A steam-powered longitudinal mill, some 87m by 14m, situated on Floor 6 of Diffwys quarry, near the head of an uphaulage inclined plane from underground workings and a counter-balance to take finished slates down to the exit railway. The earliest phase of the mill (its central section) dates from the late 1850s or early 1860s, and it was subsequently extended to the west in the 1880s and east between 1892 and 1899. Its location meant that water-power was not an option. The whole of the mill became disused in 1924 and was scrapped in 1956. It is one of the earliest ‘integrated’ mills – ones that include all the processes for producing roofing slate and it survives as a roofless rectangular building. Its ruins crown the skyline. 

The original central part is constructed of un-sawn slate blocks with the additions in circular-sawn blocks. An extension to the south provided roofed cover for a trimming waste railway and attached to the north are an engine and boiler house (the steam engine was replaced by two 15hp electric motors in 1913), chimney and smithy (a later addition by c 1900) that contains two hearths and which is attached to the earlier drum-house for the counter-balance incline. Within the central and eastern section of the mill are stelinau (working platforms) for the initial reduction of blocks and a series of waste chutes. During operation the mill is thought to have contained at least 10 circular saws and guillotine dressers.  

Though no machinery survives, the process flow and the means by which slate blocks were processed are evident.  

Statement of Significance:

The Diffwys Slate Quarry mill and its chimney is a prominent sky-line feature above the town of Blaenau Ffestiniog. Located on the ridge which forms floor 6 of the quarry where there was no water to power machinery, it is an early example of a steam-powered mill in the slate industry of north-west Wales, and is also one of the first ‘integrated’ mills – geared to the production of roofing slates, rather than slate-slabs, by incorporating the un-mechanised splitting process within the building itself

This site is part of the Slate Landscape of Northwest Wales World Heritage Site, Component Part 5: Ffestiniog: its slate mines and quarries, slate town and railway to Porthmadog. Inscribed July 2020.

Sources:  

• Louise Barker & Dr David Gwyn, March 2018. Slate Landscapes of North-West Wales World Heritage Bid Statements of Significance. (Unpublished Report: Project 401b for Gwynedd Archaeological Trust) 
• Tirwedd Llechi Gogledd Orllewin Cymru / The Slate Landscape of Northwest Wales. Nomination as a World heritage Site (Nomination Document, January 2020) 
• Wales Slate World Heritage Site https://www.llechi.cymru/   

Hannah Genders Boyd, RCAHMW, January 2022