Why polish granite? Polishing brings out the colors and patterns of granite and also produces a dark surface that contrasts well with a lighter steeled (hammered) surface. Polishing "closes up" the granite, producing a reflective glass-like and more waterproof surface. Most stones, including slate and limestone, can not be polished to a mirror surface. Some marbles can be polished to a lustrous, even reflective, surface. Polishing is labor intensive and costly but a polished surface has a richness and beauty for which the customer has been willing to pay extra.



Some of the earliest polished granite surfaces are found on Egyptian obelisks that used granite from the Elephantine quarry on the Nile. This quarry yielded a very hard reddish syenite granite (actually granodiorite) of fine texture that could be polished to a surface with high luster and permanence. The Egyptians had a range of hand tools for working granite, including the circular polishing stone used with sand, corundum, or pumice abrasive. More recently, the Joseph Smith Monument, quarried and finished in Barre, was reputed to be at the time of its erection in 1905 the world's tallest polished obelisk.



By the early 1700s, Mr. Collis of Kilkenny, Ireland was employing powered polishing machines. The stone to be polished was placed on a table and an iron plate was driven back and forth on top of the stone by a waterwheel-powered crank and pitman rod. A succession of abrasives was used: sharp sand, fine sand or whetstone dust, and limestone dust (Tripoli). A buffer with tin oxide putty was used for the final polish.



Much of the progress in granite finishing can be credited to advancements in abrasive technology. Natural abrasive materials were used from ancient times for polishing, 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 19th century, manufactured abrasives began to appear, including flint shot, cast iron shot, chilled cast iron shot, broken iron shot, chilled steel shot, broken steel shot, and silicon carbide. During the 20th century, many new manufactured abrasive materials entered the market, including artificial diamonds, aluminum oxide, boron carbide, cerium oxide, and tungsten carbide. Manufactured abrasive, although more expensive than the same amount of natural abrasive, is more uniform, can do far more work, and is more cost effective. In the late 1960s, Steven Schaub of Barre locally manufactured small contained abrasive bricks which were more economical and convenient than loose abrasives for many polishing applications.



In the latter half of the 19th century, the gate-type polisher was invented for flat-surface polishing and was the subject of a number of patents, including one issued to Medad and Prentiss Wright of Montpelier in 1878 which describes the first of a long line of successful polishing machine designs. The gate-type polisher consisted of a horizontal cast iron polishing wheel supported by an articulated arm that allowed the wheel to be moved, via an operator handle, to any position in a horizontal plane. The arm framework, which supported a system of pulleys and flat belts to drive the wheel, was attached to a shed post or wall and depending on size had a radial swing of from five to eleven feet. A pair of cone pulleys allowed several polishing wheel speeds - slower for initial grinding and faster for final buffing. The arm and polishing wheel could be raised and lowered to accommodate work at various heights. This polisher was called the "Jenny Lind", after the celebrated singer who toured the U.S., because it emitted a pleasing humming sound.



In order to save time and abrasive during polishing, the surface was prepared by gang saw and pneumatic surfacing machine and, as technology progressed, by wire saw or diamond circular saw. Polishing took place in three stages: initial grinding with sand or iron shot, closing up with emery or Carborundum (silicon carbide), and buffing with zinc powder or zinc oxide. A typical 18-inch polishing wheel was designed to rotate at 200 rpm, required a ten-horsepower engine, and with an experienced operator could polish 30 to 40 square feet in an eight-hour day. Operating a polishing machine was not easy - it took a strong and sure hand and hence there was considerable variation in productivity from one operator to another.



A variety of polishing wheels was used depending on the polishing stage and abrasive used: broken scroll, cast scroll, emery ring, concentric ring, contained abrasive brick, hemp rope buffer, coco mat (from India), and felt buffer. Improved designs for polishing wheel surfaces, in which a number of discontinuous curved grooves or raised ribs were arranged in a spiral pattern that forced the abrasive between the wheel and the stone, were patented by Samuel H. Mills of Montpelier in 1907 and 1923 and William Milne of Barre in 1927 and 1941.



Bed setters placed multiple stones in a level bed so that they could be simultaneously polished by the gate-type polisher. Paper was stuffed into the cracks between the stones and, using a wooden paddle, the remaining cracks were filled with plaster just below the top of the stones. The plaster both held the stones in place and kept the abrasive on the surface and in action.



Large and expensive "automatic" line polishers were employed by a few companies that polished a high volume of stone. For example, Rock of Ages (ca. 1915-20) operated a machine in which an electric motor-driven vertical shaft, supported by a steel bridge-like framework, drove four 4-foot diameter polishing wheels. Stone-supporting carriages moved along a runway under the polishing wheels. The polishing wheels overlapped and thus covered the full width of the carriage.



At the other end of the spectrum was the manual polisher that was only suitable for occasional small jobs such as might be found at a granite retailer. An example in the Vermont Granite Museum's collection has a 68-inch long arm with a polishing wheel at one end. The arm can be moved so the polishing wheel will reach any point on a horizontal plane. The polisher is operator-powered by a crank handle geared to the polishing wheel. The polisher came with three wheel types: an iron wheel with four spiral grooves for initial grinding, an iron wheel with a smooth surface for closing up, and a wooden wheel to which a buffing cloth was attached for final polishing.



Gate-type polishers took up less shed floor space than the large line polishers. Also, only the number of gate-type polishers needed for the polishing volume had to be purchased. The two-bed gate-type polisher, in which one bed was set while the other was polished, provided much of the continuous operation benefits of the line polishers and as a result mostly replaced the latter over time.