The History of Quarrying -  The Slate Industry of Ffestiniog

THE SLATE INDUSTRY

OF

FESTINOG
(N. Wales)

With the Compliments of

The Associated Slate Quarries

 The Slate Industry of North Wales

The Festiniog Slate Quarries

Composite bird's-eye view of a Festiniog Slate Quarry (diagrammatic section), showing order of slate veins.

Although commonly spoken of as "slate quarries," the Festiniog [Ffestiniog] group in Merionethshire are actually slate mines worked on the underground principle of alternate openings, locally termed chambers, and walls or supporting pillars of solid slate. In this they differ from their sister workings in Caernarvonshire, which are mostly of the open kind and consist of a series of terraced steps ; somewhat analogous in lay-out to the old Roman amphitheatres.

The dip of the slate veins at Festiniog [Ffestiniog], at an average of 30° from the horizontal, in hilly country, necessitates the winning of the slate rock by burrowing below the surface in order to follow the course of the slate-producing strata, and some of the lowest workings are as much as 1,500 ft. or more down. As will be readily understood, the outlay in driving the adits, or as is more usual to-day, in sinking inclines on the angle of dip of the slate vein, and horizontal levels at 50 to 60 ft. vertical intervals, from which to open out the chambers where the slate blocks are produced, together with the accompanying cost of haulage, compressed air equipment for drilling, and pumping to keep the workings clear of water, is very great. There are advantages and disadvantages in underground quarrying as compared with open working. On the one hand, the open slate quarries are liable to stoppage or to having their operations slowed down in bad weather by snow, frost and rain, and suffer from the necessity to remove heavy " overburden " ; on the other hand, having once removed the overburden, they are able to win 100 per cent. of the slate rock. Whereas, in the Festiniog [Ffestiniog] slate mines one must have 40 per cent. to 50 per cent. of exploitable slate ungotten in the pillars supporting the mine, though, to counterbalance this the total tonnage mined is very much less for every ton of actual slate produced, and inferior rock need not always be removed.

THE SLATE-BEARING STRATA

The slate-bearing strata of Festiniog [Ffestiniog] belong to the Ordovician or Lower Silurian systems, and are of an uniform soft blue-grey colour, although the North Vein or topmost bed shows a tendency to be slightly darker. The chemical analysis of a standard Festiniog [Ffestiniog] slate shows silica 55 per cent., alumina 25 per cent., iron oxide 11 per cent., the remainder being composed of various elements of which the chief is magnesia. The lime contents - an excess of which often characterises an inferior slate - are negligible.

Entrance to underground workings, showing slate chamber and pillar left in position to support the superimposed mountain.

The finely-developed cleavage allows the slate rock to be split into amazingly thin laminae by hand-for instance, a strip only 1/6 ins. thick can be split into 26 strips, a remarkable tour de force and example of extreme manual craftman-ship. A unique feature of Festiniog [Ffestiniog] slate is its resilience, which permits of one of these strips 3 ft. long or more, only 1/20-in. thick, being bent this way and that as a similar strip of steel. Indeed, if it were possible to obtain delicate enough tools, the slate rock could be split even thinner. Clever quarrymen make fans out of a solid piece of rock, beautifully turned out in fretwork designs, capable of opening and shutting like the real article. Such properties yield a roofing slate of unique elasticity and strength, and it is the possession of these two qualities, combined with chemical inertness and its inability to absorb moisture, which enables the Festiniog [Ffestiniog] slate to yield a roofing material of ample strength and great durability when split to a thickness of no more than 1/6 in. This is borne out by the following figures, the result of a number of tests :-

and a Festiniog [Ffestiniog] slate, 12 ins. wide, 1/6 in. thick, placed on supports 22 ins. apart, load applied at centre, yielded :-

 A Festiniog [Ffestiniog] slate is also practically non-absorbent, as, after being dried and then immersed for 24 hours, showed the remarkably low figure of 0.016 per cent. absorption by weight.

The several slate deposits worked at Festiniog [Ffestiniog] are of considerable thickness, often from 150 ft. up to as much as 300 ft., measured on the horizontal, inter-stratified with beds of chert and other rocks of superior hardness to the workable slate. These slate veins or beds are known in their ascending order as the New or Deep Vein, the Old Vein, Back or Middle Vein, and North Vein. In addition, there is the Manod Vein under the New or Deep Vein again, but this, at present, is only being worked to quite a small extent.

Quarrymen drilling underground on the cleavage or bedding plane with a pneumatic hammer drill for insertion of explosive charge.

MINING THE SLATE

The following description of Festiniog [Ffestiniog] slate mining methods, evolved through several generations, gives a clear idea of the standard practice adopted and followed to-day in Festiniog [Ffestiniog], and was written by Mr. E. Andrewes, A.M.Inst.C.E., for The Sheffield University Mining Magazine.

" The normal process of opening up and working one of the veins has been as follows. Access is first obtained by means of an adit level which, if circumstances permit, would in the first instance be driven under the overlying hard of the vein. Otherwise it will be driven to reach the hard and then continued as a drift along it. From this level chambers are opened in an upward direction, at appropriate intervals under the hard. To appreciate fully the process of opening and working the chambers, it is necessary first to detail some features of the slate rock. That essential feature, the cleavage, approximately coincides in strike with the bedding of the slate, but has a somewhat steeper dip, so that the cleavage planes cut the hard at a slight angle. An equally constant feature of the Festiniog [Ffestiniog] slate beds is what is known as the ' pleriad ' or ' pillaring.' This consists in a tendency in the rock to rend with comparative readiness and regularity along a plane at right angles to the cleavage and with vertical dip. The slate is also cut by frequent joints, which lie at various angles, but some of the principal of which, and the most helpful in quarrying operations, strike and dip approximately at right-angles to the bedding cleavage. All these attributes of the slate rock are made use of in quarrying.

" In opening a chamber, the first operation is to drive in the slate what is known as a roofing shaft upwards under the hard, following the direction of the pillaring. The roofing shaft having been driven a certain distance, the next operation, called widening, is the cutting out of the top layer of the slate for the full width of the chamber. The whole process so far is in the nature of development work, and is carried on by men classed as miners, who use some form of dynamite as an explosive.

" These are now followed by rockmen, who carry out the actual quarrying operations, and use gunpowder for charging the holes. Their function is to quarry the rock in blocks suitable for working up in the slate mills, with as little waste and injury to the good rock as possible. They must begin by working a ' free side ' to the rock. This is done by cutting a vertical slice on the right-hand side of the chamber, unless some natural feature of the rock offers an advantageous opportunity of doing it elsewhere. Having achieved a free side, it is necessary also to obtain a free end to the rock before a block can be quarried. Convenient joints may supply this, but as a rule a certain amount of cutting away of rock, in the -form of a trench traversing the width of the chamber, has to be done. Where the rock is ' large ' (i.e., has no joints) near the foot of a chamber, a channeller is used for cutting free bottom.

" The quarrying of a block can now be commenced. This is accomplished by driving a splitting hole along the cleavage, and a pillaring hole at right-angles to it, and firing them in turn with a suitable charge of gunpowder. The holes are placed about half-way between the free bottom and the next joint. The latter may be anything from a couple of feet to twenty feet or more from the free bottom, but a single hole properly charged will ' run ' for the whole of such a distance, and if the cleavage and pillaring are good, very clean and regular blocks are produced. It will be readily understood that there is scope for a considerable degree of skill and judgment on the part of the rockman in placing and charging his holes. 

" When one chamber has been worked far enough forward to clear the level, the latter is driven further forward and another chamber started in due course. The distance normally allowed, measuring at right-angles to the line of pillaring, for a chamber and pillar is 85 ft. Of this width, 45 ft. would be chamber and 40 ft. pillar. But in actual practice there is a good deal of variation, which may be due either to the roof not being solid enough to permit of a chamber being opened to the full width of 45 ft. or to a fault or other irregularity in the ground making it inadvisable to open a chamber in a certain position.

" We have so far dealt with the opening and working of a single level or floor only, but as soon as such a floor is fairly launched and has a chamber or two at work, steps would be taken to open a floor below it. This, in the initial stage of development of a quarry, was sometimes done by driving another adit level from the surface, but the days are long past when such a course was feasible, and the new level has to be reached by sinking.

" The method adopted has been to sink an incline shaft under the chert overlying the Narrow Vein. A floor is then opened on the new level in the same manner as described above, care being taken that the chambers on this floor line up with those above. The process is continued downward on succeeding floors, the final result being, as regards any particular chamber that, where conditions permit, the whole of the slate rock is removed, and it becomes one continuous excavation from floor to floor. The quarry, in fact, becomes a series of alternate chambers and pillars, or walls. 

" Where more than one slate vein is worked in the same area, the work goes on simultaneously in each vein, the walls and chambers in each vein being kept in alignment and superimposed. This, of course, entails careful surveying, but is a very essential feature of sound working. All the principal slate mines in Festiniog [Ffestiniog] have suffered to some extent from falls (subsidences on a large scale), which in some cases have had disastrous results as regards the future working of the quarries. These falls have been caused chiefly by careless and unsystematic working in past years. Chambers have been made too wide, where profitable slate was to be found, and pillars in some cases well-nigh obliterated. The stability of the ground has further often been adversely affected by total lack of care as to the placing of the chambers and pillars in one vein vertically above and in alignment with those in a vein above or below it."

A 20-ton block of solid slate rock.

The slate blocks won in the rough are next hauled to the surface, often by electric or mechanical traction on the flat, and up the inclined haulages, whence they are transported similarly to the mills, and, after being first split into convenient slabs, usually 3 to 4 ins. thick, are placed on circular saw tables where they are reduced to dimensions suitable for man-handling by the splitter. Having been split to the appropriate thickness for roofing purposes, the still unshaped slate is passed to the " dresser," who puts it though a power-driven revolving knife dressing machine, from which it emerges in the true rectangular form so well known to the trade and public, and characteristic of English roofing slate practice. Variations in size are obtained by means of a graduated gauge on the dressing machine.

Slate mining and production is thus seen to be a most highly-skilled occupation, with a special technique of its own, and one which is largely hereditary, being carried on by generations of slate quarrvmen, who grow up with the instinctive " knack " or " flair " which no mere training can give, nor machines reproduce ; an outstanding example of real craftsmanship.

A splitter and dresser at work.

Bird's-eye view of a Festiniog [Ffestiniog] slate quarry showing old surface workings, and main entrance to mine at A.

Festiniog [Ffestiniog] slate, by reason of its light weight, unique qualities and high non-conductivity, has found its way to all parts of the globe-Iceland, Russia, South and East Africa, Nigeria, South America, Australia and New Zealand, to mention but a few-while on the Continent many important buildings are roofed with it, notably the Hotel de Ville at Brussels, Hamburg Cathedral, and Peace Palace at The Hague.

Apart from the manufacture of roofing slates, slate is widely used for billiard table beds, monumental purposes, brewery tanks, aquariums, electric switch panels, flat slate roofs (common in America) and pavements, and even for " honing " razors, slate giving the finest cutting edge possible, while three years ago a secret process was discovered and patented by members of the Festiniog [Ffestiniog] group, as a result of long research, by means of which slates can be treated most satisfactorily by a colouring process, by which the most delightful pastel shades, hitherto unobtainable in any roofing material, may be obtained. Exotic tints such as peacock blues and greens, are also available ; in fact, they can be coloured or tinted to suit each architect's or user's individual taste, in a single colour or in any desired combination of colours and shades. These coloured slates withstand the action of boiling in acids and alkalies up to and beyond twice normal concentration. Mineral, or earth, colours are alone used and these withstand treating to very high temperatures. By a chemical action, definitely beneficial to the slate, the colour film becomes practically part and parcel of the slate surface, hardening it and becoming harder and more permanent with age.

PRINCIPAL QUARRIES

The principal quarries in the Festiniog [Ffestiniog] group are the Oakeley-incorporating the Votty & Bowydd Quarries the Maen Offeren - incorporating the Rhiwbach-the Llechwedd (Messrs. J. W. Greaves & Sons, Ltd.), the Manod and the Craig Ddu Quarries. Up-to-date machinery, usually electrically driven, is used. Great care and expense are involved in keeping workings clear of water, an important item where the average annual rainfall varies between 80 and 170 ins.

The slates are entrained at Blaenau Festiniog [Ffestiniog], which centre is served by both the London, Midland & Scottish and Great Western Railway Companies, and at Minffordd Junction (G.W.R.) and shipped coastwise or abroad at Portmadoc [Porthmadog], whither they are taken by a wonderfully constructed narrow gauge railway laid in the early part of last century on the 2-ft. gauge-at the time the pioneer of all narrow-gauge railways, on which was modelled later the Darjeeling railway in India, and many others.

In conclusion, a good Welsh slate is the "Rolls Royce" of roofing materials, and being a natural rock must necessarily be more lasting than any artificially produced substitute, however good. It cannot be mass-produced, nor adulterated, but remains constant in quality and durability. A genuine Festiniog [Ffestiniog] slate will easily outlast the building it covers, and there are many instances of its re-use on new buildings after removal from very old buildings on demolition. While admittedly not as cheap initially as some artificial " roofers," on the basis of the real cost of a roof, namely, the initial cost plus maintenance costs over a period of sixty years or more, it is " facile princeps," maintenance costs being practically negligible, always provided suitable nails are used. It can be said, with truth, that the first cost is the last cost, and now that the only objection to its use, that of a tendency to drabness- more often than not caused by a lack of imagination and suitable architectural design rather than to any fault of the slate itself-no longer holds good, and the universal cry for colour and yet more colour can be aesthetically met, slate should come, and is coming, once more into its own, and taking pride of place as of yore.

A length of solid slate opened out into many laminae, thus showing the extreme resiliency of this material.

OUTPUT OF SLATES. While slates are sold by number and covering capacity their output is usually expressed in tons, and to give some idea of this, the present output of Festiniog [Ffestiniog] slate is round about 40,000 tons per annum, equivalent probably to at least 32,000,000 actual slates in the various standard sizes, 24 ins. by 14 ins. to 10 ins. by 6 ins., or (say) 1,500,000 square yards of slating at 3 in. lap. All British material, produced by British labour - and it is up to the British public to insist on having their houses covered with them to their own and their children's children's lasting satisfaction.

It is no idle boast that good Welsh slates are the finest and most lasting roofing material, but a proved and indubitable fact, as evidenced on every side and in every part of the world, for everyone to see for themselves.

It may be of interest to our readers to know that in the Festiniog [Ffestiniog] group's largest slate mine there are more than 30 miles of tram roads, of which one-half are probably underground, and it would take a good walker several days to go through all the workings. Air pipe-lines run into many miles ; as also do electric cables. A hearty welcome is also extended to any who may be in the vicinity of Blaenau Festiniog [Ffestiniog], in North Wales, near the centre of Snowdonia, to visit one or other of the group's slate mines, and to see for themselves their unique characteristics. We can promise them an interesting and highly instructive time, and a unique experience, for they are to be counted among the modern wonders of the world, the slate chambers being easily the largest individual openings to be found in any type of mining.

A circular saw table at work.

Printed by LOXLEY BROTHERS LIMITED, 50, Southwark Bridge Road, London

Source: Reprinted from the Building Times, 1936.