dissolving the writing fluid and affecting the underlying paper or
parchment as little as possible.

If the erasure be effected by scratching or rubbing, this removes also
the surface of the paper, which consists of some sort of "size" or
paste with resin soap, which is pressed into the upper pores to give
the paper a smooth appearance, and to prevent the writing fluid from
"running," or entering the pores and blurring the edges of the lines.

If the paper were left as it exists when the scratching or rubbing is
completed, it would be very easy to see that it had been tampered
with, for not only would the parts thus abrased show the running of
any fluid which was subsequently laid upon them, but the surface would
appear rough to the eye in comparison with adjacent parts of the
paper, and the place would appear thinner by transmitted light. Even
to the touch the surface would reveal differences from the ordinary
condition of other parts of the paper.

But the forger usually endeavors to overcome these difficulties by
applying to the scratched area sandarach, resin, alum, paste, or two
or three of these together, the effect being to prevent an unusually
large flow of ink from the pen and its abnormal absorption by the
paper.

The paper should be placed between the observer and a strong light, by
which means, either with or without a magnifying-glass, a distinct
increase in the brightness of the suspected area may be noticed,
indicating a thinning, and even traces of letters, or marks which have
escaped the erasing-tool, may be seen.