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Paper--More than Meets the Eye
A) We are surrounded by so much paper and card that it is easy to forget
just how complex it is. There are many varieties and grades of paper materials,
and whilst it is fairly easy to spot the varieties, it is far more difficult to
spot the grades.
B) It needs to be understood that most paper and card is manufactured for a
specific purpose, so that whilst the corn-flake packet may look smart, it is
clearly not something destined for the archives. It is made to look good, but
only needs a limited life span. It is also much cheaper to manufacture than high
grade card.
C) Paper can be made from an almost endless variety of cellulose-based
material which will include many woods, cottons and grasses or which papyrus is
an example and from where we get the word "paper". Many of these are very
specialized, but the preponderance of paper making has been from soft wood and
cotton or rags, with the bulk being wood-based.
Paper from Wood
D) In order to make wood into paper it needs to be broken down into fine
strands. Firstly by powerful machinery and then boiled with strong alkalies such
as caustic soda, until a fine pulp of cellulose fibers is produced. It is from
this pulp that the final product is made, relying on the bonding together of the
cellulose into layers. That, in a very small nutshell, is the essence of paper
making from wood. However, the reality is rather more complicated. In order to
give us our white paper and card, the makers will add bleach and other materials
such as china clay and additional chemicals.
E) A further problem with wood is that it contains a material that is not
cellulose. Something called lignin. This is essential for the tree since it
holds the cellulose fibres together, but if it is incorporated into the
manufactured paper it presents archivists with a problem. Lignin eventually
breaks down and releases acid products into the paper. This will weaken the bond
between the cellulose fibers and the paper will become brittle and look rather
brown and careworn. We have all seen this in old newspapers and cheap paperback
books. It has been estimated that most paper back books will have a life of not
greater than fifty years. Not what we need for our archives.
F) Since the lignin can be removed from the paper pulp during manufacture,
the obvious question is "why is it left in the paper?" The answer lies in the
fact that lignin makes up a considerable part of the tree. By leaving the lignin
in the pulp a papermaker can increase his paper yield from a tree to some 95%.
Removing it means a yield of only 35%. It is clearly uneconomic to remove the
lignin for many paper and card applications.
G) It also means, of course, that lignin-free paper is going to be more
expensive, but that is nevertheless what the archivist must look for in his
supplies. There is no point whatsoever in carefully placing our valuable
artifacts in paper or card that is going to hasten their demise. Acid is
particularly harmful to photographic materials, causing them to fade and is some
cases simply vanish!
H) So, how do we tell a piece of suitable paper or card from one that is
unsuitable? You cannot do it by simply looking, and rather disappointingly, you
cannot always rely on the label. "Acid-free" might be true inasmuch as a test on
the paper may indicate that it is a neutral material at this time. But lignin
can take years before it starts the inevitable process of breaking down, and in
the right conditions it will speed up enormously.
I) Added to this, as I have indicated earlier, paper may also contain other
materials added during manufacture such as bleach, china clay, chemical
whiteners and size. This looks like a bleak picture, and it would be but for the
fact that there are suppliers who will guarantee the material that they sell. If
you want to be absolutely sure that you are storing in, or printing on, the
correct material then this is probably the only way.
J) Incidentally, acids can migrate from material to material. Lining old
shoe boxes with good quality acid-free paper will do little to guard the
contents. The acid will get there in the end.
Paper from Rag
K) Paper is also commonly made from cotton and rag waste. This has the
advantage of being lignin-free, but because there is much less cotton and rag
than trees, it also tends to be much more expensive than wood pulp paper. You
will still need to purchase from a reliable source though, since even rag paper
and card can contain undesirable additives.
L) A reliable source for quality rag papers is a recognized art stockiest.
Many water color artists insist on using only fine quality rag paper and
board.
M) The main lesson to learn from this information is that you cannot rely
on purchasing archival materials from the high street. The only safe solution is
to purchase from specialist suppliers. It may cost rather more, but in the end
you will know that your important and valuable data and images have the best
home possible.
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发表于 2018-5-31 22:48:16