ABRASIVE MATERIALS 649

 Artificial abrasives.—Production of artificial abrasives has declined
since 1929, although up to that year the trend was decidedly upward. In common
with other relatively new abrasives, vigorous market expansion has been aided
by lower selling prices, brought about by progress in manufacturing practice.
 Artificial abrasive products were made possible by cheap hydro-. electric
power, a prime consideration in the manufacture of both silicon carbide and
synthetic aluminum oxide. Great impetus was given the industry by war-time
activities, but growth has been substantial since the business slump of 1921.
 Artificial abrasives have several advantages over natural products for certain
industrial purposes. They are much harder than such minerals as quartz and
garnet and rival corundum, which is next to the diamond in Moh's scale. Artificial
abrasive grains can be bonded and manufactured into many special types of
grinding equipment essential to the exacting requirements of industry. Special
grain size, cutting speed, and odd sizes of wheels or stones can be provided
on demand for any particular job. Uses developed outside the strictly abrasive
industry, such as refractory material, floor tile, and stair treads, also
are important.
 To summarize, the utilization of artificial abrasives and the group composed
of natural siliceous products is growing with the expansion of the manufacturing
industries and the intensive development of new markets. On the other hand,
special quarry products and garnet and emery have lost many old markets and
gained few new ones. The following sections give more detailed current data
in regard to the individual commodities.

NATURAL SILICEOUS ABRASIVES

 Diatomite.—The Bureau of Mines is not at liberty to publish the annual
production figures on cliatomite since 1926. Permission has been obtained,
however, to combine the data into 3-year periods. The total production for
the period 1930—32 was 248,273 short tons valued at $3,902,126.
 A large market for diatomite has been developed in the sugar-refining industry.
Sugar meltings declined only 14 percent in 1930—32 compared with 1927—29
and only 10 percent in 1932 compared with 1931. Detailed data are not available
on the quantity of diatomite consumed in the refining of sugar, that used
as insulating material, or that sold to battery manufacturers (reported to
be the three major uses), but these markets are comparatively new and account
for the rapid growth in the use of diatornite for other than abrasive purposes.
 The economic importance of cliatomite depends mainly upon its unusual physical
properties—fineness and uniformity of texture, light weight, high porosity,
and relative chemical inertness. Dry lump material weighs about 28 pounds
per cubic foot. Dry, loose powder usually ranges in weight from 7 to 16 pounds
per cubic foot, depending upon the character of the material and the type
of the deposit.
 Diatomite (known also as diatomaceous earth, infusorial earth, and kieselguhr)
is a fine-grained hydrous or opaline fossil form of silica composed of skeletal
remains of diatoms. Deposits may be of marine or fresh-water origin. The
organic structure of the particles, which is plainly visible under a microscope
of moderately high power, affords