These are the light brown stains that detract from the pleasing appearance of maps and prints.
They are caused by exposure to pollution in the atmosphere, smoke, moisture, acid, printing inks and quality of paper.
Foxing is a term given to random spots of light brown
discolouration.
The spots can range from minute and faint to large and strong. The very
faint spots can only be seen by holding the paper up to a light for
back illumination. Foxing can be local, patchy or wide spread. In
bad cases it is not just spots, but a uniform brown
discolouration. The paper may have been affected by moisture
during storage, but sometimes the paper may be damp or contaminated at
the time of printing. In may cases books and publications were
assembled from old stocks of maps and prints. It is not
surprising to find a book published in 1820 containing maps or prints
printed anywhere between 1800 and 1820. These old stocks would
have been kept in cellars and store rooms subject to temperature and
humidity fluctuations. The foxing we see today may have been set
in motion long before the item was bound into a book or atlas.
Rust spots are not to be confused with foxing. Both are brownish in
colour, but foxing is generally wide and gradual in appearance, whereas
rust spots are small and sharp. They are caused by small
particles of iron or steel have found their way into the paper pulp at
the time of manufacture. It can sometimes be caused an iron filing on
the printing plate being pressed into the paper during printing. Either
way, it is well and truly embedded in the paper and can only be removed
surgically, leaving an unsightly exit hole in the paper.
Offset
Transfer is a term used to describe an image impression left
from an adjacent page, or from a fold in a page. Its effect is usually
seen as
a mirror image of the original imprint. It is caused by the
chemicals
(or moisture) from the original image leeching into the virgin paper,
and may be a factor of the page being folded shortly after the printing
process, or high pressure of the opposing pages during long term
storage. A likely contribution is from the printing or hand
colouring procedure, where the sheets were processed then heaped in
large piles ready for sorting or binding. The combination of fresh
moisture and pressure from being stacked in a pile may result in an
early offset impression. Some publishers used to place a thin
page of tissue paper in front of images and pictures to absorb the
transfer process and prevent the adjacent page from being marked.
Another source of discolouration is acid. Thankfully, the addition of
acid to paper manufacturing is a fairly recent innovation. Anyone
familiar with the old and cheap paper-backs of the 1950s and 60s will
remember the uniform brown discolouration of the paper - this is caused
by the acid content, a self destruct mechanism in modern paper
manufacturing. Cheap mounting card has a lot of acid and will degrade a
framed map or print within a decade. Always insist on acid-free
mounting card. Do not store old maps and prints with acid rich paper or card.
The foxing and offset transfer stains can be removed by a chemical
cleaning process that does not harm the paper or the original ink
impression. It does, however, remove any applied hand colour. In
the case or original hand colour it is sometimes preferable to suffer
the small discolourations to retain the decorative colour. In
some cases,
not all, the offset transfer can be removed with a pencil eraser, but
with great care - only rub in one direction. A rough bi-directional rub
can cause a surface snag and result in a crease in the paper.
Any newly acquired item with these kinds of discolourations will not
deteriorate further when stored in today's modern homes and
offices. It's not like rust - it will not spread to eventual
terminal destruction. Most of the damage was done in the early
years and in any prolonged periods of cold, damp storage (imagine
decades of storage in a Dickensian cellar).
The description of the severity of discolouration is purely
subjective. A dealer will want to convey this imperfection to a
buyer in order to avoid possible disappointment or to have a returned
sale item.
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