The weekly herald. [volume] (Cleveland, Tenn.) 1876-1888, 1878-05-30
How Granite Columns are Polished.

The substances used in grinding and
polishing granite shafts are beach sand
and emory, applied in certain quantities
at a certain velocity Of revolution for
the column; as the work is done In a
lathe similar in some respects TO the
lathes for turning and polishing metals
and woods except that the spindles both
reyolye, that In the foot-stock as we'll
as that in the head. stock. The lathe
is driven at different speeds to suit the
diameter Of the shaft to be ground and
polished, for a uniform surface speed Of
two hundred and forty revolutions per
minute is required for work, and 1t is
evident that while a shaft Of twelve
Inches diameter would have TO revOlve
about seventy-seven times per minute,
one of three feet diameter would re-
quire only one-third that rotary speed
or twenty-five turns. The centres of
the columns are found on the ends by
measurements, and proper boxes are
secured into cavities made for the pur-
pose, and the column is then swung in
the lathe. At the rear of the lathe, ex
tending Its entire length, is a ~bulkhead,
OF wall, against which rests the ends
of a series of polishing blocks which lie
on the top Of the revolving column, the
direction of the revolution of which is
toward the bulkhead, thus holding the
polishing blocks in place. These blocks
are of cast iron about three inches wide,
long enough to reach across the column,
and of sufficient thickness to weigh
about one hundred pounds each for a
column of forty inches In diameter, and
of fifty pounds for n Of twenty inches.
The under side of the block is conceived
to nearly a segment of a circle repre-
sentIng the cross section of the column,
and when in place for work are ranged
closely side by side covering the entire
length of the column. Their motion
may be likened to that of the keys of a
piano, particularly when the work is
first begun and until the ''stunt"
bruises, or tool marks of the chisel, are
removed. An attendant feeds these
blocks with sand and water until all the
tool marks are removed and a uniform
surface is obtained. Then9pery is used,
the grade being what is XII wn in ON
merce and among mechanics as that of
number forty, and from that to number
sixty according to the relative hardness
of the stone. The emery used must all
be weighed and put in at n time,
otherwise, the addition of new emery
would make scratches, for when the
stone is finished the emery becomes an
almost impalpable mass, finer than the
finest emery flour. The amount weigh
ed is estimated by the surface to be pol
ished, being at the rate of about half a
pound per superficial foot. Thus for a
column ten feet long and three feet d.
ameter, equaling about ninety superb
cial feet, the amount of emery required
would be from forty five to fifty pounds.
After the emery has been once fed
through and fallen into a trough under
meath the revolving column, it is all
carefully removed into a tub and re-fed
to the blocks. After a good surface is
secured, the final polish is obtained by
means Of oxide of tin and water, the
bearing surfaces of the polishing-blocks
being faced with felt. The time en-
ployed in the entire processes varies
according to the size of the column and
the quality Of the stone; a Westerly
granite shaft ten feet long and fourteen
inches diameter requiring from forty
five to fifty hours actual working time.
The result is a surface polish of a re-
markable brilliancy contrasting in
architectural work elegantly with
carved caps and dressed bases.