Plug and Feathers
Many of Nova Scotia’s most beautiful buildings were built with stone over a century ago. Stone used to erect buildings needs to be in solid blocks so it looks good and lasts. But how was building stone extracted from a deposit without damaging it?
“Plug and feathers” is an old way of extracting stone that leaves it in solid, undamaged blocks.
Historical quarries, like those in the Wallace area, drilled holes into the rock face in a straight line. Feathers (metal wedges) were placed in the holes and plugs (long metal pins) were hammered into the holes. As the plugs were forced down by the hammering, they pushed the feathers outward. This put pressure on the rock and caused blocks to break off in a straight line without damaging the stone.
The principles of the plug and feathers method are still used today but the technology has evolved. For example, Wallace Quarries now uses hydraulic plug and feathers, in which the wedges and pin are combined in a mechanical unit. A line of holes is drilled in the stone, each one usually about a foot apart from its neighbouring drillholes. The hydraulic splitter is put into each drillhole, and the feathers/wedges are pushed outwards by a pin that is forced down between them by hydraulic power. This creates the outward pressure on the rock that the hammering used to provide.
Wallace Quarries also uses a chemical expansion product to split rock. Water is mixed with a lime-based powder and poured into the drillholes. The mix slowly swells, generating thousands of pounds of pressure per square inch, and forces the rock to split, usually within hours.
After the chemical reaction is complete, the environmentally-friendly product is just hydrated lime, which has a wide range of uses such as raising the pH level in acidic soils.
The big slab shown in the picture below was extracted using the chemical expansion method, which is how Wallace Quarries does all its primary cuts. (The vertical lines in the stone are the drillholes. The white in the drillholes is the remains of the hydrated lime.) Then the slab was cut into smaller pieces using the hydraulic plug and feathers method.
The quarry has natural vertical and horizontal seams, from four feet to 30 feet apart, along which they cut. Some of Wallace Quarries’ primary cuts are over 20 feet long and eight feet tall, which illustrates how extraordinary the sandstone deposits are in the Wallace area. Most rock has faults, joints or other issues that would make it impossible to extract such large, solid blocks.
In quarries that provide rock like gypsum for use in wallboard, or aggregate for use in construction, we blast the rock because that is the most efficient way to break it off the quarry’s working face. The rock is then crushed as part of preparing it for manufacturing or construction, so it does not matter that we break the rock down in blasting.
Blasting also causes microfractures throughout the rock for some distance from the blast, so it affects not just the specific stone being removed, but also surrounding stone.
That is why blasting is not used to extract building (aka dimension) stone. We need those big, solid blocks to make big, solid buildings!
Traditional plug and feathers.
The vertical lines in the stone are the drillholes. The white in the drillholes is the remains of the hydrated lime that was used to extract it. The slab was then cut with the hydraulic plug and feathers method.
Hydraulic plug and feathers.
