50 Chapter 3
18–20°C. On exhibition, light levels are recommended to be as low as possible
(50–150 lux usually) with no UV or at least less than 75Watts lumen^1.
Some paper-based objects may need a change in these recommendations, e.g.
colour photographs are generally stored at lower temperatures.
Materials for the repair of paper should be tested by accelerated ageing to
ensure that they will be same after they have been applied to objects. There
are several desirable features for conservation materials; they should be eas-
ily removable again; they should not discolour; lose flexibility; be or become
acidic or emit a substance harmful to artefacts in the collection. A few examples
will illustrate the hazards of using bad materials. Inappropriate storage mate-
rials can contribute to the deterioration of paper. Bad quality mounting boards
can contain acidic or coloured materials that can migrate to works of art and
discolour them. Some adhesives can become so insoluble in common solvents
that they are impossible to remove. In the case of rubber-based adhesive tape,
the adhesive becomes totally insoluble and brown. Some poly(vinyl acetate)
adhesives emit acetic acid when they age and this can act as a catalyst for the
corrosion of lead in seals attached to documents. Some papers emit aldehy-
des and peroxide that cause fading of black and white photographic images
by attacking the silver micro-particles.
10 Methods for Monitoring the Deterioration of Paper
10.1 Physical Measurements
The deterioration of paper is manifested in several ways, for example, exten-
sive discolouration and loss of mechanical strength can occur. The early
detection of changes and their measurement needs special apparatus.
Colour change on dark ageing produces discolouration of the paper to yel-
low/brown. Light ageing similarly produces this type of discolouration but
may, less commonly, produce bleaching. Not only may dyes and pigments in
the paper bleach, but initially discoloured paper may also become whiter.
Light spectroscopy can be used to measure the entire visible reflectance spec-
trum of the paper, but usually the reflectance at a single wavelength near the
blue/violet end of the spectrum is used; 400 and 457 nm can be found in the
conservation literature. Possibly the most convenient method is to use a tris-
timulus colour measuring meter which reports the hue as three parameters;
many systems for colour measurement have been devised, at present the most
used is the CIELAB system. Total colour change or a change in b*(which rep-
resents yellowness) have been used when studying discolouration.
Tensile strength is one of the commonly measured mechanical properties.
Tensile testing machines are very versatile and used for all types of