Slate has been mined in north Wales for several centuries — this was recently confirmed by the discovery in the Menai Strait of the wreck of a 16th century wooden ship carrying finished slates.
Large-scale commercial slate mining in North Wales began with the opening of the Cae Braich y Cafn quarry, later to become the Penrhyn Quarry near Bethesda in the Ogwen Valley in 1782. Welsh output was far ahead of other areas and by 1882, 92% of Britain's production was from Wales (451,000t): the quarries at Penrhyn and Dinorwic Quarry produced half of this between them.
The men worked the slate in partnerships of four, six or eight and these were known as "Bargain Gangs". "Bargains" were let by the "Bargain Letter" when a price for a certain area of rock was agreed. Adjustments were made according to the quality of the slate and the proportion of 'bad' rock. The first Monday of every month was "Bargain Letting Day" when these agreements were made between men and management. Half the partners worked the quarry face and the others were in the dressing sheds producing the finished slates. In the Glyndyfrdwy mines at Moel Fferna each bargain worked a horizontal stretch of 10 by 15 yards. Duchesses, Marchionesses, countesses, Viscountesses, Ladies and Small Ladies, Doubles and Randoms were all sizes of slates produced.
Rubblers helped to keep the chambers free from waste: one ton of saleable slate could produce up to 30 tons of waste. It is the mountainous heaps of this very same waste that is perhaps the first thing to strike someone visiting the old regions nowadays. The men had to pay for their ropes and chains, for tools and for services such as sharpening and repairing. Subs (advances) were paid every week, everything being settled up on the "Day of the Big Pay". If conditions had not been good, the men could end up owing the management money. At Moel Fferna a team could produce up to 35 tons of finished slate a week. In 1877 they received about 7 shillings a ton for this. After paying wages for the manager, clerks and 'trammers' the company could make a clear profit of twice this amount. This system was not finally abolished until after the Second World War.
Early workings tended to be in surface pits, but as the work progressed downwards, it became necessary to work underground. This was often accompanied by the driving of one or more adits to gain direct access to a Level. In some rare instances, such as Moel Fferna, there is no trace of surface workings and the workings were entiely underground.
Chambers were usually driven from the bottom, by means of a "roofing shaft" which was then continued across the width of the chamber: the chamber would then be worked downwards. Slate was freed from the rockface by blasting in shot holes hammered (and later drilled) into the rock.
Slate would be recovered from the chamber in the form of a large slab, which would be taken by truck to the mill where it would be split and cut into standard-sized roofing slates.
Slate mines were usually worked in chambers which followed the slate vein, connected via a series of horizontal "Floors" (or "Levels"). The chambers varied in size between mines and were divided by "pillars" or walls which supported the roof. The floors were connected by underground "Inclines" which used wedge-shaped trolleys to move trucks between levels.
In some mines, where slate was worked away below the main haulage floor, the route was maintained through the construction of a wooden bridge across the chamber, often supported from chains attached to the roof above. These bridges could be as much as 100 feet/30 m above the floor below.
In the Blaenau Ffestiniog area, most of the workings were underground as the slate veins are steeply angled and open cast workings would require the removal of a massive amount of rock to gain access to the slate. The larger mines in the Ffestiniog area include:
There were also a number of slate mines in the Llangollen area which produced a much darker "black" slate:
Another cluster of mines were found in mid Wales centered on Corris. These all worked a pair of slate veins that ran across the Cambrian mountain range from Bryn Eglwys in the west through Corris and Aberllefenni in the Dulas Valley to the mines around Dinas Mawddwy in the east.
Most underground slate mines in north Wales were closed by the 1960s although some open-cast quarries have remained open, including the Penrhyn Quarry and the untopping work at Oakeley in Blaenau Ffestiniog. Work also continues at Berwyn near Llangollen. The final large-scale underground working to close was Maenofferen Quarry (which is owned by the Llechwedd tourist mine) in 1999 although opencast quarrying continues at this location.
Many of the mines are now in a state of considerable decay and those that are accessible should not be entered as they are on private property and contain many hidden dangers. The lower levels of many mines are now flooded and collapses are commonplace, for example the hillside above the Rhosydd workings has many pits where the roofs of the chambers below have collapsed.
During the last 500 years, much slate extraction has taken place the Lake District at both surface quarries and underground mines. The major workings are:
The slate industry in the United States is concentrated in the Taconic Mountains region of Vermont and New York, known as the Slate Valley. Slate was first quarried in 1839 at Fair Haven, Vermont. An influx of immigrants from the North Wales slate quarrying communities saw a boom in slate production that peaked between the late 1850s and early 1900's. The slate of the region comes in a variety of colors, notably green, gray, black and red. Some production continued in 2003 with 23 operating full-time mines employing 348 people
Large scale slate quarrying also took place around the town of Monson, Maine where an extensive series of quarries flourished from the 1860s onwards. A small scale quarrying and dressing operation continues in Monson into the 21st. century.