PHOTOGRAPHIC MANIPULATIONS.
The chemicals being prepared, they must be placed in the dark-room; the
collodion bottle should be placed on a convenient shelf close to the
door, and on a handy peg hang a broad camel’s-or badger-hair brush, for
dusting the clean plate just previous to coating it with collodion; the
brush must not be used for anything else upon any pretence whatever.
The silver bath must be kept, both in use and out of use, in some
convenient corner, where there is no risk of getting any other chemical
splashed into it. For a bath holder a deep 15 by 12 porcelain or papier
maché dish, with a wooden or stout cardboard lid, should be provided,
and this dish must always be thoroughly washed, and wiped dry with a
clean damp leather (kept for the purpose), both before use and after
pouring the silver solution back into its bottle or jug. Or, if the
upright form be preferred, be sure and have it large enough.
The developer should be placed on a shelf, on the right of the sink,
the developing cup close by.
The fixing or clearing solution should be kept in an upright bath of
glass, porcelain, or ebonite.
The intensifier No. 1 may be kept in a flat dish, or in an upright
holder, at discretion; but if in a flat dish, the solution should be
poured back into its bottle, after each day’s work.
Keep the intensifier No. 2 in a stoppered bottle close to the sink.
The water varnish also is kept in a bottle or jug.
On a shelf close to the collodion, the polished or albumenized plates
may be stacked, with the clean or prepared side facing the wall.
A pneumatic holder for attaching to the back of the plate during the
operation of coating with collodion will be found very handy.
Of course, it is thoroughly understood that there is a sink in the
dark-room, and a good water supply is indispensable.
Then, again, the window through which the light is transmitted, must
be |25| covered with a non-actinic medium, such as two thicknesses of
golden fabric, or one of canary medium; do not stint the amount of
light, but have as much as possible, so long as it is non-actinic.
No arbitrary plan for the dark-room need be followed, but the novice
will find the annexed plan useful to him in contriving for himself. It
is self-explanatory.
[Illustration: Fig. 2.]
Everything being in readiness, we will now proceed to make a line
negative, reserving a description of half-tone work to follow.
To begin: wash the bath holder thoroughly, and wipe it dry, then pour
into it sufficient silver solution to make the depth about half an
inch; dust the inside of the cover, and put it over the dish; now take
a clean glass plate, if it be polished, see that it is edged with
India-rubber, and if albumenized, be sure and clean the back—and attach
it to a pneumatic holder; then with a broad camel’s-or badger-hair
brush (kept for this purpose only) remove any particles of dust,
back and front; now hold the plate in a horizontal position and pour
upon it, toward the right hand corner furthest away from the body, a
little more collodion than it is judged will be sufficient to cover
it; let the collodion run to the corner, then incline the plate to the
left, and the collodion will run into the upper left-hand corner. Now
by inclining the plate toward the body the collodion will flow into
the lower left-hand corner, and thence it is guided into the funnel
placed in the bottle prepared for its reception; during the time that
this surplus is draining, the plate must be gently rocked sideways,
so that the lines formed by the collodion, in draining, are merged,
leaving the film quite homogeneous. As soon as the collodion ceases
to drip, release the plate from the pneumatic |26| holder, and touch
the lower ridge of the collodion; if it is quite set, close the door
of the dark-room, remove the lid from the bath holder, then—holding
the plate in the left hand—with the right hand raise the end of the
bath holder, as far as is possible without risking any of the silver
solution running over the end of the dish resting on the bench; now
lay the plate (with the collodion side up) on the bottom of the dish,
then drop the plate into the solution, and, at the same time, lower the
end of the dish held up by the right hand, which will cause the silver
solution to flow in one even wave over the collodionized surface. This
operation must be done with dexterity, as if the silver solution does
not flow in an even wave the film will be marked, and, in the case
of a half-tone negative, the image will be spoilt. Therefore, it is
necessary to see that the plate and dish are lowered simultaneously, so
that the solution goes over the collodion film in one steady wave. If
the upright bath holder be used, the plate is placed upon the ledge of
the dipper, and lowered slowly and steadily into the solution.
The plate will require an immersion of about three minutes for proper
sensitizing, and this interval—the bath being covered up to prevent
the access of white light—may be utilized to give the final adjustment
to the camera, and to see that the drawing or print to be copied is
properly in focus.
Use a medium diaphragm in the lens for focussing, and examine the
image all over before deciding that it is sharp. Focus by preference
about halfway between the centre of the picture and the edges, and
then, after moving the rack to and fro until the proper sharpness is
obtained, take out the diaphragm and substitute the smallest sent out
with the lens; or if the picture to be taken be small and from a large
original, then the second, or third, smallest diaphragm may be used.
Now remove the focussing screen from the camera (note that the ground
side of the glass has been next the lens) and place the cap upon
the lens; close the door of the dark-room, open the dark slide, see
that the proper carrier is in it, then raise the plate from the bath
slowly, using a silver-wire hook for the purpose. If the solution flows
evenly over the film, the plate is ready; but if, on the contrary,
the solution flows greasy, then again lower the plate and rock the
dish gently for a minute or two. The film being fully sensitized, the
plate is carefully lifted out of the solution, both hands holding
the plate in such a manner as to avoid touching the film; allow the
solution to drain from the plate as much as possible, then lean the
plate against the wall, the lower end resting upon a pad of clean
blotting-paper, so as to catch any further draining; |27| now with a
pad of blotting-paper or papier-joseph wipe the back of the plate as
dry as possible; this will prevent stains, and keep the dark slide from
rotting by the action of the silver solution.
The plate being drained, lay a piece of thin filtering paper in each
lower corner of the carrier, so that the plate will rest upon the
filtering paper, which will catch any further drainings from the film,
and be a safeguard against stains.
Now lay the plate face down in the carrier of the dark slide, close the
door, and fasten it.
The dark slide is now carried to the camera, and inserted in the
grooves previously occupied by the focussing screen, taking care that
the sliding shutter is next the inside of the camera; when pushed quite
home, cover the back with the focussing cloth; pull out the shutter,
when the plate is ready for the exposure.
The time of exposure will vary very much with the time of the year, and
with the amount of light falling upon the object being photographed.
The exact time can be found out only by actual practice, but the
operator, in time, by observation, will be able to guess it pretty
correctly.
In taking the cap off the lens, be sure that the camera is not shaken,
else the picture will be spoilt.
The exposure being made, close the sliding shutter, remove the slide
to the dark-room, and close the door; lay the dark slide (with sliding
shutter down) upon the bench, open the back shutter, and, placing the
pneumatic holder in the centre of the plate, lift it out of the carrier.
Take the plate to the sink, and holding it face up, in the left hand,
take the developing cup in the right, and, with a quick motion,
pour over the plate sufficient developer to cover the whole film;
the developer must go all over the film in one even wave, for any
hesitation in this will result in a stain; pour upon the film only
sufficient to cover it, and be careful to spill as little as possible
into the sink, or else the image will be thin, on account of the
absence of free silver washed off by the wave of developer.
Now rock the plate gently, so as to keep the developer flowing to and
fro over the film (but keep it there, don’t spill it off) and the
image will soon appear, faint at first, but growing gradually darker,
the black lines of the drawing being white and the white paper black.
If the development be continued too long the lines will eventually
be veiled; therefore, as soon as the finest details are out and well
defined, place the plate under the tap and allow |28| the water to
well sluice the film all over; as soon as it is properly washed, which
can be seen by the water flowing evenly over, without greasy looking
lines, hold the negative up in front of the window, and examine for
spots or stains. If there be any present that at all interfere with the
picture, at once reject the negative, placing it in a pan filled with
water, to be washed off at leisure. Then start afresh.
The negative being satisfactory, give it another rinse under the tap,
and place it in the cyanide bath, where the unaltered iodide will
speedily be dissolved. Then it is withdrawn, and well washed under
the tap. Whilst the water is washing over the front, rub the back
with the fingers or a sponge, and then turn the plate over and allow
the water to wash the back also. It is now ready for immersion in the
intensifier No. 1, where it remains until the film is bleached almost
white. Then it is thoroughly washed, drained, and flooded with a small
quantity of intensifier No. 2, which will at once turn the film to an
intense black, or if the action does not penetrate through to the back
of the film, continue the application until it does; again wash, then
place the negative on a rack to drain. When the water ceases to drip,
flood the plate twice with the filtered water varnish. Then place the
negative on the rack to dry. When dry, it is ready for printing on
zinc. The negative may be varnished with a benzole varnish, sold by
the dealers under the name of positive or ferrotype varnish. Negative
varnish may be used, but unless very thin it is apt to be sticky and
render the negative easily marked by the heavy pressure it is subjected
to in the printing-frame.
Now lay the finished negative face down upon a piece of clean white
paper, when, if it is a good one, all the details of the original will
show through, clearly and distinctly; but if any of the details are
missing or veiled over, the negative will not do, and another must be
made.
This process does not always work satisfactorily, but with care and
cleanliness no serious fault should arise.
Sometimes the bath may give what are called foggy images, which are
indicated by a veil over the lines. These can often be wiped off with
a pledget of cotton-wool. The addition of half a drachm of nitric acid
to the eighty ounces of bath solution, thorough mixing, and a rest for
a few hours, will end this difficulty. If the dark-room window be not
of the proper color, fog will ensue; therefore, the non-photographic
experimentalist had better call in the aid of a professional
photographer in case of any difficulty in getting clear negatives.
_Wilson’s Photographics_ will be a profitable investment, as it was
published |29| before the advent of dry-plate photography, when the
wet process reigned supreme.
At the end of each day’s work the silver solution should be carefully
poured into the jug or bottle, and allowed to stand all night; then in
the morning, just before using, it should be filtered. The dish also
must be carefully washed out and put away in a corner where it has no
chance of being soiled.
Such are the manipulations necessary for the production of a negative
from a line subject—_i. e._, a drawing in black and white, in which the
whole of the picture is obtained by means of lines, or stipple, or by
cross hatching. Flat washes of color not being permissible, all effects
of half tones, etc., are obtained by lines, either thicker, or thinner,
or closer, or further apart. Such a negative is available for printing
from, direct upon zinc, in either albumen or in bitumen, for etching
into relief, or for printing from as a lithograph.
THE USE OF GELATINE DRY PLATES IN PHOTO-ENGRAVING.
For making negatives for collographic printing either direct or by
transfer, gelatine dry plates may be used with as much certainty as the
wet collodion, so long as the operator can produce a result giving all
the gradations of tone, from the deepest shadow to the highest light;
and as there are so many excellent brands of plates in the market,
it would be worse than useless to name any brand here as having any
distinct advantage over the rest, more especially as the brand that
the writer would be inclined to name, would perhaps, by the majority
of readers, be classed as a plate that they could not work. This being
so, for half-tone negatives, the author leaves the choice of the plate
and the manner of working to the operator, contenting himself with
saying that the criterion of suitability of a negative for yielding
good results by any of the processes treated of in this book, leaving
out of the question entirely negatives for line work, etc. (treated of
in Parts I., II., and III.), is the same for giving soft, delicate, and
brilliant prints, either in platinum, carbon, or on albumenized paper,
resting assured that a negative yielding a hard black and white, or a
dirty flat print, by any of these three methods, will not give results
any different in collographic or transfer methods.
For line work negatives for printing upon zinc, for etching, or for
transfers to stone, ordinary gelatine dry plates are not useful, as
it is almost impossible to get that freedom from veil over the lines
that it is absolutely necessary to have; here the wet collodion is best
as it is simplest, easiest, and cheapest. |30| Still there are times
when the most ardent wet-plate worker is obliged to confess that the
process has its shortcomings, most especially in dull weather, when it
is necessary, to get the result required, to give exposures from thirty
to sixty minutes; then, what with the plate drying and the want of
pluck from the bad light, it is impossible to get a result suitable for
the work. In this case the plate called the “Process” plate is a great
boon. It is fairly rapid, and gives intense negatives, without veil on
the lines.
For making transparencies same size of original negatives, the best
plan is to place a large sheet of white paper in front of the copying
camera, and rack the lens out of focus, taking care that the white
sheet of paper is sufficient to illuminate the full size of the plate.
Then in the dark-room place the negative in the carrier of the dark
slide, film side up, and carefully dust the surface (also seeing that
the back of the negative is clean) with a clean camel’s-hair brush;
dust also the film of a gelatine dry plate, and place it face down
upon the negative, close the slide, and insert it into the grooves of
the camera and expose to the light reflected through the lens from the
white sheet of paper for from five to twenty seconds, according to the
density of the negative and the intensity of the light; close the lens
and the shutter, and remove it into the dark-room for development.
By exposing a plate in this manner there is a far better chance of
getting absolute contact between the two plates, as the rays of light
are parallel.
For making transparencies for printing upon copper-plates, first of all
make a grained negative the proper size, then when this is varnished
and touched up, make from it as above a transparency upon a gelatine
chloride dry plate, by which process it is more certain to get a
suitable result for the process than on a gelatine bromide dry plate.
No formula will be given here for developing these gelatine chloride
plates as each packet has full instructions for development which must
be followed implicitly.
For developing gelatine bromide dry plates for negatives, or for
transparencies, the following, a modification of the Ilford formula,
will be found better than using formula where the ammonia and bromide
are mixed together.
_Solution No. 1._
Pyrogallic Acid 1 ounce.
Meta Bisulphate of Potash 1 ounce.
Bromide of Potassium 1 ounce.
Water 80 ounces. |31|
_Solution No. 2._
Liquor Ammonia 1/2 ounce.
Water 10 ounces.
For a normal exposure equal parts of these are mixed just before using,
and for an uncertain exposure begin with two parts of No. 1, one part
of water, and one part of No. 2.
THE REPRODUCTION OF NEGATIVES.
For the production of suitable collotype prints, the first
consideration is the negative, from which the printing surface is to
be made. The requirements are, a negative in which all the half-tones,
both in the lights and in the shadows, are reproduced in their proper
gradations; the lights must not be too dense, or the shadows too clear;
the blackest portion of the negative must be that part representing
the highest light, and the only clear glass, that which represents the
deepest shadow; these negatives may be either wet collodion, or on
ordinary gelatine-bromide plates and reversed—_i. e._, the mirror must
be used behind the lens. If it is desired to produce a collographic
block from a negative already in existence, and which may not be
retaken, then it will be necessary to make another negative. This may
be done in many different ways; but whichever way is adopted, the first
and most important step is the production of a transparent positive.
This transparent positive may be made on an ordinary gelatine-bromide
plate, or on a gelatine-chloride plate, and then from the transparent
positive a negative is made by transmitted light (in the enlarging
camera), either the same size, reduced or enlarged in size; or instead
of using this class of transparency, by far the finest results can be
got by making the transparency in carbon tissue, which can be used for
both transmitted light, for enlarging or reduction in size, and it can
also be used for contact printing, when the reproduced and reversed
negative is required to be the same size as the original.
On page 29 directions are given for the manipulations when gelatine dry
plates are used, so here we will confine ourselves to the production
of negatives from the carbon transparency. First of all, procure some
carbon transparency tissue, ready sensitized, cut this into sizes at
least half an inch larger all round than the negative, then cut a brown
paper mask to fit all round the negative, and to protect the margin
of tissue from the action of light; this acts as a safe edge to the
tissue, enabling it to adhere to the glass support during the operation
of development. If it is only intended to reproduce a portion of the
|32| original negative, the safe edge may be made by putting the mask
just round that portion of the image, but if the whole of the negative
is required, put it in a printing frame with a plate glass front, fix
the brown paper mask outside the negative, then place the sensitive
carbon tissue the black side next the negative film, and put in the
backing, and back, and fasten it up. Next expose it to the light for
four or five times the time requisite to get an ordinary silver print.
The carbon tissue having been exposed to light for the proper time, it
is taken into the dark-room, the edges are turned up so as to form a
dish, and pinned upon a small board; it is now coated with thin plain
collodion, the surplus being poured into a bottle kept for the purpose,
and the board gently rocked to and fro, so as to prevent the collodion
running in ridges, or streaks. Now allow the collodion to dry, then
place the coated and exposed tissue in clean cold water until it is
quite limp, then slide underneath it a gelatinized glass plate a little
larger all round than the tissue, bring the gelatinized surface of the
glass plate and the collodionized surface of exposed tissue in contact
under the water, taking great care not to abrade the collodion; then
lift the two out and lay the glass down upon a bench, and cover it over
with a piece of thin India-rubber, or mackintosh, or oil-cloth. Hold
the cover firmly on the bench with the left hand; with the right use a
strong squeegee vigorously. That will expel all the water from between
the two surfaces (collodion on the tissue, and gelatine on the glass),
and so bring them into absolute contact; now remove the cover, lift the
plate and examine the tissue through the glass, and if there are no
air-bells between the two surfaces the operation has been successful;
but if air-bells are present, _at once_ plunge the plate into cold
water, strip off the tissue, and try again, but be careful to be quick
about it, or there is a risk of the tissue absorbing too much water, in
which case great difficulty will be experienced in getting it to lie
flat upon the gelatinized glass.
The gelatinized plate upon which the exposed and collodionized tissue
is mounted is prepared as follows: Glass plates free from scratches and
stains, are placed in a weak mixture of hydrochloric acid and water,
and rubbed with a clean rag until free from grease and dirt; they are
then washed under the tap, and whilst wet are covered with:
Gelatine 1 ounce.
Water 20 ounces.
Bichromate of Potash 15 grains. |33|
Soak the gelatine in the cold water until it is soft, then melt at
gentle heat, add the bichromate, and when dissolved, filter, and coat
the cleaned plate two or three times, throwing the surplus away each
time of coating; now stand the coated plate on a rack to dry. Any
quantity of these plates may be prepared at a time, and used when
required. When all the batch has been coated, the rack may be removed
into a warm corner, free from dust, until the plates are dry, then
expose the plates to the light for at least twenty minutes, when
they are ready for use for mounting the exposed and collodionized
transparency tissue upon.
After the tissue is mounted upon the glass plate, it is allowed to
stand a few minutes, then it is placed in a dish containing hot water
(100° F.), and as soon as the tissue compound shows signs of dissolving
by exuding from under the paper, lift the paper away, then rock the
dish, and, if necessary, add more hot water; then raise the plate and
gently lave it with the hot water, until the whole of the soluble
pigmented gelatine is washed away, leaving a transparent positive on
the glass, which merely requires washing under the cold water tap, and
drying, to be finished. Now examine it closely, and if it contains the
whole of the details in the negative, and is free from dust spots, it
may be put away to dry, but if underexposed, or overexposed, or marred
by spots not in the original, then another must be done; but if care is
taken to dust the surface of the tissue, and the film of the negative
with a camel’s-hair brush before printing, and before collodionizing,
and also to see that both the collodion and the gelatine mixture are
properly filtered, and that the water in which the coated and exposed
tissue is soaked is free from dirt, then there will be no difficulty in
getting a perfect transparency.
The transparency being obtained, if it is desired to make a negative
larger or smaller than the original, the transparency is placed in the
enlarging camera with the carbon film inside, and the negative made the
requisite size, either on a dry plate or on a wet collodion plate.
If a negative the same size is required, the transparency is placed
face up in the carrier of the dark slide, and the surface dusted; then
take a dry plate, and having also dusted its film, place it face down
in contact with the transparency, then close the dark slide and remove
it to the camera, which should be previously adjusted opposite a large
sheet of white paper; now put in the dark slide, draw the shutter, and
expose to the sheet of paper, and develop as directed on page 30.
By exposing in this manner, instead of direct to the light (day or
gas), the |34| rays projected through the lens fall upon the dry plate
quite parallel, so securing sharp negatives.
In making reversed negatives for collographic printing when wet
collodion is used, it is not absolutely necessary to use the mirror, as
they may be made upon polished glass (_i. e._, not albumenized) dried,
then coated with gelatine, again dried, and stripped.
Some brands of dry plates can also be stripped, by first of all, after
drying, coating them with India-rubber solution, then with collodion,
and soaking in dilute fluoric acid; but great care and patience must
be exercised, the preliminary trials being made upon negatives of no
particular value.
STRIPPING COLLODION FILMS.
Negatives made by the wet collodion process can be easily stripped
from the glass support, and in such a condition are extremely handy,
especially for printing on collographic plates, or on zinc or copper,
as, the film being flexible, contact is more easily secured.
When it is intended to strip the film, the glass plate must be
polished, _not albumenized_, and after the negative is dried, strips
of thick paper are pasted along the four sides of the negative, with
starch, or a solution of gum tragacanth; the negative is then placed
on a levelling stand and a sufficient quantity of the following warm
solution of gelatine is poured upon the plate (which should be slightly
warmed first) and spread all over it by means of a glass rod:
Gelatine 2-1/2 ounces.
Sugar 1-1/2 ounces.
Water 20 ounces.
Soak the gelatine in the water until soft, melt by gentle heat, then
strain through two or three thicknesses of fine muslin, and pour upon
the plate, using three ounces for a plate 12 × 10; more or less, in
proportion for different sizes. The plate must be allowed to stay upon
the levelling stand until the gelatine has quite set, then put it on a
rack to dry, in a current of cold air, for if heat be used the gelatine
will melt.
When the gelatine is _quite dry_, pass the blade of a penknife all
around underneath the strips of thick paper, when the negative film may
be lifted away from the plate.
In this condition the negative may be stored away without risk of being
broken, the only condition being, it must be kept free from dampness.
|35|
Another plan of stripping is to coat the dry negative with a thick
solution of India-rubber in benzole, and allow it to dry on a levelling
stand; then coat with a thick plain collodion, to which a few drops of
castor oil have been added. When the collodion film is quite dry, place
the plate in a dish of clean water, strip off the film from the glass,
then place the film between sheets of clean blotting-paper to dry, or
it may be allowed to dry on the plate.
Old collodion negatives that have been varnished may be stripped, but
it is rather a risky operation, and should never be attempted until a
really good carbon transparency has first been obtained.
The first step will be to remove the varnish, which is effected by
soaking the plate in
Alcohol 1 part.
Ammonia 1 drachm.
until all the varnish is removed, then rinse in alcohol, wash well
under the tap, dry, paste slips of brown paper around the margin, and
finally level and cover with the warm gelatine, dry and strip.
THE REVERSING MIRROR.
The mirror is a piece of perfectly plane glass, coated on the surface
with a thick film of pure silver, and highly polished. This silvered
glass must be mounted in a mahogany box, and fitted on the front of the
camera, the lens being fitted in front of the mirror, see Fig. 1. This
mirror is placed in grooves at an angle of forty-five degrees from the
axis of the lens. So situated, it receives the image projected by the
lens, and reflects it on the focussing screen, or sensitive plate in
the camera. By this means a negative is obtained, which, when looked
through with the film between the operator and the glass, will present
the image in its proper position, whereas, if the lens be used without
the intervention of the mirror it will be necessary to have the glass
between the eyes and the film, to get the image in its proper position,
and as the generality of photographic negatives are taken without the
intervention of the mirror, they are called ordinary negatives in
contradistinction to those made through the mirror, which are called
“reversed” negatives.
The silver surface of the mirror requires great care and attention to
preserve its lustre. If tarnished, it will make the exposure in the
camera longer, besides which, the cost of resilvering is too great to
allow of the surface being spoiled through carelessness. |36|
At the end of each day’s work, remove the mirror from the box and warm
it in front of the fire (not over a gas flame) just slightly, then wrap
it carefully in a piece of fine velvet, which has also been previously
warmed, then wrap it up in a piece of India-rubber or macintosh cloth,
and put it in an air-tight box; by doing this, the mirror, if well
silvered at first, will last for a year or two.
If the surface should become tarnished, get a square of very fine
chamois leather, and place in the centre a pellet of cotton-wool; then
gather up the leather and tie the wool in the centre, making a small
globe about an inch and a half in diameter; now warm the mirror, and
after dipping the leather globe into fine rouge, proceed to polish
gently with a quick circular motion, using little or no pressure—take
care in doing this that the mirror, the leather, and the rouge are
quite dry, else the silver coat will come away—_verb sap_.
In purchasing a mirror be sure and get it large enough to take the cone
of rays from the lens it has to be used with; for a lens three inches
in diameter, the plane mirror should measure about 8 × 3-1/2 inches.
The box to hold this mirror should be made square at the side, fitting
into the rabbet of the camera front, so that when objects are to be
photographed that will do better lying on the ground than when fixed
against the wall, the mirror can be placed to look down upon the object.
For silvering the glass, one cannot do better than to quote the
directions for working Common’s process given by Major Waterhouse in
the _Photographic News_.
“The solutions recommended by Mr. Common are three:
(1) Nitrate of Silver 1 ounce.
Distilled Water 10 ounces.
(2) Caustic Potash 1 ounce.
Distilled Water 10 ounces.
(3) Glucose 1/2 ounce.
Distilled Water 10 ounces.
The above quantities are suitable for 250 square inches, consequently,
an ordinary copy mirror 8 × 6 would require rather more than two ounces
of each solution, and other sizes in proportion.
The caustic potash and distilled water must be quite pure. Ordinary
caustic potash will not answer at all. The best to use is known as
_pure by alcohol_.
The glass surface to be silvered is carefully cleaned with strong
nitric acid, applied, as recommended by Mr. Browning, with a Buckle’s
brush, then well |37| washed in clean water, and after rinsing with
distilled water, laid, face downward, in a dish of distilled water
until wanted.
Before cleaning the glass, it will be necessary to arrange for
supporting it face downward in the depositing dish, so that the surface
to be silvered may be quite horizontal, and just below the level of the
fluid, which should be about half an inch above the bottom of the dish.
I have generally used a large cork, about four inches in diameter,
cemented to the back of the plate, and fitted with three strings, by
which it could be suspended in a level position and adjusted to any
height by winding the string over a roller placed at a convenient
height above the dish. When this arrangement is not available, I fix on
the back of the plate two ordinary wide-mouthed bottle corks of equal
thickness, in the positions shown in the figure, and to these corks
attach thin slips of bamboo running transversely across the plate, and
of sufficient length to rest on the sides of the dish, thus:
[Illustration: Fig. 3.]
The slips of bamboo give the arrangement a certain amount of spring, by
which the height of the plate can easily be regulated, by putting on
weights until the surface of the plate is just below the level of the
fluid in the dish.
To prepare the silvering solution: A sufficient quantity of the
silver solution, No. 1 (two ounces), is put into a perfectly clean
glass. Ammonia is dropped in until the precipitate first formed is
just redissolved. The same quantity of potash solution, No. 2, as of
silver is now mixed in, and the precipitate again dissolved by ammonia.
A little more silver solution is then added to produce a distinct
turbidity, and distilled water to make up the quantity necessary to
fill the depositing dish to about three-eighths or half an inch, and
the mixture is then filtered through cotton into another clean glass
vessel.
The same quantity (two ounces), of filtered solution of glucose, No.
3, as was taken of silver and potash, is now mixed in, and the whole
is poured into a depositing dish (which should preferably be of glass,
well cleaned with nitric acid).
The glass plate is then taken out of the distilled water and laid
face downward on the silvering solution, being supported, as before
described, just above the surface, so that the solution does not cover
its back.
Mr. Common places the requisite quantity of distilled water in the dish
in |38| which the mirror has been remaining face downward, and then,
having lifted the mirror up, pours in the undiluted silvering solution,
together with the glucose solution, stirs well together, and then
carefully lowers the mirror again into the dish.
Almost immediately after the immersion of the plate, the silvering
action begins, and, if things are going on well, a brilliant reflecting
surface will be seen at the back of the plate, and in forty minutes,
or even less, a good deposit of silver will be obtained. It is usually
recommended to stop the action as soon as the silvering fluid appears
clear and free from turbidity, but it is not always easy to see this.
After silvering, the plate is well washed, finishing with distilled
water, and dried off quickly. A slight cloudiness of the surface may
appear, and must be removed by polishing before the mirror can be
used. It is better to allow the mirror to remain a day or so before
polishing, in order to harden the coating.
To polish the plate, it should be slightly warmed, and perfectly dry,
and rubbed very gently in small circles with a piece of very soft and
dry chamois leather, afterward using a little jewellers’ rouge.
Mirrors should always be kept in a dry place, and will require
repolishing from time to time.
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