The morning news. [volume] (Savannah, Ga.) 1887-1900, 1892-07-17

placed in the cooling oven, and
the molding table Is run along the
track to the oven, where the glass
plate ia deposited and allowed 20 stand
Is evenly and slowly cooled. The plate is
then taken out, examined for any flaws,
and TIPS according TO requirement.. In both
processes of manufacture crude mirror
glass the glass is IN " rought aud i'm perfectly
transparent In order TO even the rough
surface the grinding apparatus is used, and,
| after being promptly ground the polishsr
imparts the clear, shining surface TO 1t.
Both the grinding and polishing Of mirror
! glass is done by machinery, while the
| smoothing is best done by hand; but even
the smoothing process is effected by means
of machinery in the case Of large-siZed
plates.
stone rests on a solidly constructed wooden
base. and on the top Of this stone layer of
burned and sittsd gypsum IL placed, which
is then converted into q pasty cement by
adding water. This cement is spread evenly
on the stone, and the glass is laid upon IL
and pressed trnly until it gradually adherea
10 the gradually hardening gypsum. A
somewhat smaller stone, called the upper
stone, is similarly prepared, and another
plate Of glass is laid upon it when the two
stones are brought together so that the glass
surfaces touch each other. A two-fold
motion is given to the upper stone
about its own axis and in a circle
from right to left. Sand and water are
then poured between the two glass surfaces,
and the grinding begins by setting the
upper stone il motion, the power being
supplied by machinery. The first grinding
begins, and the surface of the glass gradu-
ally becomes sm Doth, the elevations in the
glass being reduced by sand. As the two
plates Of glass begin to snow an even sur
face, finer sand Is used and the so-called
second grinding takes place. After one
side of the glass is ground, it is turned over
and the reverse side submitted to the same
operation. The sand used has seven differ
ent grains, from the coarsest to the finest.
When both surfaces have been made even
in the manner described, there is still too
rough a surface TO permit successful pol
ishing, and they must be subjected to the
smoothing process. Here, again, one glass
cement Of continually increasing
fneness used, the cement being
applied to the moist and rough glass sur
face, and the surfaces carefully rubbed to
gether by hand. After the smoothing pro-
cess is finished the glass is carefully exam.
ined and all f1awless pieces given to the
polishers, while those containing flaws are
cut up into small sizes. Polishing is done
by specially constructed machinery, and is,
properly speaking, an advanced stage Of
smoothing. It differs from the smoothing
process IN that the glass surfaces are not
applied to each other, but a wooden or iron
surface covered with cloth, is substituted,
aud further, that a fine mixture of oxide of
iron replaces the hard cement. The polisher
fixes the glass by means of plaster or CO.
menb to a firm support, usually q fiat
stone, and moistens the cloth-coVered rub
bling implement which receives q for
ward and backward motion by connection
with mechanical apparatus. The polishing
implement is kept In motion until the por-
ton of the glass ia moved 80 as to expose
another portion OF its surface to the polisher.
When the entire surfase is polished, and it
contains no flaws, it is ready for the silver
ing process. The large mirrow glasses are
often put on the market. however, In the
polished condition, especially the large
rolled plates which are used for show win
dows. In describing the foregoing different
processes for preparing mirrow glass, it re-
mainssto be said that a so-callsd circular
grinding apparatus has come into use lately
in Bohemia, especially in the grinding of
rolled glass. This apparatus is expensive,
but II does the work at least five times as
rapidly as by the other process. For srolled
& ass machinery rs also used in the smooth
ing process, and n circular polishing ap-
paratus has been introduced. This apparatus
Is used in Bohemia only for rolled glass.
AAtor Che poiLsninR ~ IL only necessary 10
put face on the glass and it goes to the
slverer, who gives the essential qualities of
the mirror to il. In Bohemia both silver
and quicksilver is used for this purpose.
The quicksilver process is really an ANAL
garGtion of tin to the glass surface, the
silver process the precipitation of nitrate Of
silver on the glass. TIE quicksilver process
is being almost entirely replaced by the
silver process in late years, both on account
OF the greater facility of the process aud on
account Of the less injurious effect on the
health Of the workmen, 80 that most all Of
the mirrors are now silvered ones