The cost of silvering is trifling, and with cleanliness and ordinary
care very little difficulty will be experienced. The apparatus and
chemicals required consist of the following articles:—
APPARATUS.—A Silvering Vessel:—This should be a flat-bottomed
circular glass or glazed dish, 1 inch or more larger in diameter than
the speculum to be silvered, and sufficiently deep to allow of a stratum
of fluid of an inch or rather more between the face of the mirror and
the bottom of the dish, the top of the mirror being nearly level with
the edge of the dish.
A Mixing Vessel:—A 40 oz. glass measure will answer well, but care
should be taken not to scratch the sides whilst stirring, or the
glass is liable to fly. Should a measure not be procurable, any
receptacle of sufficient size may be used, but a glass one is best,
as it will allow of the action of ammonia being better observed.
A Box of Scales and Weights.
A Glass funnel and filtering paper.
Two Glass Rods for Stirring purposes.
A Five-ounce Glass Measure.
A Test tube 3/4 or 1 inch in diameter.
Some Clean Cotton Wool.
Some very fine Wash-leather.
A Support of Wood, on which to cement the speculum, described
further on.
All these articles should be perfectly clean and free from dust, and
the Glass ones well rinsed with distilled water just before using.
CHEMICALS.—Nitrate Silver.
Potash pure, precipitated by Alcohol.
Sugar of milk powdered.
Nitric Acid, pure.
Liquor Ammoniæ.
Distilled water, pure.
Pitch.
Fine Rouge.
Turpentine.
Procure a strip of wood an inch or so less in diameter than the
mirror, and of sufficient length to rest securely on the opposite
sides of the silvering vessel. Pour on this piece of wood some
melted pitch, and whilst it is still hot, place on it the back of
the speculum moistened with a little turpentine; when cold, reverse,
and lay the cemented speculum face downwards, suspended in the dish.
Should the distance between the face of the mirror and the bottom of
the dish be less than an inch, raise the mirror by means of thin
wedges, placed between the strip of wood and the edges of the dish.
But if, on the contrary, the mirror should be considerably more than
an inch away from the bottom, which will occur if the silvering
vessel is very deep, the speculum must be cemented to a block of
wood of sufficient height, screwed to the suspending strip, instead
of being cemented directly to the strip itself.
When the speculum is properly placed in the dish, namely, with the
front surface about one inch from the bottom, pour in water till the
fluid reaches about 1/4 inch up the side of the mirror. Measure this
quantity, as it will indicate the total amount of the silvering
solution required to be prepared.
To prepare the Silvering Solutions—make 3 standard solutions as
follows—
No. 1. {Nitrate Silver in crystals 100 grains.
{Distilled water 4 oz.
No. 2. {Potash pure by alcohol 1 oz.
{Distilled water 25 oz.
No. 3. {Powdered sugar of milk 1/2 oz.
{Distilled water 4 oz.
Solution No. 3 must be made just before using. The others will keep
if the distilled water employed in their preparation is pure, and
the solutions, when made, are kept in glass stoppered bottles.
Suppose it is desired to silver an 8-1/2 inch mirror, proceed as
follows:—the total amount of solution required having been
ascertained, as before described, pour 2 ozs. of solution No. 1 into
the mixing vessel previously well washed and rinsed with distilled
water, and cautiously add Liquor Ammoniæ. A grey precipitate will be
formed; continue to add the ammonia, drop by drop, till the
precipitate is just dissolved and the solution becomes clear. The
solution should be well stirred with a glass rod whilst adding the
ammonia. Now add 4 oz. of solution No. 2, and re-dissolve the brown
precipitate which is produced with Liquor Ammoniæ as before
described. There will now be about 6-1/2 oz. of solution. Subtract
the 6-1/2 oz. from the total amount previously ascertained to be
required, and the remainder will be the amount of distilled water to
be added. Add half this quantity of distilled water to the 6-1/2 oz.
of solution, and add a drop at a time of solution No. 1, till there
is a slight precipitate, which cannot be re-dissolved by a
considerable amount of stirring (say for 2 or 3 minutes); then add
the remaining half of distilled water and cover up from dust the
vessel containing the solution, so as to allow the slight
precipitate to settle. There is a point of importance to be attended
to, namely, that no more ammonia is employed than is absolutely
necessary. The total amount of ammonia required in the 6-1/2 oz. of
solution is about 2 drs.
To Clean the Mirror.
Fill the end of a test tube with cotton wool, leaving plenty outside
the tube. Having poured a small quantity of strong nitric acid on
the front of the mirror, rub the acid well all over the front and
sides with the cotton wool brush. Place the speculum under a water
tap for a few minutes till the acid is washed away, and finally well
rinse with distilled water; then place it in the silvering vessel
(previously thoroughly cleansed) and pour in distilled water till it
reaches 1/8 inch up the side of the speculum.
To Mix the Solutions.
The precipitate having settled, pour into a clean vessel all that is
clear of the solution, leaving about 2 oz. behind, which will be
turbid with the precipitate, and therefore useless. The total amount
will be afterwards made correct by the addition of the same quantity
of solution No. 3. Having filtered solution No. 3, warm it to about
100° F. by allowing the bottle to stand in warm water, or by heating
in a small flask. When everything is quite ready, add 2 oz. of the
filtered solution No. 3, whilst warm, to the clear solution
described above, and thoroughly mix.
To immerse the Mirror.
Remove the mirror from the distilled water, taking care not to touch
the surface of the mirror, and wipe the back and edge with some
clean cotton wool. If this precaution is not taken the water is
liable to drain down the sides of the mirror whilst silvering, and
cause streaks at the edge of the film. Having poured away the
distilled water in the silvering vessel, substitute the mixed
solutions, and directly the solution becomes slightly inky, gently
immerse the mirror, taking care that no air bubbles, or specks of
any kind, remain between the surface of the mirror and the solution.
The mirror should not be removed from the bath until all the silver
has been exhausted from the solution. This may be known by the
solution being clear below the silver film on the surface of the
liquid. The time required will vary from 45 minutes on a hot
summer’s day, to 90 minutes when the thermometer shows a low degree
of temperature. In the latter case it is better to have the
silvering bath in a warm room.
Immediately the mirror is removed from the bath, the silvered
surface should be well washed by allowing ordinary water to flow on
it from a tap, for five minutes or more, then finally rinse with
distilled water and place the mirror to dry with the silvered
surface resting on some blotting paper. It is as well if the mirror
can be left undisturbed for a day or two, as the film will be
firmer, but it may be polished, if desired, after drying for a few
hours.
To Polish the Silvered Surface.
Make a couple of polishing pads by filling two pieces of very soft
wash-leather about six inches square loosely with cotton wool, and
tie them into balls. Gently remove any dust that may have settled on
the film with some loose cotton wool, and then go over it with one
of the pads in small circular strokes for about 15 minutes. This
will consolidate the film and fit it for polishing. Spread a little
of the finest rouge on a sheet of writing paper, and impregnate the
other pad with it. Go over the film with the rouged rubber with the
same circular strokes till it is perfectly polished, which will take
another 15 minutes or so. Never commence with the rouged pad, as the
surface may be injured. When once the film has been consolidated it
will remain so, and can be repolished many times with the rouged pad
should it get tarnished. The pads should be kept from dust in
wide-mouthed bottles for future use. With care the film will last
for a long time, especially if it is not allowed to get damp, and
consequently the mirrors should never be brought uncovered from the
cold air to a warmer temperature.
The “flat” may be silvered and polished in the same way as the
speculum, using a smaller appropriate vessel for the silvering
solution.
To Separate the mirror from the Wooden Support.
Should the mirror be attached directly to its support, insert a
chisel between them, when one or two gentle blows will cause them to
separate, but, should the mirror be cemented to a block, stand the
mirror on edge, when a slight tap on the block will detach it.
Scrape off any pitch that remains on the back of the mirror, using
finally some turpentine to wipe it clean. Great care should be taken
not to finger the film.
Martin’s process for Silvering.
Prepare four solutions of any quantity. Keep in stoppered bottles.
_Solution 1._—Dissolve 175 grains of pure nitrate of silver in 10
oz. of distilled water.
_Solution 2._—Dissolve 262 grains of pure nitrate of ammonia in 10
oz. of distilled water.
_Solution 3._—Dissolve 1 oz. avoirdupois of pure caustic potash
(prepared by alcohol) in 10 oz. of pure distilled water.
_Solution 4._—Dissolve 1/2 oz. avoirdupois of pure sugar candy in 5
oz. of distilled water, then add 32 grains of tartaric acid and boil
in a flask or other clean glazed vessel for 10 minutes, when cool
add 1 oz. of alcohol and then dilute with distilled water, so as to
make up the volume to 10 oz.
For silvering use equal parts of each, mix solutions 1 and 2
together, and 3 and 4; when the mirror is ready mix the whole
together in the silvering vessel and quickly suspend the mirror.
In the summer time, if the solution cannot be kept in a very cool
place, the mirror must be quite ready to be placed in the bath, as
the solutions turn instantly when mixed together.
[ Illustration: decorative ]