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Tracing Process On Metal








We call the attention of metal engravers to this process. It is well
known that wood engravers have their original designs photographed on the
block in order to save considerable time by not making the drawing
themselves; moreover the cost is nominal, so to say, and the copy more
true and perfect than it can be done by hand. Why should not the copper
engraver and the aquafortist avail themselves of the same advantages? A
few do it secretly, no doubt, but the generality not knowing the process,
or, if so, not having tried it, think it is not possible or that it may
spoil their plates. This is an error. It can be done and very easily by
adhering to the following instructions:

Dissolve 2 parts of ammonium bichromate in 100 parts of water, and in this
let soak for an hour or so 10 parts of Coignet's best gelatine, then
dissolve on a water bath, filter through flannel, and the solution is
ready for use.

Before being coated, the plate should necessarily be cleaned free from
oxidation and greasy matters. This is done by immersing the plate for a
few moments in a warm solution of common potash, then rinsing and rubbing
it with chalk moistened with a little water, when after rinsing again and
draining the plate should be immediately prepared.

To spread the gelatine solution in an even and thin layer, a tournette is
employed. The most simple consists of a round wooden stick of which the
upper part is carved in the form of a cup with an edge, or rim, about one
quarter of an inch broad. On this rim is melted some gutta-percha, upon
which the plate is pressed into contact and adhers quite firmly when the
gutta-percha is solidified. The stick is perforated at the lower end and
revolves on an iron pivot fixed at the bottom of the support, being held
in the opening on the platform of the same, as shown in the diagram on the
following page.

The plate being fastened to the tournette, the warm gelatine solution is
flowed over it and spread to the edges by means of a glass rod or a piece
of cardboard, avoiding air bubbles. This done the tournette is set into
motion, and when the film is equalized, which is done in a moment, the
plate is detached, placed on a leveled stand and slowly dried with the
spirit lamp.

[A Tournette]

By a good light the exposure on the shade does not exceed twenty minutes
with a pretty intense transparency, and should be regulated with a
photometer. When the insulation is sufficient, the image is slightly
visible, and should be so. The plate is then bordered with banking wax
and bitten-in with a solution of ferric chloride at 45 deg. Baume, or--

Ferric chloride, crystal 20 parts
Hydrochloric acid 1 part
Water 100 parts

The parts of the gelatine film the most acted on are impermeable, so to
say, and consequently do not allow the etching fluid to penetrate to the
copper; while those the least impressed are permeated according as to
their degree of insolation, Therefore, when the ferric chloride solution
is poured upon the film and carefelly brushed over with a soft brush, in a
few moments the image progressively appears, the deep blacks first, then
the half tints, and lastly the most delicate details, the whole requiring
but a few minutes. It is now that the etching action should be stopped by
washing under the tap. However, should by excess of exposure, or any
other cause, the details not appear within five or six minutes, the ferric
chloride should nevertheless be washed off, for then it may find its way
under the film and the plate would be spoiled. After washing the gelatine
is dissolved in a solution of potash, etc., when the image would be found
slightly engraved.

Should the image be in half-tints, it would be advisable to apply a grain
of rosin on the gelatine film just before etching. To engrave on steel
the operations are the same, but on its removal from the printing frame
the plate should be soaked with water renewed several times until the
bichromate is washed off. The film is then dried spontaneously and
afterwards flowed for about two minutes with the Solution A, then, this
being thrown away, with the Solution B, which is allowed to act for a
similar period.

A. Nitric acid, pure 120 parts
Silver nitrate 6 parts
Alcohol, 95 deg 50 parts
Water 75 parts
B. Nitric acid, pure 5 parts
Alcohol, 95 deg 40 parts
Water 60 parts





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Previous: Printing On Wood Canvas Opal And Transparencies



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