Cracking a Block
Since the bedding planes are parallel with the quarry floor only in a very few quarries, the slate-quarrying industry has no general term for the separation of large masses of slate from the floor. In marble and sandstone quarrying the term "bed lifting" is used, since such separations commonly parallel the bedding.
Oliver Bowles, The Technology of Slate, 1922
In order to "crack a block", quarrymen involved needed to have a solid understanding of the weaknesses of the rock within the specific quarry in which they worked. Knowing what direction or "dip" the cleavage planes ran, as well as the grain or "sculp" was key to the process. A slab of slate, having its broad faces parallel to the cleavage, could be broken transversely in one direction more readily than at right angles to this direction. The direction of easiest breaking was called the "grain" also known as the "sculp" and knowing this direction was very important.
To begin the process of removing a block, operations first cleared the cleavage surface that forms the floor or steeply-sloping "back" of the opening. In Pennsylvania quarries the floor was rarely horizontal and generally sloped southward at angles not exceeding 25°. Slate was first removed from one corner or part of the quarry to furnish a more or less vertical face, known as the "key" face, to allow the rest of the slate making up the floor to be addressed.1
The combination of processes employed in a number of typical quarries varied because the conditions for each quarry was different but the following is a description provided by Oliver Bowles in 1922 for how a block would be extracted from the Blue Mountain Slate quarry near Slatington which had a unique example of a synclinal fold.
The intended block to be split is defined by the outer corner between the two dashed lines X and Y.
The slatey cleavage dips 60° to 70° south and is practically parallel with the axis of the fold. The top of the bench is an inclined joint dipping north about 30° ; the east side of the bench (A) is parallel with the grain or "sculp" and dips 80° to 85° east.
(a bench is a portion of the quarry floor which is generally all the same level, although not always level)
The front of the bench (B) is parallel with the slaty cleavage and dips about 70° south. The ribbons (c) and (d) dip about 30° south.
A back split (created at the dashed line Y) is first made in line with the slaty clevage and about 2 feet back from the face by driving wedges in shallow holes, (as shown at e). A loose ribbon (d) makes an open floor. A hole one-half inch in diameter and 2 feet deep is then drilled at right angles to the front face (as shown at f). The hole is nearly filled with black blasting powder and tamped with about 1 inch of soft slate. The charge is fired with a fuse and the blast fractures the rock along the sculp, (as shown by the dotted line X).
If for any reason a split can not be made in the direction of the slaty cleavage by wedging, blasting is done in a drill hole (like the one at f). This is known as "split-hole" blasting; a small charge of black blasting powder is put in the bottom of the hole and most of the hole is filled with powdered slate. When used as a "sculp hole" almost the entire hole is filled with explosive. If through absence of joints or loose ribbons, two breaks approximately at right angles must be made, this is sometimes accomplished by firing a charge in a single triangular hole, the rock breaking from the corner. Such holes are drilled by hand.
As the syncline pitches east and the grain of the rock dips east, the slate blocks tend to have sharp angles rather than right angles. For this reason there is considerable waste, particularly in the thin beds. Structural slate can be cut from thin beds to better advantage than roofing slate, and of late years the increasing demand for structural slate has resulted in more advantageous work with a lower percentage of waste.
Reducing the Mass
The process of "cracking a block" was not always used for extracting pieces of slate directly from the quarry but rather for splitting a large portion of the actual living rock from the quarry wall that could be worked over time. This block was then broken down into manageable sections known as mill blocks, each part being individually raised out of the quarry. The image above shows a massive block which had been split away from the quarry wall.
When the large mass was set free it was then subdivided into slabs (mill blocks) small enough to be hoisted to the surface. Notches are cut in the bench face in a line parallel with the slatey cleavage about 18 inches or 2 feet from the top of the bench, and a split is made by drilling wedges into the notches. Vertical holes are then drilled in line with the grain direction and a fracture made with plug and feather. Breaks across the grain are made in the same manner.
