and upon it is poured in
excess the liquefied and quite warm solution, which must be allowed to
penetrate in the pores of the wood by letting it gelatinize, when it is
wiped off clean. Nothing must remain on the surface of the wood. This
done, and while still damp, the preparation is rendered insoluble by
pouring over a solution of alum at 5 per 100 of water. The object of this
preliminary operation is to render the wood impervious, and therefore to
prevent the sensitizing solution to penetrate its texture. The wood is
then heated again and its surface whitened with a little silver white or
sulphate of barium, diffused in a small quantity of the following warm
solution:

Gelatine 1 parts
Alum 0.1 part
Water 100 parts

While wet, this is smoothed with a jeweler's brush, taking care to leave
on the wood, a very thin layer of the mixture, only sufficient to obtain a
white surface which, by contrasting with color of the wood assists the
engraver in his work. The wood should now be allowed to dry thoroughly,
when it is coated with a tepid solution of

Isinglass 3 parts
Water 100 parts

and dried.

Now the sensitizing process differs according as whether the cliche is
positive or negative. In the former case the preparation is sensitized
with the solution employed in the black process, proceeding afterwards as
usual; in the latter, that is, when the cliche is negative, the best
process is the cuprotype.(12)

For printing, special frames are employed to permit one to examine the
progress of the impression from time to time without the possibility of
either the wood block or the cliche moving. These frames open in two.
The upper frame is provided with screws on the four sides to hold firmly
the block when it is placed into contact with the cliche by means of the
screws fixed on the cross bars. As to the cliche, if it is made on a
glass plate, it is secured on the thick glass plate of the lower frame by
two wooden bars against it pushed by screws.

When the block is ready for printing, the prepared side is usually
concave. It is straightened by slightly wetting the back and resting it
on one end, prepared side against the wall.

_Printing on Canvas.--_The canvas should be first brushed with a solution
of aqueous ammonia in alcohol, 1:3, to remove greasiness until the thread
just commences to show, then, when rinsed and dry, rubbed with fine sand
to give a tooth, dusted, washed with a spo