Immediate photographing, as in the case of iodine prints, is not
always essential, since the prints are permanent and become illegible
only through eventual clouding of the background. Prompt photographing
is recommended, however, as, in exceptional instances, silver-nitrate
prints have become illegible in a matter of hours. Darkening
ordinarily will occur slowly if the paper is preserved in absolute
darkness, and silver-nitrate prints so preserved more than 10 years
have been observed to be quite clear.

Items such as cardboard cartons, newspapers, road maps, large pieces
of wrapping paper, or smooth, unpainted wood surfaces, too large for
dipping, may be treated by brushing the solution over the surface with
a paint brush (fig. 423). Brushing does not damage or destroy latent
impressions on surfaces of this type. Cardboard boxes may be slit down
the edges and flattened out to permit easy placement under the light.

[Illustration: 423. Silver nitrate solution being applied with paint
brush.]

Wet paper should be handled with extreme care to prevent tearing. In
treating very thin types of paper the solution is best applied with a
cotton swab or brush.

Photographs, Photostats, and blueprints of any value should not be
treated with silver nitrate, since the developed prints or stains
cannot be removed without destroying them.

In working with silver nitrate, wear rubber gloves or handle all
specimens with tweezers; avoid spilling it on clothing. It will cause
dark brown stains on clothing, skin, and fingernails. Such stains are
not easily removed. Areas of the skin subjected to prolonged contact
are deadened, will turn black and peel.

If removal of silver nitrate prints (called "de-silvering") is
desired, this may be accomplished by placing the specimen in a
2-percent solution of mercuric nitrate in a tray similar to that used
for the silver nitrate.

To prepare a small amount of this solution, dissolve two-thirds of an
ounce of mercuric nitrate crystals in 1 quart of distilled water and
add one-third of a fluid ounce of nitric acid. Shake well. This
solution, too, may be used several times before losing its strength
and is not necessarily discarded after each use. It is not necessary
to keep it in a dark bottle.

The specimen bearing silver nitrate prints is immersed in this
solution until all traces of the prints disappear. It should then be
rinsed thoroughly in water to remove all mercuric nitrate. If thi