By W.B. PARSONS, JR., C.E.
I send you a description of a device that I got up for the N.Y., L.E.,
and W.R.R. division office at Port Jervis, by which I overcame the
difficulties incident to large glasses. The glass was 58 inches long,
84 inches wide, and 3/8 inch thick. It was heavily framed with ash. In
order to keep the back from warping out of shape, I had it made of
thoroughly seasoned ash strips 1" x 1". Each strip was carefully
planed, and then they were glued and screwed together, while across
the ends were fastened strips with their grain running transversely.
This back was then covered on side next to the glass with four
thicknesses of common gray blanketing. Instead of applying the holding
pressure by thumb cleats at the periphery, it was effected by two long
pressure strips running across the back placed at about one quarter
the length of the frame from the ends, and held by a screw at the
center. The ends of these strips were made so as to fit in slots in
the frame at a slight angle, so that as the pressure strips were
turned it gave them a binding pressure at the same time. In other
words, it is the same principle as is commonly used to keep backs in
small picture frames. This arrangement, instead of holding the back at
the edges only, and so allowing the center to fall away from the
glass, distributed it evenly over the whole surface and always kept it
in position. The frame was run in and out of the printing room on a
little railway on which it rested on four grooved brass sheaves, one
pair being at one end, while the other was just beyond the center, so
the frame could be revolved in direction of its length without
trouble. In order to raise the heavy back, I had a pulley-wheel
fastened to the ceiling, through which a rope passed, with a ring that
could be attached to a corresponding hook at the side of the back, in
order to hoist it or lower it. Although that is an extremely large
apparatus, yet by means of the above device it was worked easily and
rapidly, and gave every satisfaction.
The solution used was of the same proportions as had been adopted in
the other engineering offices of the road:
Citrate iron and ammonium 1-7/8 oz.
Red prussiate potash (C.P.) 1-1/4 oz.
Dissolve separately in 4 oz. distilled water each, and mix when ready
to use. But by putting mixture in dark bottle, and that in a tight box
impervious to light, it can be kept two or three weeks.
In some frames used at the School of Mines for making large blue
prints a similar device has been in use for several years. Instead,
however, of the heavy and cumbrous back used by Mr. Parsons, a light,
somewhat flexible back of one-quarter inch pine is employed, covered
with heavy Canton flannel and several thicknesses of newspaper. The
pressure is applied by light pressure strips of ash somewhat thicker
at the middle than at the ends, which give a fairly uniform pressure
across the width of the frame sufficient to hold the back firmly
against the glass at all points. This system has been used with
success for frames twenty-seven by forty-two inches, about half as
large as the one described by Mr. Parsons. A frame of this size can be
easily handled without mechanical aids. Care should be taken to avoid
too great thickness and too much spring in the pressure strips, or the
plate glass may be broken by excessive pressure. The strips used are
about five-eighths of an inch thick at the middle, and taper to about
three-eighths of an inch at the ends.
The formulæ for the solution given by Whittaker, Laudy, and Parsons
are practically identical so far as the proportions of citrate of iron
and ammonia and of red prussiate of potash, 3 of the former to 2 of
the latter, but differ in the amount of water. Laudy's formula calls
for about 5 parts of water to 1 of the salts, Whittaker's for 4 parts,
and Parson's for a little more than 2 parts. The stronger the solution
the longer the exposure required. With very strong solutions a large
portion of the Prussian blue formed comes off in the washwater, and
when printing from glass negatives the fine lines and lighter tints
are apt to suffer. The blue color, however, will be deep and the
whites clear. With weak solutions the blues will be fainter and the
whites bluish. Heavily sized paper gives the best results. The
addition of a little mucilage to the solution is sometimes an
advantage, producing the same results as strength of solution, by
increasing the amount adhering to the paper. With paper deficient in
sizing the mucilage also makes the whites clearer.--_H.S.M., Sch. of
M. Quarterly._
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