54 Chapter 3
corresponds to the stretching of CH bonds, while a peak at 1740 cm^1
corresponds to a stretching of a carbonyl group. As the strength of the CH
peak remains more or less constant during the ageing of paper, the ratio of the
strength of the carbonyl peak with the CH peak can be used to follow the
oxidation of the paper. The ratio of the peaks at 1372 cm^1 (CH) with that
at 2900 cm^1 can be used to monitor the crystallinity of the cellulose.
The FTIR spectrum of a piece of paper can be measured non-destructively
using an FTIR-microscope or, probably more reproducibly, by taking a sam-
ple and either placing it directly in a diamond cell or grinding it with infrared-
transparent potassium bromide and pressing it into a disc to go into the
spectrometer.
11 Characterisation of Paper
There are many parameters that can be used to describe paper, and this sec-
tion describes some of the scientific ones often used. Paper is made of cellu-
lose fibres. Some fibres have characteristic features when viewed on a
microscope and an experienced person can identify the source of fibres in a
paper sample and maybe even estimate the percentages in a mixture.
Identification is greatly facilitated by macerating the paper and separating the
fibres. There are many techniques for doing this, but many involve adding a few
drops of alkali or boiling the paper in an aqueous solution. Although some
scientists prefer to use a scanning electron microscope, most fibre identifica-
tions are performed on a polarising light microscope as interior features can
be seen. Fibres are laid out on a glass microscope slide and mounted under a
cover-slip using a liquid mountant. Several features of the fibre, and the asso-
ciated botanical debris, are used by the microscopist who may also use stain-
ing techniques. Many fibres can be easily identified, e.g.cotton and linen,
and it is easy to tell if the fibre is coniferous wood, however, it is more diffi-
cult to distinguish different wood-derived fibres from one another. The trad-
itional Oriental paper fibres, e.g. kozo, mitsumata, gampi, etc., are very
difficult to differentiate.
Many standards describing paper to be used for conservation purposes will
specify that the papers should be free of alum and lignin. Several spot tests
can be used to detect these and other materials found in paper; one of each
will be described. One test for alum uses a solution of a chemical called alu-
minon. The pale pink solution is applied to the surface of the paper and
allowed to dry. A pale pink spot is produced when no aluminium salts are
present; a darker pink or red is seen when such salts are present. Lignin can
be detected using a freshly prepared solution of phloroglucinol in hydrochlo-
ric acid. A drop of the colourless solution is applied to the paper and a red
colouration indicates the presence of lignin. There are several tests for rosin.