MAGIC PRINTS.
To Professor Husnik (to whom collotypists are greatly indebted for
so many unselfish publications), we owe the origination of this
interesting method of producing “magic” prints. It is founded on the
reaction well known to chemists that two substances, each colourless
in itself, when mixed, enter into fresh chemical combinations and
produce other substances of intense colour. Herschel and Hunt made
use of many of these reactions, both to develop the invisible image,
and to give to others, already visible a more agreeable colour or
tone. Impressions other than those in fatty ink may be obtained from
the Collotype plate—as in the process patented by Edwards—by means
of colour, soluble in water, to which the addition of a small amount
of gum has been made. In such cases it will not be those portions of
the layer which have been rendered insoluble by the action of light
which will give the impression (as it would if ink were used) but on
the contrary, the unexposed parts, or those which remain soluble. On
this account the resulting print will be a negative, and to obtain a
positive, the printing must take place under a positive. If a Collotype
plate so produced be damped with a linen rag which has been moistened
with an aqueous solution of a salt readily absorbed by gelatine, and
which will produce in combination with a second solution an intensely
coloured compound (the use of the ink roller being entirely omitted),
the impression on the printing paper will be almost or even entirely
invisible until brought in contact with the second solution, when
the picture will be immediately developed and rendered visible. For
instance, dampen the plate with an aqueous solution of red prussiate of
potash, surface dry it, print upon paper, allow to dry thoroughly, and
place upon a solution of protosulphate of iron, when the picture will
immediately appear of an intense blue colour. Solutions of bichromate
of potassium or nitrate of silver will produce brown images; the
former developed with extract of logwood, alazarine or sugar of lead,
all produce different colours. Not only is this process interesting,
applied in the manner already described, but it is of use in calico
printing. Care must however be exercised in the selection of a suitable
solution for damping the plate, as many salts would tan the gelatine
layer, and such are of course utterly unsuited for the purpose. Among
the mordants in common use, salts of tin, alum, iron, &c., may be
mentioned as exercising a hardening influence upon gelatine, and
therefore useless for moistening the layer. There remain, however, many
others which produce no damaging effect upon the plate, and at the same
time produce useful colours.