To coat plates perfectly, says H. S. Starnes in _British Jour. of
Photography_, I found the following points were necessary:
1. That a certain quantity of emulsion should be flowed in one even
stream all over the plate, instead of pouring the emulsion in a pool in
the center of the plate and then dispersing it over the whole surface;
because in the latter mode of coating large plates the gelatine is apt
to commence setting before it is equally distributed, and an unequally
coated plate is the result.
2. The plate ought to be put on the leveling-table before coating, and
not be moved before the gelatine is set; because in the dull light of
the dark room it is so difficult to prevent the emulsion running off
the plate when putting it down on the leveling-table.
3. I found that if the emulsion be rubbed (so to speak) on to the glass
there is much less chance of frilling, etc., than if it were poured
on. I think it is because in the former case the gelatine is in firmer
contact with the glass. When the gelatine is poured on to the plate
the cold glass instantly chills it, and by the time the emulsion has
reached the edges of the plate it has so far set as to have partially
lost its power of adhesion to the smooth surface of the glass.
[Illustration: Fig. 1.--Showing melted emulsion in coater ready for
coating.]
Two or three years ago, when it was the practice to warm the plates
before coating, I found from a series of experiments I then made that
when a plate was warmed before being coated the emulsion commenced
setting on the surface of the film, and of course in setting
contracted, thereby leaving a partial vacuum between the film and the
glass. On development frilling was the consequence. I found, however,
that, when pouring the same emulsion on cold glass, on the portion
of the plate where it was poured on, the film instantly chilled and
commenced to contract on to the glass, and it never frilled there; but
toward the edges of the plate, as the emulsion had commenced to chill
before they were covered, the film was not in such perfect contact with
the glass. Any person can try the experiment by first coating a plate
in the ordinary way, and on the second plate just pour a small pool
of emulsion on the center; let both dry, and he will then see after
exposure which frills the easier on development.
[Illustration: Fig. 2.--Showing emulsion flowing through the slit on to
the glass.]
After a series of experiments I found that by brushing a substratum of
emulsion on to the cold plate (with a brush made by binding a strip
of wash-leather at the end of a strip of glass), and then pouring the
full quantity of emulsion on to the substratum (for quarter-plates I
used a small silver teaspoon, which held sufficient to cover that size
of plate), I found I could coat plates far better and quicker and as
easily as when coating with collodion, and I got over the difficulties
of having frilling plates.
When only a few small plates are required--such as for experimental
purposes--I believe this method is as quick and good as any; but when
several dozen plates are wanted, any plan of coating them separately
takes a long time. With my plate-coater I can coat a dozen plates in
about the time I formerly took to coat one. When coating a number, I
thought it would be best to lay them in rows on the leveling-shelves
and draw the receptacle containing the emulsion over them, rather than
keep the latter a fixture and run the plates under it either on an
endless band or sliding shelves; because by the first mode the plates
can be fixed close together, and the emulsion is less likely to get
between them.
The coater is a species of wooden tray (of which the diagrams show the
section), having a small slit in one of the bottom edges through which
the emulsion passes in one even wave the whole width of the plate. The
width of the coater is the same as that of the plate, though one six
and a half inches wide can be used for either half or whole plates.
[Illustration: Fig. 3.]
I find the best way of making it, so as to get the slit an equal
opening the whole length of it, is to put the back, bottom, and two
sides together first, as in Fig. 3. Then by putting a piece of very
thin paper (A B) on the angle piece when the front piece of wood is put
tight down on the paper and fixed in its place, and the paper is drawn
out, it will be found that the slit is very even. In one coater I made
I had the slit a little too wide an opening, and to reduce it I glued
a piece of muslin over it. This I found was a great improvement, as it
not only acted as a strainer, but it checked and caused a more even
flow of the emulsion over the plate. I varnished the wood and muslin
(except over the slit) with black Japan.
To coat the plates I put them close together in rows on the
leveling-shelf, as shown below:
[Illustration: Fig. 4.]
A is a thin, narrow ledge of wood. B B B are thin pieces of wood, in
the center of each of which is a small slot and thumb-screw. The plates
are pressed against A by the pieces of wood, B, and the thumb-screws
are then fastened. The plates are thus kept from slipping about. All
this, of course, can be done in ordinary white light. The light is then
made non-actinic; the melted emulsion is poured into the reservoir of
the coater, which is put to the left hand edge of the outer row of
plates. It is then lifted up on edge, as in Fig. 2, and drawn slowly
over the row of plates, and so on until the whole of the rows are
coated. Of course when not coating plates it is kept in a horizontal
position, as in Fig 1. The emulsion on the plates is allowed to set
without being disturbed; the shelf is then slipped into the drying-box
until the plates are dry, so that they are not touched from the time
they are coated until they are dry and ready for packing.
I am at present engaged in making a modification of this coater to hold
a much larger quantity of emulsion at one time, when a large number of
plates require to be coated. It is something the shape of a flat teapot.
[Illustration: Fig. 5.--A A is a piece of curved glass. B a piece of
coarse ground flat glass, ground side uppermost, sliding in two grooves
in the wooden side. C is the handle fixed to the wooden back.]
A piece of thin paper is placed on the curved glass, and the ground
glass pushed close up and fixed by two small wedges, D. The paper
is then slipped out, leaving a narrow, even opening between the two
glasses. The width of this opening can be varied by using thicker paper
if the plates require to be coated with a thicker film. By using this
form the coater can be more easily cleaned, as the ground glass can be
slipped right out at the back, and probably in passing from the opening
to the plates over the curved glass the wave of the emulsion will be
equalized as well as when passing through the muslin.