Penwyllt is a Welsh hamlet located in the upper Swansea Valley, to the east of the Black Mountain.

A former quarrying village and quicklime production centre, its fortunes rose and fell as a result of the industrial revolution within South Wales.

Background
The Geology of South Wales provides it with the basic raw materials structure to be at the centre of the industrial revolution. The large natural coal fields enclose significant deposits of iron ore and limestone, each basic to the production of materials via production methods.

The best coal in South Wales is found in the eastern sections beneath Rhondda Fawr, where the pressure is highest. The greatest deposits of limestone are found above the western section, around the northern section of Swansea Valley where it borders Breconshire. There were also coal deposits below the lime layer, and the coal which lay underground at Clydach, Ystradgynlais, and Abercraf became more valuable as the Industrial Revolution of the Victorian era led to a huge demand for iron and steel, and giving the area prosperity.

Industrialisation
Penwyllt developed primarily as a result of the need for quicklime in the industrial processes in the lower Swansea Valley, taking limestone from the quarries and turning it into quicklime in lime kilns.

Subsequently Penwyllt also supported the Penwyllt Dinas Silica Brick company, which quarried silica sand at Pwll Byfre from which it manufactured refractory bricks, a form of fire brick, at the Penwyllt brick works (closed 1937 or 1939). The bricks were destined for use in industrial furnaces. A narrow gauge railway, with a rope worked incline, transported silica sand and stones to the brickworks, which was adjacent to the Neath and Brecon Railway (which on 1 July 1922 became part of the Great Western Railway).

A detailed account of the history of Penwyllt and its industries is provided by Matthews(1991).

Christie
In 1817, Fforest Fawr (English - Great Forest of Brecon) was enclosed or divided up into fields, and large parts of it became the property of John Christie, a London businessman. Christie had already developed a limestone quarry at Penwyllt, and decided to develop lime kilns there as well. In 1820 he moved to Brecon, and developed the Brecon Forest Tramroad. This network consisted eventually of over 100miles of tracks connecting the farms of Sennybridge and the Fforest Fawr (where Christie wanted to improve the land through application of lime), with the charcoal burning centres and coal extraction below Fforest Fawr, with the lime kiln at Penwyllt and iron works at Ystradgynlais, and the Swansea Canal dock for other industries down stream. Unfortunately, before he could complete the system, he went bankrupt.

Claypon
The Claypon brothers took over Christie's assets, and quickly came to the conclusion that shipping lime, coal, iron ore and quicklime south to the larger industrial premises in the southern Swansea Valley was more productive than trying to create an integrated industry over 100 miles of steep track.

They quickly sold or abandoned the farms and developments north of Fforest Fawr, and concentrated on expanding the lime kilns at or around Penwyllt. In total there were fifteen lime kilns at Penwyllt:

• Penwyllt quarry: two lime kilns created in the railway age by "Jeffreys, Powell and WIlliams", dated 1878
• Pen-y-foel: a bank of four kilns near the Penwyllt Inn erected in around 1863 to 1867 by, it is thought, the Brecon Coal & Lime Co. There is a loading bank for railway wagons in front of the kilns
• Twyn-disgwylfa: Built by Joseph Claypon between 1836 and 1842, the bank of seven kilns has been largely destroyed by quarry tipping. Only one draw arch can now be seen
• Twyn-y-ffald: The 1825 and 1827 kilns built by Joseph Claypon have been largely demolished, although the single draw arch can still be seen

Latter half of the 18th century
On 29 July 1862, an Act of Parliament created the Dulais Valley Mineral Railway, to transport goods to the docks at Briton Ferry, Neath built by Isambard Kingdom Brunel. The population of Penwyllt grew on this increased transport ability to over 500 citizens by the 1881 Census.

After being authorised to extend the railway to Brecon, it changed its name to the Neath and Brecon Railway. The railway agreed to co-operate with the Swansea Vale Railway to create the Swansea Vale and Neath and Brecon Junction Railway linking the railway fully into Neath, as well as the South Wales Railway mainline. An early and unsuccessful purchaser of the new Fairlie locomotive, when in 1863 the railway reached Crynant, coal mining quickly expanded. At Crynant several new mines were opened including the Crynant colliery, Brynteg colliery in 1904, Llwynon colliery in 1905, Dillwyn colliery, and Cefn Coed colliery 1930

The passenger station at Craig-y-nos/Penwyllt was in part funded by opera singer Adelina Patti, who lived at Craig-y-Nos Castle. She built a road from the castle to the station, and a separate waiting room. The railway supplied her in return with her own railway carriage, which she could request to go anywhere within the United Kingdom.

Decline
Penwyllt was created on the back of the industrialisation of the Swansea valley. As the industrialisation declined with reducing economic stocks of coal, iron ore and limestone and the development of new technologies on a larger scale on the coast of South Wales, particularly at Port Talbot and Llanwern, Penwyllt declined.

By 1870 the seven blast furnace ironworks of Ynyscedwyn had only one working furnace. Penwyllt was in decline from the turn of the century, but World War II created the final closure, as the need to scale production upwards for the larger coastal meant the heavily manual process of Penwyllt quarry was uneconomic compared to other British and foreign facilities which could bulk ship by sea.

The Penwyllt Inn, or 'Stump' as it was often known, closed in 1948, and in October 1962 all passenger services were withdrawn by British Railways from Neath and Brecon Railway line. The line north of Craig-y-nos/Penwyllt station closed to Brecon on closure of Brecon station, and by the end of the 1960s the population had fallen to 20 people. The railway line remained open south to Neath until 1977 to serve the quarry, until its closure.

Today
Many of the former industrial buildings, commercial properties and houses of Penwyllt were demolished in the early 1980s, being both beyond economic repair and unneeded. The former pub survives as private accommodation for cavers. The former Craig-y-nos/Penwyllt station survives in good repair as a private holiday cottage.

The only group of former terrace houses now standing are in Powell Street and form the headquarters of the South Wales Caving Club, and the West Brecon Cave Rescue Team

Beneath Penwyllt and the surrounding area is the extensive limestone cave system of Ogof Ffynnon Ddu, part of which was the first designated underground National Nature Reserve in the UK. A corresponding area on the surface is also part of the National Nature Reserve.

The quarry, though not the railway, re-opened in 2007 to provide limestone for the works associated with the new gas pipeline being laid through South Wales.