other possible reasons, the paper also has differing properties in the thickness
of the paper (ZD). The paper conservator will be aware of the different sur-
face textures that occur on the different sides of paper.
A simple experiment can be used to determine MD. Take a sheet of paper
from a newspaper or magazine and cut it into a square, keeping the sides of
the square parallel to the original edges of the page. Bending the paper into a
U-shape first along, then across the sheet will show that the paper bends more
easily in one direction, the direction of the “tunnel” made by bending the
paper is the MD. The paper also tears more easily in the MD. Gently bending
a corner of a page of this book will probably show that the MD is parallel to
the spine; this enables the pages to be turned more easily. There are some
other MD-related properties that are related to the changes of paper when it
interacts with water; these will be described later.
Papers come in different types; they often contain fillers that may be min-
erals or polymers. Papers may have surface coatings to provide a good sur-
face for printing, to texture them or may have a layer of adhesive. Thick
sheets of paper can be formed by casting a lot of fibres onto the web or lamin-
ating thinner papers together with adhesives to form cardboard. Some papers
are dyed, waterproofed, fireproofed or perfumed. Paper is the support for
photographs and can be made into books and magazines, or can be made into
all manner of objects, including boxes. Paper with printing, drawing or paint-
ing on it and paper constructions are much more complicated than just a sheet
of paper and any ageing and conservation process of a sheet of paper will be
influenced by the media on the surface and vice versa. These examples are
intended to convey the realisation that the subject of paper conservation is
very wide-ranging and it is beyond the scope of a single chapter to attempt a
comprehensive overview of all the scientific aspects of the work of the paper
conservator.
3 How Paper Interacts with Water
The cellulose molecule is liberally supplied with hydroxyl groups, which
form hydrogen bonds with both water molecules and hydroxyl groups on
other cellulose molecules. Cellulose molecules prefer to bond more with
other cellulose molecules than water, so cellulose fibres do not dissolve in
water. In a fibre, some regions contain regularly ordered cellulose molecules
that are in crystalline domains, while other areas are disordered and said to be
amorphous. Natural cellulose fibres will give a good X-ray diffraction pat-
tern, which can be used to study the crystallinity of fibres. In the crystalline
areas, the molecules of cellulose are so closely packed that even small water
molecules cannot gain access, however, the water will go into the amorphous
areas and cause swelling and plasticise the fibre, making it more flexible.
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