Tools and Machinery of the Granite Industry, Part III

Chronicle of the Early American Industries Association, Inc., The, Dec 2006 by Wood, Paul

Granite Finishing

A small number of basic finished dimension stones made up the great majority of granite shed production. For gravestones and private monuments, there were dies (the main stone on which the lettering and ornamentation was cut), bottom bases, second bases, markers (a small stone set either flush to or raised from the ground level), posts (indicating the corners of a cemetery plot), boulders (natural-shaped stones, usually rock face finished), tablets (a die whose lower portion is buried underground), crosses, shafts, and columns. For mausoleums and vaults, there were roof stones and sidewall stones. For buildings and large public monuments, there were ashlars (four- to twelve-inch thick blocks that were carefully dressed on top, bottom, and sides so they could be set in a wall with uniform and tight joints), columns, capitals, steps, foundations, bas-relief panels, and statuary. The finished surfaces applied to these stones were rock face-an irregular natural looking surface produced by chipping out pieces of stone with a chisel; hammered-a powdered or steeled surface produced by hand or pneumatic bush hammer and of varying degrees of smoothness; polished-a mirror-like finish produced by a polishing machine; and carved-a wide variety of surface shapes and textures produced by hand tools, by a small pneumatic carving tool, or by sandblasting.

The finishing of granite involves only two basic processes-shattering and abrasion. Shattering is the crushing and breaking of granite by the impact of a steel tool. The bull set, hand set, hand point, chisel, circular saw, surfacing machine, cutting lathe, and the first two stages of polishing machine use are examples of tools and machines that work by shattering. Abrasion is the wearing away of granite by an abrasive forced under pressure along the stone's surface. The gang saw, wire saw, Carborundum saw, grinding machine, polishing lathe, and the last stage of polishing machine use are examples of tools and machines that work by abrasion. Sandblasting appears to employ a combination of the two processes.

Much of the progress in granite finishing can be credited to advancements in abrasive technology. Natural abrasive materials were used from ancient times, including beach sand, whetstone dust, red limestone powder (Tripoli), emery powder, tin oxide putty, garnet dust, and iron filings. In the latter part of the nineteenth century, manufactured abrasives began to appear, including flint shot, cast iron shot, chilled cast iron shot, broken iron shot, chilled steel shot (Figure 1), broken steel shot, and emery bricks. During the twentieth century, artificially synthesized abrasive materials entered the market, including artificial diamonds, silicon carbide, aluminum oxide, boron carbide, cubic boron nitride, cerium oxide, tungsten carbide, and contained abrasive bricks. Contained abrasive bricks are molded blocks of abrasive contained in a binding-matrix material such as magnesite and chloride. They are used for the initial stages of polishing and are more economical to use than loose abrasives.

Evolution of Shed Architecture

Many farmers harvested granite boulders from their fields and shaped the stone during the winter slow time in unused spaces in a barn or shed. The earliest commercial stone sheds were designed around the boom derrick-either a round shed with a centrally located inside derrick that could reach any point in the shed or a horseshoe-shaped shed that defined a semi-circular yard with an outside derrick that could reach all the shed doors and any point in the yard (Figure 2). The final form was the straight shed having a rectangular footprint and designed for an inside overhead traveling bridge crane (Figure 3). One or two cranes could run along tracks that ran the full length of the shed and by this means reach any point in the shed. Whereas granite quarries were typically located at higher hilltop elevations where much of the overburden had been glacially removed, granite sheds were usually located in the valleys, often in an existing town, where worker housing and water or electric power were available.

Granite sheds needed to be provided with a variety of supporting services such as compressed air, water, heat, light, and dust removal. Compressed air was usually supplied via 4-inch diameter threaded iron pipes that went down both sides of the shed. Each pipe had a smaller-diameter steam pipe inside to warm the air and lower its relative humidity to prevent freeze up of pneumatic tool exhaust ports. Heating also yielded an added quantity of compressed air at a lower cost compared to that produced by a compressor. Smaller feeder hoses went to each granite cutter's or carver's workstation- called a banker-and surfacing machines with a shutoff valve for each (Figure 4). Water pipes, with either well or cityprovided water, went down both sides of the shed. The horizontal and vertical grinders required large quantities of water to keep the dust down. Considerable water was also used in the tool grinding room.