The first difficulty comes when the nature of the spot is not easily
recognized. This yellow spot which resists both washing and bleaching, may
perhaps be formed by some greasy body or by some metallic oxide, and one
must proceed carefully on any hypothesis which may be formed. In such
cases, where experiments must be tried, it is necessary to know some
chemical substance which can be first applied, to the end that, if the
spot persists, the chemicals used in attempting its removal will not, at
least, render it impervious to further efforts. It is not possible to set
positive rules for this. I have tried indifferently the action of an acid
before that of an alkali, and vice versa. Only, I have been careful,
before renewing any experiments to soak the print for several hours in
cold water to stop the action of any chemicals already used and to annul
their traces and effects.

The first attempt to make upon a spot of unknown origin, is to soak the
print for several hours in cold water and then rub the spot gently with a
finger or a small brush. It sometimes happens, especially when the paper
has been well made and well sized, that the spot will yield to this gentle
rubbing, slide off and disappear. When the spot becomes thick and pasty,
it is at least weakened even if it does not come off. This is, in any
case, a necessary first operation. But it should be carried out with care,
in order not to injure the surface of the print. Before soaking a print in
water or chemicals, it is best to clip a few small shreds from the margin
and soak these in a small glass test-tube to note the effect.[5]

It sometimes happens that there appears on a page or print a single spot
which it is desirable to remove without going to the trouble and risk of
soaking the whole sheet. A spot on the corner offers few obstacles; the
part is simply dipped in a vessel containing the proper solution. If the
spot is in the middle of the sheet, I usually make use of a shallow
porcelain cup having sides slanted in toward the centre, such as is used
for water-colors. By means of such a cup, any part of a sheet can be
brought into contact with the solution. The chemical may also be applied
directly to the spot by means of a small brush.

M. de Fontelle advises the use of blotting paper from which a hole, a
little larger than the spot, has been cut. This is placed over the spot
and the chemical liquid dropped in. The blotter around the spot will
absorb the