with both aqueous and ethereal peroxide, but the ethereal solution has the
advantage that no cockling, swelling of paint or local redistribution of water-
soluble materials occurs.
Sodium borohydride is the most widely used of the reducing bleaches.
Short treatments reduce aldehyde and carbonyl groups back to alcohol
groups and cause some bleaching and stabilisation of the cellulose against
future ageing. Longer treatments reduce carbon–carbon double bonds by
adding hydrogen to them. Two other borohydride salts can also be used, the
tetraethylammonium and tetramethylammonium borohydride, however, sodium
borohydride is the most effective bleach. All three salts are soluble in water
and ethanol. Unfortunately, borohydrides decompose in water with the evo-
lution of hydrogen bubbles with associated possible physical damage to the
paper and image. The most stable solution is the tetraethylammonium salt.
Certain metals present as impurities in the paper or as pigments, such as iron,
copper and manganese, can catalyse this decomposition. The borohydrides
are used in the concentration range 0.01–2.0% and produce solutions of
about pH 9.
7 Foxing
Old paper may appear uniformly yellowed due to age, but another phenom-
enon is foxing where yellow/brown spots appear on the paper. For many years,
a controversy raged as to whether the spots were caused by microbiological
agents or were tiny particles of iron in the paper that had corroded to brown
iron corrosion products. The truth is that both explanations are valid. Under
ultraviolet light, the two types of foxing can be distinguished. Spots that
fluoresce brown, and may have a pale blue edging, are caused by fungi; some
spots in this category appear pale blue in UV and are invisible in daylight.
Foxing caused by iron particles appears black against the fluorescence of
the paper.
Iron particles are believed to have been introduced to the paper from the
water used for the papermaking process or come from the machinery used.
The latter explanation has some validity as there are some papers with brass
(Cu/Zn alloy) particles in them; these have often corroded to black or green
copper corrosion products that may include chlorine from residues of bleach-
ing agents.
Microbiological foxing is formed very slowly and is thus difficult to
reproduce artificially in the laboratory. Besides, identifying the fungi respon-
sible by sampling it from paper is fraught with problems as the samples
always include other species in the paper. Whatever mechanism is causing
foxing, the production of spots is slowed down or prevented by storage in low
RH conditions.
Paper 47