Arkansas

Description


Arkansas hones are mined in the Ouachita Mountains in Arkansas. They come in grits from around 500 to roughly 5000 (although exact figures vary depending on who you ask); the ordering is as follows from coarse to fine: Washita, Soft Arkansas, Hard Arkansas, Black Hard Arkansas (also known as Surgical Black) and Translucent Arkansas.

Annual report of the Geological Survey of Arkansas Vol 3

Ouachita (Washita) stone


The Ouachita stone has grains equally as fine as the Arkansas stone, and so will give equally as fine an edge when properly used; its porous structure, however, renders it unsuitable for fine instruments, but makes it much more effective with larger tools. It is a hard stone but wears away comparatively fast, and both cuts away the steel rapidly and produces a fine edge. Owing to the peculiar cutting property of the edges of the pores this stone,* the finest results will not be obtained by both a forward and backward movement of the tool edge over the stone. With the Ouachita stone the forward movement involves considerable irregular cutting, while the backward movement produces scratches alone; thus a fine edge can be obtained by the backward movement. Since this movement is the one employed in honing razors(sic), the Ouachita stone can be and is utilized as a razor hone; it is better indeed for this purpose than the Arkansas, which will glaze under a honing treatment.

A hard stone, the Ouachita for example, can be scratched by the knife point; while a very hard stone, like the Arkansas, receives no scratch from the knife point.

Composition


The following is an average of several chemical analysis made of the Arkansas stone :

Average Analysis of Arkansas Novaculite:
Silica 99.5%
Alumina 0.20%
Iron (ferric) oxide 0.10%
Lime 0.10%
Magnesia 0.05%
Potash 0.10%
Soda 0.15%
Loss on ignition (water) 0.10%
Total 100.30 per cent.


Silica is the only mineral seen in the microscopic section of novaculite, but its arrangement is a matter of interest and importance. The groundmass is composed of densely crowded grains of silica, but here and there are little cavities generally appearing on first sight to be irregular, but which on closer examination are found to have rhombic outlines. These cavities show that lime in the form of calcite was formerly present in the stone, but that it has been leached away. The cavities are very small, averaging about .05 of a millimeter in diameter; a large one measured .12 by .07 of a millimeter. Sometimes they are filled with secondary silica, forming a grain of larger size than usual; other large grains of silica are occasionally found having a jagged outline and a diameter of about .05 of a millimeter.

These larger grains of silica may have some influence on the abrasive quality of the Arkansas stone, though it would appear that the cavities had more, since stone which is identical in other respects, but is almost entirely without cavities, has a finer grit. Thus in the Arkansas stone the abrasion by the minute silica grains is materially assisted by the sharp cutting edges of the rhombic cavities about which the grains are closely packed.

Hardness


The Arkansas novaculite cannot be scratched by a knife point. Quartz will scratch it, however, and in turn it will scratch quartz; hence its hardness may be considered identical with that of quartz.


Analyses of the Ouachita stone show it to have the same chemical composition as the Arkansas stone; and experiments on the solubility of the silica show a percentage of soluble silica slightly smaller than that of quartz crystals, so the soluble silica is reduced to a minimum in this stone. Under the microscope silica is found to form the groundmass as in the Arkansas stone, and its grains are equally fine. Fine particles of iron are visible, and specks of carbon occur, but these minerals are in such small quantity that they do not affect the color, and to the eye the stone appears perfectly white. A very few flakes of muscovite mica are seen. The larger grains of silica seem more plentiful than in the Arkansas stone, and frequently give polarization colors; but this is because the rock sections are thicker than those of Arkansas stone.

The remarkable qualities of the Ouachita stone depend upon the existence of many cavities in a dense groundmass. As in the Arkansas stone these cavities have a rhombohedral form, but they play a much more important part in the Ouachita stone because they are somewhat larger and much more numerous.* On account of the larger size and greater number of the cavities in the Ouachita stone they are much more easily studied in that stone than in the Arkansas; in fact they were first noted in the Ouachita. In size these cavities are very constant and average about .07 of a millimeter in diameter. The outlines are often obscured by the presence of silica which has either been pushed into the cavity since the disappearance of the calcite or has been deposited there from solution, yet the original form of the cavity can usually be traced. Small cavities .03 or .04 of a millimeter in diameter are sometimes completely filled with secondary silica, but the large ones are never so filled. There is a sufftcient number of perfect rhombic forms left to prove beyond a doubt that the imperfect forms have had the same origin. Sections of the Ouachita stone seem to contain a large amount of foreign matter in the form of dust, but the chemical analyses do not indicate the presence of such matter, and as it usually appears in the cavities it is probably the dust from the grinding material used in preparing the microscopic sections.

The outline of the rhombohedral cavities is singularly accented in many cases by an arrangement of the silica grains with their long axes perpendicular to the sides of the cavity and very closely packed together. The grains are angular but the arrangement gives an appearance like brick work in a mass of rubble. This layer of silica grains is only about .01 of a millimeter thick; beyond it is the usual groundmass of silica. It is evident in almost all cases that the silica is more compact about the cavities than elsewhere though it may not show the brick-like structure. As a result of this compactness the sides of the cavities present fine cutting edges to a tool rubbed upon the stone. That these act as cutting edges must be the case for there is nothing else in Ouachita stone different from the Arkansas stone to render it capable of producing the much faster abrasion which the Ouachita accomplishes. These edges then do the rapid work of abrasion, and at the same time there is a constant fine scratching going on from contact with the grains of silica; so that the Ouachita stone produces a fine edge in a short time. By moving the edge backward over the stone the cutting effect of the edges of the rhomb is lost, and the scratching alone gives a finish to a tool edge as fine as can be obtained on an Arkansas stone. It is for this reason that the Ouachita stone can be used as a razor hone. The reason why the Ouachita stone cannot be used for fine-pointed tools lies in the fact that the partitions between the cavities are thin and easily broken away by a pointed instrument; such use soon produces an irregular surface and ruins the stone.