937 the note gives it as included in Blaenavon.

What remains

No account of the slope itself as it now stands has been found. The Forgeside area is part of the Blaenavon Industrial Landscape World Heritage Site, and the ground around Big Pit survives as an industrial landscape with considerable buried archaeology; the Forgeside ironworks site itself was cleared and landscaped.

None recorded for the slope.

Related mines

• Big Pit — Forge Slope's workforce was returned as included in Big Pit from 1933, two years after it closed; Big Pit absorbed the older Blaenavon levels and slopes as it grew.
• Blaenavon Ironworks — The slope existed to feed the Blaenavon works, and the Forgeside plant it served is where the basic steel process was proved in 1877.

Nearby mines

• Big Pit — 0.2 km W
• Blaenavon Ironworks — 1.0 km NE
• Cinder Pits — 1.1 km E
• Milfraen Colliery — 2.5 km NW
• Marine Colliery — 3.1 km SW, Blaenau Gwent

Straight-line distance, within 15 km. Show on the map.

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• The official returns give Forge Slope Colliery as opened in 1878 and closed in June 1931, owned by the Blaenavon Iron and Steel Co., Ltd. and then the Blaenavon Co., Ltd. Employment was 124 in 1894, 123 in 1896, 136 in 1900, 149 in 1910 and 741 in 1923; the Horn seam is named to 1900 and the Rock in 1910. ¹
• From 1933 the returns record nobody employed at Forge Slope and the note 'inc. In Big Pit'; the 1937 return gives 'inc. In Blaenavon'. ¹
• Coal working to supply the Blaenavon Ironworks dates from around 1800, initially by means of levels or drifts following the outcropping seams dug into the hillsides. Examples include Forge Level, driven in 1812, Mine Slope, sunk about 1840, and Dodd's Slope. Big Pit was developed from 1860 on the site of an earlier mine, Kearsley Pit, and other workings were later incorporated such as Mine Slope and Forge Level and the shafts of the neighbouring Coity Pit. ²
• In December 1877 the site at Forgeside was used for larger scale experiments which succeeded in using a basic lining to convert iron made from phosphoric ores into steel, previously unachievable using the Bessemer process. The work was associated with Percy Carlyle Gilchrist and Sydney Gilchrist Thomas. ²

Photographs & further reading

Sources

1. (secondary) Forge Slope Colliery — Durham Mining Museum
2. (primary) Forgeside and Big Pit — Historic Landscape Character Area — Heneb (Glamorgan-Gwent Archaeological Trust)

Record created: 11 September 2026 · Last researched: 11 September 2026