Tools and Machinery of the Granite Industry

Introduction

This article, the first in a series of four on granite working, deals with granite as a material, an industry, and a product and begins the description of the granite quarrying process. The second article will complete the account of granite quarrying. The final two articles concern the process of finishing granite and will conclude with a discussion of power sources, toolmaking, patents, granite workers, labor unions, and safety and health issues. The four articles will appear in consecutive issues of The Chronicle.

Granite Composition

Granite is a very common stone found world-wide. Granite has been commercially quarried in the United States, Canada, Scotland, Finland, Italy, Ukraine, India, China, and Africa. Granite is found throughout the United States and has been commercially exploited in every New England state (Figure 1). Granite is composed primarily of quartz and feldspar with smaller amounts of mica. Quartz contributes to granite's strength, hardness, and luster, and acts as a cement binding all the elements together. Feldspar, granite's principal ingredient, occurs in a number of forms, mostly sodium/aluminum/silicon-rich plagioclase and potassium-rich microcline. In addition to contributing to strength and hardness, feldspar primarily determines granite's color, resistance to discoloration and decay, and ability to receive a polish. Mica (mostly white muscovite and black biotite) is present in much smaller amounts. The relative amounts of white and black mica are an important factor in both the color and commercial value of the granite. If white mica predominates, the granite will be light-colored, and if the black predominates, the granite will be dark, often approaching black. If the white and black occur in roughly equal amounts, the granite will be speckled. Since mica does not polish well nor does it retain its luster, excessive amounts of mica decrease the commercial value of granite. As an example, granite of the Barre, Vermont, area contains 31.7 percent plagioclase feldspar, 26.4 percent microcline feldspar, 23.3 percent quartz, 6.4 percent muscovite mica, 4.5 percent biotite mica, 2.7 percent orthoclase, 2.1 percent chlorite, and 1.6 percent calcite.

Granite is an igneous rock, formed by high heat and pressure from molten rock called magma. Deep magma over time forced its way up through fissures and cracks toward the surface where it cooled into columns of granite called plutons. Over millions of years, erosion, especially glaciation, exposed the tops of the plutons. In Barre, Vermont, it is estimated that the pluton is ten miles deep-plenty of granite for the future. If the cooling was slow, it produced coarse-grained, building-grade granite with large crystals, up to the size of a large pea. Monumental granite must be fine grained, with crystals the size of a pin head, to allow fine carved details and to accept a mirror-like polish. Fine monumental granite occurs, for example, at Barre, Vermont, Quincy, Massachusetts, and Westerly, Rhode Island, while building granite occurs at Woodbury and Bethel, Vermont. Only granite, certain marbles and a few dolomites can be polished. Limestone, soft marbles, and sandstones can be rubbed and honed but not polished. Monument designers exploit the marked contrast between darkpolished and light-hammered granite surfaces (Figure 2). Different quarries not only yield a range of grain sizes but also a wide variety of colors, including deep red, light pink, white, light gray, gray with blue tints, dark gray, and black. The white granite of Bethel, Vermont, is the whitest known granite, having almost the appearance of a fine white marble, and this white stone made the quarry so bright that many of the quarry workers wore dark glasses (Figure 3). Quarries also differ in the maximum size blocks that can be quarried without including a seam or defect. Most granite deposits are laminated by horizontal joints or cleavage. The distance between these joints typically increases with the depth of the quarry; hence, the deeper the quarry the larger the defect-free blocks that can be extracted.

Marble, limestone, and slate are granite's chief competitors. Marble is easy to rough out but difficult to finish. It carves beautifully and takes a sharp carved edge. However, marble does not stand up well out of doors since it dissolves, pits, and discolors in polluted air. Limestone is easy to work when freshly quarried, takes a sharp carved edge and weathers well outdoors. Slate is easy to work but due to its strong lines of cleavage is subject to flaking both while being worked and as it ages. Brownstone and sandstone were used for buildings in the nineteenth century, but these stones are very soft and subject to flaking and pitting. Granite is the hardest and most durable of the building and monumental stones and is the most difficult to quarry and to finish. The development of the granite business into a large-scale industry had to wait on the invention of more powerful and more efficient tools and machinery to deal with this obdurate stone.