I have consulted the Penrhyn Quarry Railway Society's historian who made the following coments:
"At Penrhyn Quarry, as David Gwyn has indicated, the mills at Cilgeraint were being re-equipped between 1866 and 1868. On 3rd September 1867, the Hunter Saw was commissioned. This was a frightening circular saw with two plates, cutting slabs of slate at two points at the same time. The blades were above the slab and its table, and were almost 0.8 metres in diameter, with 2.5 cm round cutters on the tips, which made a series of perforations in the slab. Once installed, it is still easy to identify slate cut with this machine by the series of grooves at the edge, similar to a postage stamp edge.
Within the quarry itself, work was most concentrated in the pit area, served by 5 Water Balances in 1867, to a depth of 271 feet at a maximum from the main Red Lion Level.
At about this level, a band of rare dark red-coloured slate had been found, and this was being extracted as quickly as possible, as it was very popular, despite its price. However with hindsight, this was a huge mistake, as it undermined the back face of the excavation, which all fell down dramatically on Sunday 7th July 1872, "The Great Fall", which almost closed the quarry, as over 2 million tons of good slate is estimated to have fallen in a single slide. Fortunately the quarry was empty at the time. Red Slate has never since been found.
For the quarrymen, their wages had been reduced to save money, as the quarry was suffering from the effects of the failure of the Bank of Overend and Gurney in London in May 1866, which had, like now, a wide range of effects on trading during the succeeding years throughout Great Britain and its Commonwealth.
Indeed in January 1867, the men (who were paid every fourth week) could not be paid on time, owing to a lack of currency at the local Bank in Bangor. The men eventually got their pay, including what was owed, in the due payment in late February.
This situation was made worse because between January & March there was a lot of thick snowfalls, restricting all work on the rock faces, and indeed water for the Balances. To keep the men busy, they were employed on building long lines of small sheds on Red Lion Level, concentrating the splitting and shaping of the slates into one area,, which previously had been scattered throughout the quarry. This site was already where the main offices were sited, thus assisting in the management of this aspect of quarrying.
To gain the rock, the men used gunpowder (Dynamite was invented in 1867, but was not yet on the market) but the drilling of the holes for blasting was still carried out manually, - an unsuccessful trial for doing this was carried out in 1869.
There is a book, "Treatise on Slate and Slate Quarrying" by David Christopher Davies initially published in 1878, which is an excellent guide to the quarrying practices in Wales at this period. It is available from Amazon, I believe."
Penrhynman