[Footnote 1: Read June 21, 1882, before the Boston Society of
Civil Engineers.]
By CHANNING WHITAKER.
The blue process is well known to the members of the society, and I need
not take time to describe it; but with the ordinary blue process
printing frame the results are sometimes unsatisfactory, and now that
the process has come to be so commonly used I have thought that an
account of an inexpensive but efficient printing frame would be of
interest. The essential parts of the apparatus are its frame, its glass,
its pad or cushion, its clamps, and the mechanism by which the surface
of the glass can easily be made to take a position that is square with
the direction of the sun's rays.
_The Blue Process Printing Frame in Common Use.--Its Defects._--The pad
of the apparatus in common use consists of several thicknesses of
blanketing stretched upon a back board. The sensitized paper and the
negative are placed between the pad and the plate glass, and the whole
is squeezed together by pressure applied at the periphery of the glass
and of the back-board. Both the glass and the back-board spring under
the pressure, and it results that the sensitized paper is not so
severely pressed against the negative near the center of the glass as it
is near the edges. If at any point the sensitized paper is not pressed
hard up against the negative, a bluish tinge will appear where a white
line or surface was expected. With an efficient printing frame and
suitable negatives, these blue lines will never appear, and it was to
prevent the production of defective work that I undertook to improve the
pad of the printing frame.
_The Printing Frame Used in Ordinary Photography._--Very naturally, I
first examined the printing frame used in ordinary photography. This
frame is extremely simple, and is very well adapted to its use. It is,
undoubtedly, the best frame for blue process printing, when the area of
the glass is not too large. The glass is set in an ordinary wooden
frame, while the back-board is stiff and divided into two parts. A flat,
bow-shaped spring is attached by a pivot to the center of each half of
the back-board. The two halves of the back-board are hinged together by
ordinary butts. Four lugs are fastened to the back of the frame, and,
when the back-board is placed in position, the springs may be swung
around, parallel to the line of the hinges, and pressed under the lugs,
so that the back of the back-board is pressed most severely at the
center of each half, while the glass is prevented from springing away
from the back-board by the resistance of the frame at its edges. Unless
the frame is remarkably stiff, it will resist the springing of the glass
more perfectly in the neighborhood of the lugs than elsewhere. It will
now be seen that, on account of the manner in which the pressure is
applied, the back-board tends to become convex toward the glass, while
the adjacent surface of the glass tends to become concave toward the
back-board; and that with such a frame, the pressure upon all parts of
the sensitized paper is more nearly uniform than when the pressure is
applied in the manner before described. With a small frame of this
description, a piece of ordinary cotton flannel is used between the
back-board and the sensitized paper, and, with larger sizes, one or more
thicknesses of elastic woolen blanket are substituted for the cotton
flannel. There is an advantage in having a hinged back-board like that
which has been described, because, when the operator thinks that the
exposure to sunlight has been sufficiently prolonged, he can turn down
either half of the back and examine the sensitized paper, to see if the
process has been carried far enough. If it has not, the back-board can
be replaced, and the exposure continued, without any displacement of the
sensitized paper with respect to the negative. This is an important
advantage.
_An Efficient Blue Process Frame, for Printing from Large Negatives, or
for Printing Simultaneously from many Small Ones._--In order to be
efficient, such a frame must be capable of keeping the sensitized paper
_everywhere tightly pressed against the negative_. Again, such a frame,
being large, is necessarily somewhat heavy. It should be so mounted that
it can be handled with ease; and, in order that it may print quickly, it
should be so arranged that it can be turned without delay, at any time,
into a position that is square with the direction of the sun's rays.
Undoubtedly, if a sufficiently thick plate of glass should be used, the
ordinary photographic printing frames would answer the purpose, whatever
the size, but very thick plate glass is both heavy and expensive.
Commercial plate glass varies in thickness from one-fourth to three
eighths of an inch, and the thicker plates are rather rare. A large
plate of it is easily broken by a slight uniformly distributed pressure.
But the pressure that is required for the blue process printing,
although slight, is much greater than is used in the ordinary
photographic process. For the sensitized paper that is used in the blue
process printing is, comparatively, very thick and stiff, and it may
cockle more or less, while the paper that is used in ordinary
photography is thin and does not cockle. Now, it is easy to see that a
pressure severe enough to flatten all cockles must be had at every part
of the sensitized paper, and that, if the comparatively thin,
inexpensive, light weight, commercial plate glass is to be used, it is
desirable to have the pressure _nowhere much greater than is needed for
that purpose_, lest the fragile glass should be fractured by it. In each
of my large frames I use the commercial plate glass; instead of the
cushion of cotton flannel, or of flannel, I use a cushion filled with
air of sufficiently high pressure to flatten all cockles, and to press
all parts of the sensitized paper closely against the negative; and
instead of the hinged back-board I use a back-board made in one piece
and clamped to the frame of the glass at its edges. Connected with the
cushion is a pressure gauge, and a tube with a cock, for charging the
cushion with air from the lungs. Experience shows what pressure is
necessary with any given paper, and the gauge enables one to know that
the pressure is neither deficient nor in excess of that which is safe
for the glass.
[Illustration: PLAN. COTTON FLANNEL REMOVED.]
[Illustration: SECTION AT CO.]
_The Construction of the Air-Cushion._--The expense of such an
air-cushion seemed at first likely to prevent its being used; but a
method of construction suggested itself, the expense of which proved to
be very slight. The wooden back-board, as constructed, is made in one
piece containing no wide cracks. It has laid upon it some thick brown
Manila paper, the upper surface of which has been previously shellacked
to make it entirely air-tight. Upon this shellacked surface is laid a
single thickness of thin paper of any kind; even newspaper will answer.
Its object is simply to prevent the sheet rubber, which forms the top of
the air-cushion, from sticking to the shellacked paper. The heat of the
sun is often sufficient to bring the shellac to a sticky state. It would
probably answer as well to shellac the under side of the paper, and to
use but one sheet, but I have not tried this plan. Around the periphery
of the pad, there is laid a piece of rubber gasket about one and a half
inches wide, and about one-eighth of an inch thick. In order that the
gasket may not be too expensive, it is cut from two strips about three
inches wide. One of them is as long as the outside length of the frame,
and the other is as long as the outside width of the frame. Each of
these strips is cut into two L-shaped pieces, an inch and a half in
width, with the shorter leg of each L three inches long. When the four
pieces are put together a scarf joint is made near each corner, having
an inch and one-half lap. It is somewhat difficult to cut such a scarf
joint as perfectly as one would wish, and it is best to use rubber
cement at the joints. Over the gasket is laid a sheet of the thinnest
grade of what is called pure rubber or elastic gum. Above this, and over
the gasket, is placed a single thickness of cotton cloth, of the same
dimensions as the gasket, and yet above this are strips of ordinary
strap iron, an inch and a half wide and nearly one eighth of an inch
thick. These strips are filed square at the ends and butt against each
other at right angles. As the edges of the strips are slightly rounded,
they are filed away sufficiently to form good joints wherever the others
butt against them. The whole combination is bound together by ordinary
stove bolts, one quarter of an inch in diameter, placed near the center
of the width of the iron strips, and at a distance apart of about two
and one-half inches. Their heads are countersunk into the strap iron. In
making the holes for the stove bolts through the thin rubber, care
should be taken to make them sufficiently large to enable the bolt to
pass through without touching the rubber, otherwise the rubber may cling
to the bolts, and if they are turned in their holes the rubber may be
torn near the bolts and made to leak. A rough washer, under each nut,
prevents it from cutting into the back-board. For the purpose of
introducing air to, or removing air from, the pad, a three-eighths of an
inch lock nut nipple is introduced through the back-board, the
shellacked paper, and its thin paper covering. Without the back-board a
T connects with the nipple. One of its branches leads, by a rubber tube,
to the pressure gauge, which is a U-tube of glass containing mercury.
The other branch has upon it an ordinary plug cock, and, beyond this, a
rubber tube terminating in a glass mouth-piece. When it is desired to
inflate the air-cushion, it is only necessary to blow into the
mouth-piece. A pressure of one inch of mercury is sufficient for any
work that I have yet undertaken. With particularly good paper, a lower
pressure is sufficient. Upon the top of the pad is laid a piece of
common cotton flannel with the nap outward, and with its edges tacked
along the under edge of the back-board. The cotton flannel is not drawn
tight across the top of the pad. The reason for employing a cotton
flannel covering is this: When the sheet rubber has been exposed for a
few days to the strong sunlight, it loses its strength and becomes
worthless. The cotton flannel is a protection against the destruction of
the rubber by the sunlight. I first observed this destruction while
experimenting with a cheap and convenient form of gauge. I used, as an
inexpensive gauge, an ordinary toy balloon, and I could tell, with
sufficient accuracy, how much pressure I had applied, by the swelling of
the balloon. This balloon ruptured from some unknown cause, and I made a
substitute for it out of a round sheet of thin flat rubber, gathered all
around the circumference. I made holes about one-quarter of an inch
apart, and passing a string in and out drew it tight upon the outside of
a piece of three eighths of an inch pipe, I then wound a string tightly
over the rubber, on the pipe, and found the whole to be air-tight. This
served me for some time, but one day, on applying the pressure, I found
a hole in the balloon which looked as if it had been cut with a very
sharp knife. That it had been so cut was not to be imagined, and on
further examination I found that the fracture had occured at a line
which separated a surface in the strong sunlight from a surface in the
shade, at a fold in the rubber. I saw that all of the rubber which had
been continuously exposed to the intense sunlight had changed color and
had become whiter than before, and that that portion of the balloon had
lost its strength. I then returned to the use of the mercury gauge, and
took the precaution to cover my pad with cotton flannel, as a protection
from the light and from other sources of destruction. This pad is upon
the roof of the Institute; and is exposed to all weathers. As a
protection from the rain and the snow, the whole is covered again with a
rubber blanket. It has withstood the exposure perfectly well for a year,
without injury. The gauge, made from flat rubber, is altogether so cheap
and so convenient that I am now experimenting with one of this
description having a black cloth covering upon the outside. The balloon
is of spherical shape, the black cloth covering is of cylindrical shape,
and I hope that this device will serve every necessary purpose. A
sectional view of the air-cushion is offered as a part of this
communication.
_The Frame, which Contains the Plate Glass_, is made of thick board or
plank, with the broad side of the board at right angles to the surface
of the glass. A rabbet is made for the reception of the glass, and four
strips of strap iron, overlapping both the glass, and the wood, and
screwed to the wood, keep the glass in position. Strips of rubber are
interposed between the glass and the wood and between the glass and the
iron. The frame is hinged to the back-board by separable hinges, so that
the glass can be unhinged from the pad without removing the screws.
Hooks, such as are used for foundry flasks, connect the frame with the
pad upon the opposite side. A frame made in this manner is very stiff
and springs but little, and its depth serves an excellent purpose. The
air-cushion and the frame are so mounted that they can be easily turned
to make the surface of the glass square with the direction of the sun's
rays. It is necessary to have a tell tale connected with the apparatus,
which will show when the surface of the glass has been thus adjusted.
The shadow of the deep frame is an inexpensive tell-tale, and enables
the operator to know when the adjustment is right. I have now described,
in detail, the construction of the air-cushion with its back-board, as
well as that of the frame which holds the plate glass, and I think it
will be evident that the first cost of the materials of which they are
made is comparatively little, and that the workmanship required to
produce it is reduced to a minimum. It will also, I think, be evident
that a uniform pressure, of any desired intensity, can be had all over
the surface of the sensitized paper for the purpose of securing perfect
contact between it and the negative. The blue copies that are taken with
this apparatus are entirely free from blue lines when the negatives,
chemicals, and paper are good.
_The Mechanism for Adjusting the Surface of the Glass, until it shall be
Perpendicular to the Direction of the Sun's Rays._--I have found many
uses for the blue copying process in connection with the work of
instruction at the Massachusetts Institute of Technology. Notes printed
by it are far better and less costly than those printed by papyrograph.
I will not detain you now with an account of the uses that I have made
of it. I will merely say that more than a year ago I found that my
frame, which has a glass 3 feet x 4 feet, was wholly inadequate to the
work in hand, and I tried to increase the production from it by
diminishing the time of printing. The glass of this frame was
horizontal, except when one of its ends was tilted off from the slides
which guided it when pushed out of the window; and I knew that it took
three or four times as long to print when the sun was low as it did when
the sun was near the meridian. I made plans for mounting this frame upon
a single axis, about which it could be turned after it had been pushed
through the window, but I saw that no movement about a single axis would
give a satisfactory adjustment for all times of the year, and I
considered what arrangement of two axes would permit a rapid and perfect
adjustment, at all times, with the least trouble to the operator. It was
evident that when the sun was in the equatorial plane, the surface of
the glass should contain a line which was parallel to the axis of the
earth; and further, that if such a glass was firmly attached to an axis
which was parallel to that of the earth, it would fulfill the desired
purpose. For the glass, being once in adjustment, is only thrown out of
position by the rotation of the earth, and if the glass is rotated
sufficiently about its own axis, in a direction opposite to that of the
earth, it will retain its adjustment. In order to have the adjustment
equally good when the sun was either north or south of the equatorial
plane, it was sufficient to mount a secondary axis upon the primary one
and at right angles to it. About this the glass could be turned through
an angle of 231/2 deg., either way, from the position which it should have
when the sun was in the equatorial plane.
[Illustration: BLUE PROCESS PRINTING APPARATUS.]
_The Construction of the Adjusting Mechanism._--I desired to have the
mechanism as compact and inexpensive as possible, and to have the frame
well balanced about the primary axis, in every position. I also desired
to have a rotation of nearly 180 deg. about the principal axis. The plan
adopted will be most easily understood by referring to the drawing which
illustrates it. The axes are composed chiefly of wood. They are built up
from strips which are 3 inches x 7/8 inch, and from small pieces of 2
inch plank. They are stiffly braced. A pair of ordinary hinges permit
the secondary rotation to occur, while a pair of cast iron dowel pins
with their sockets, such as are used in foundry flasks, serve as pivots
during the primary rotation.
_The Adjustments._--The adjustment about the secondary axis does not
need to be made more frequently than once a week, or once a fortnight.
In order to prevent rotation about this axis when in adjustment, two
cords lead from points which are beneath the back board, and as far
removed from the secondary axis as is convenient. Each cord passes
forward and backward through four parallel holes in a wooden block which
is attached to the primary axis. The cords can be easily slipped in the
holes by pulling their loops, but the friction is so great that they
cannot be slipped by pulling at either end. It takes about twice as long
to make the adjustment as would be necessary if a more expensive device
had been used; but this device is at once so cheap, so secure, and has
so seldom to be used, that it was thought to be best adapted for the
purpose. To prevent rotation from occurring about the primary axis when
it is not desired, a bar parallel to the secondary axis is attached by
its middle point to the primary axis near one end. A cord passes from
either end of this bar through cam shaped clamps, which were originally
designed for clamping the cords of curtains with spring fixtures. These
clamps are cheap. They are easily and quickly adjusted, and are very
secure.
The whole apparatus can be located upon the roof of a building, or, if
convenient, it can be mounted upon slides, and pushed through an open
window when it is to be exposed to the light. If it is to be used upon a
roof, a small hut, or shelter of some sort, near by is a great
convenience to the operator, particularly in winter.
_An Inexpensive Drying Case for Use in Coating the Paper._--When the
apparatus is in continuous use, time may be saved by having a convenient
arrangement for drying the sheets that have been coated with the
sensitizing liquid. I have made an inexpensive drying case which serves
the purpose very well. It consists simply of a light-tight rectangular
case of drawers. There are twenty-five drawers in all. They are
constructed in an inexpensive manner, and are the only parts of the case
that are worth describing. They are very shallow, being but 1-7/8 inches
deep, and as it appeared that the principal expense would be for the
materials of which the bottoms of the drawers should be composed, it was
decided to make the bottoms of cotton cloth. This cloth is stretched
upon a frame, the dimensions of which are greater than that of the paper
to be dried. The stock of which the frame is made is pine, 11/4 inches
wide, and three-eighths of an inch thick. The corners are simply mitered
together and attached to each other by means of the wire staples that
are commonly used for fastening together pages of manuscript, and which
are called "novelty staples." Eight staples are used at each miter, four
above and four below the joint. Two of the staples, at the top and near
the ends of the joint, are set square across it, and two others, at the
top and near the middle of the joint, are placed diagonally across it.
The staples at the bottom are similarly placed. The joint is quite firm
and strong, and is likely to hold for an indefinite period with fair
usage. The cloth, stretched upon the frame, is fastened to it by means
of similar staples. A dark colored cloth not transparent to light is to
be preferred. A strip of pine, 1-13/16 inches wide, and three eighths of
an inch thick, forms the vertical front of the drawer, and prevents the
admission of much light from the front while the sheet is drying. Two
triangular knee pieces, three-quarters of an inch thick, serve to
connect the front board with the frame, and four small screws with a few
brads are used in attaching them. The lower edge of the front board
drops one-quarter of an inch below the bottom of the drawer. My case
stands in a poorly lighted room, and paper dried in this case and
removed to a portfolio as soon as it is dry does not seem to be injured
by the light that reaches it. With the case in a well lighted room, I
should prefer to have outer doors to the case, made of ordinary board
six or eight inches wide, hinged to one end, and arranged to swing
horizontally across the front of the case. These would more completely
prevent the admission of light. The opening of any one of the doors
would allow three or four of the drawers to be filled, while the rest of
the case would be comparatively dark at the same time.[2]
[Footnote 2: Since this paper was read, I have seen in the
office of the City Engineer of Boston a drying case which is
similar in some respects to the one that I have devised. It has
been longer in use than my own. The drawers are simply the
ordinary mosquito netting frames covered with cotton netting.
They have no fronts, but a door covers the front of the case,
and shuts out the light.]
_The Portfolio for Protecting the Sensitized Paper from Exposure to
Light._--The sensitized paper is very well protected from exposure to
light, if kept in a portfolio or book, the brown paper leaves of which
are considerably larger than the sensitized sheets. The sheets may be
returned to such a book after exposure, and washed at the convenience of
the operator. They can be washed more quickly and perfectly if _two_
water-tanks are provided in which to wash them. A few minutes' soaking
will remove nearly all of the sensitizing preparation which has not been
fixed by the exposure. If the soaking is too long continued in water
that is much discolored by the sensitizing preparation, the sheets
become saturated with the diluted preparation, and they may become
slightly colored by _after_ exposure. If the first soaking is not too
long continued, and if the sheets are transferred at once to a second
bath of clean water, which is kept slowly changing from an open faucet,
they may remain there until the soluble chemicals have been entirely
extracted, and there will be no risk of staining by after exposure.
Washing in two tanks is of more consequence when the ground is white and
the lines blue, than when the ground is blue and the lines white.
_The Grades of Paper that are well Adapted for Blue Process Work._--I
have tested many grades of paper, to ascertain if they were well adapted
for blue process work. Some grades of brown Manila are very good; others
have little specks embedded in their surfaces which refuse to take on a
blue tint; still others, when printed upon, have white lines that are
wider than the corresponding black lines of the negative. The blue
obtained upon bond paper appears to be particularly rich, and the whites
remain pure; but bond paper cockles badly, and the cockles remain in the
finished print. Weston's linen record is an excellent paper. It is
strong, cockles but little, and dries very smooth. A paper that is used
by Allen & Rowell, for carbon printing, is comparatively cheap, and is
an excellent paper. It is not so stiff as the linen record, and the
whites are quite as pure. It does not cockle, neither does it curl while
being sensitized. It comes in one hundred pound rolls, and is about
thirty inches wide. The best papers are those that are prepared for
photographic work. The plain Saxe and the plain Rives both give
excellent results. Blue lines on a pure white ground can be obtained on
these papers, from photographic negatives, without difficulty. None of
the hard papers of good grade require the use of gum in the sensitizing
liquid. The liquid penetrates the more porous papers too far when gum is
not used, and without it good whites are seldom obtained upon porous
paper.
_The Best Chemicals for this Work_ are the _recrystallized_ red
prussiate of potash and the citrate of iron and ammonia, _which is
manufactured by Powers & Wightman_, of Philadelphia. If the red
prussiate has not been recrystallized, the whites will be unsatisfactory
and the samples of citrates of iron and ammonia which have come to us
from other chemists than those named, have all proved unreliable for
this process.
_The Sensitizing Liquid.--Its Proportions._--The blue process was
originally introduced from France, by the late Mr. A. L. Holley. I was
indebted to Mr. P. Barnes, who was with Mr. Holley at the time, for an
early account of it, and I had the first blue process machine that was
in use in New England. Since 1876, instruction in the use of the blue
process has been given to the students of mechanical engineering of the
Massachusetts Institute of Technology, and they have caused its
introduction into many draughting offices. The proportions of the
sensitizing liquid, as originally given me by Mr Barnes, were as
follows:
Red prussiate of potash............. 8 parts.
Citrate of iron and ammonia......... 8 parts.
Gum arabic.......................... 1 part.
Water.............................. 80 parts.
_Results of Experiments._--In our use, it first appeared that the gum
might be omitted from the preparation when sufficiently hard papers were
used. Next, that a preparation containing
Red prussiate of potash........ 2 parts,
Citrate of iron and ammonia.... 3 "
Water......................... 20 "
printed more rapidly. This preparation I continue to use when much time
may elapse between sensitizing and printing; but, when the paper is to
be printed immediately after sensitizing, I use a larger proportion of
citrate of iron and ammonia. Before arriving at the conclusion that
these proportions were the best to be used, I made a series of purely
empirical experiments, beginning with the proportions:
Red prussiate of potash.......... 10 parts.
Citrate of iron and ammonia....... 1 part.
Water............................ 50 parts.
and ending with the proportions:
Red prussiate of potash............... 1 part.
Citrate of iron and ammonia.......... 10 parts.
Water................................ 50 "
I found the best plan for conducting these experiments to be: To coat a
sheet of the paper with a given mixture; to cut the sheet into strips
before exposure; to expose all the strips of the sheet, at the same
time, to the direct sunlight without an intervening negative; and to
withdraw them, one after another, at stated intervals. I found that with
each mixture there was a time of exposure which would produce the
deepest blue, that with over-exposure the blue gradually turned gray,
and that if a curve should be plotted, the abscissas of which should
represent the time of exposure, and the ordinates of which should
represent the intensity of the blue the curves drawn would have
approximately an elliptical form, so that if one knew the exact time of
exposure which would give the best result with any mixture, one might
deviate two or three minutes either way from that time without producing
a noticeable result. I have found that, with the same paper, the same
blue results with any good proportions of the chemicals named, provided
a sufficient weight of both chemicals is applied to the surface; that an
excess of the red prussiate of potash renders the preparation less
sensitive to light, and very much lengthens the necessary time of
exposure; that the prints are finer with some excess of the red
prussiate; that an excess of the citrate of iron and ammonia hastens the
time of printing materially; that a greater excess of the citrate causes
the whites to become badly stained by the iron, while a still greater
excess of the citrate, in a concentrated solution causes the sensitized
paper to change without exposure to light, and to produce a redder blue
or purple, which does not adhere to the paper, but may be washed off
with a sponge. I have found that the cheapest method of reproducing
inked drawings that have been made on thick paper is not to trace them,
but to print the blues from a photographic glass negative; and also,
that the dry plate process is well adapted to such work in offices, when
one has become sufficiently experienced. Printed matter can also most
easily and inexpensively be reproduced by the same means, when a small
issue is required on each successive year. For the reproduction of
manuscript by the blue process, the best plan that I have found has been
to write the manuscript upon the thinnest blue tinted French note-paper,
with black opaque ink--the stylographic ink is very good--and,
afterward, to dip the paper into melted paraffine, and to dry the paper
at the melting temperature. This operation, if cheaply done, requires
special apparatus. For positive printing from the glass negative, I use
a multiple frame, by the aid of which I can print from 16 negatives at
the same time, upon a single sheet of paper. This frame is
interchangeable with the one that contains the plate glass. The
negatives are so arranged in the frame that the sheets can be cut and
bound, as in the ordinary process of book binding. The time required for
exposure, when printing from glass negatives, varies with the negative;
and, in order to secure satisfactory results with the multiple frame it
is necessary to stop the exposure of some, while the exposure of others
is continued. I insert wooden or cloth stoppers into the frame for the
purpose of stopping the exposure of certain negatives. When paraffined
manuscript is to be printed from, I find it convenient to have it
written on sheets of small size, and to have these mounted upon an
opaque frame of brown Manila paper, printing sixteen or more at a time,
depending upon the size of the printing frame. Many small tracings may
be similarly mounted upon a brown paper multiple frame, and may be
printed together upon a single sheet.
* * * * *