Much money was invested in development work, but volumes of useful slate produced were small, amounting to just 226 tons in 1868.

Access to the underground workings was by a single adit, and the surface mill was powered by two water wheels.

In 1895, the quarry reopened under the direction of Moses Kellow, a fearless innovator who set about modernising working practices and methods.

This ceased when the Central Electricity Generating Board became aware of it, and feared that an underground explosion would damage the dams of the Ffestiniog pumped storage power station.

[1] Between 1868 and 1869, Beaumont, Appleby & Ashwell, a company from London, worked on a contract to cut shafts and tunnels.

The work was hampered by water flowing into the mine faster than it could be pumped out, and in 1873 the shareholders declined to advance any more money.

[4] Prior to the opening of the Croesor Tramway, slate was carried by horse and cart to Penrhyndeudraeth station on the Ffestiniog Railway.

In order to improve the journey, a long straight road, which now forms part of the A4085, was built south from Garreg by the quarry company.

When the Croesor Tramway opened in 1864, a long incline was built from the quarry to the valley floor, and carriage by railway reduced transport costs from 7s 10.5d (39p) per ton to 2s 6.5d (12.5p).

Local opinion was that a new tunnel, at the level of the lowest chamber, would need to be cut, and a London-based engineer had calculated that pulsometer pumps could empty it in about six months.

This enabled the mill to be run at full capacity most of the time,[13] with productivity rising by 25 per cent in the first month after it was introduced.

[14] Kellow introduced compressed air drills, which were fed by a small water-powered compressor located near the foot of the incline to the nearby Rhosydd quarry.

Standard British practice at the time was to use 550-volt direct current for such work, but Kellow thought this was too high for distribution underground, and the cost of cabling for a lower voltage system was prohibitive.

He found references to a new system of three-phase alternating current, but was unable to find British manufacturers who thought it was sensible, or who would make equipment to work with it.

He produced a full specification for a three-phase plant in 1902, and toured Europe, talking to electrical manufacturers, who he found more supportive of his plans.

[17] The quarry was lit by electricity, using 2000-candlepower arc lamps, and he installed reversible winches, which were driven by 10 hp (7.5 kW) motors.

[18] Other innovations included a new type of planing machine which he invented, which worked between six and ten times faster than traditional designs.

[11] At the time, percussive drills powered by compressed air were being introduced in many quarries, but Kellow disliked them because they were very noisy, created clouds of dust, and were uncomfortable to the men who had to use them for long periods.

The drill was powered by a small Pelton wheel, supplied with water at 500 psi (34 bar), equivalent to a head of 1,100 feet (340 m).

Next he redesigned the bit, which used a replaceable D-shaped cutter, making it much easier to manufacture than a twist-bit, and this was covered by a patent obtained in 1910.

[22] The adit was worked by three Ruston & Hornsby 4-wheeled diesel mechanical locomotives, originally built in 1939 and 1941, and obtained second-hand.

[23] Several buildings were constructed in the marshalling area at the far end of the adit, and a massive haulage winch partially blocked access to the incline to the levels above.

[24] In 1971, the Central Electricity Generating Board (CEGB), operators of the Ffestiniog pumped storage power station, became aware of this use, and calculated that if there was an explosion underground, either of their two dams could be damaged.

They therefore drained both reservoirs, and Cookes began removing about 250 tons of explosives per week, which were taken to Penrhyndeudraeth or by rail from Blaenau Ffestiniog railway station to various other ICI Nobel works in the United Kingdom.

Planning permission for this was not granted and in the later 1970s most of the remaining mine infrastructure was removed for use in the Oakeley Quarry, also owned by the Ffestiniog Slate Company.

[citation needed] Croesor is connected to the nearby Rhosydd quarry by a tunnel that was built to aid surveying and settle boundary disputes.

Fixed ropes, home-made suspension bridges, zip wires, and inflatable boats have been installed which make it possible to complete the journey in comfort although not necessarily in safety.

[26][27][28] The Ffestiniog region contains five major slate veins, which are on top of a thick layer of feldspathic porphyry, called the "Glanypwll Trap".

When the chambers were used for the storage of explosives, a large amount of masonry was installed to house a haulage winch, and this makes it difficult to access the incline to the upper floors.

[24] Croesor is notable amongst Welsh slate quarries for being almost entirely underground, with no significant surface workings.