Thickness is now measured in microns.
1000microns = 1 millimetre.
This has superceded the old “sheet” terminology
where the thickness was described by the number of sheets of
paper glued together to make the board eg 6sheet board from
6 sheets of paper. Unfortunately the thickness depended entirely
on how thick the original sheets of paper were. All boards
except Museum boards are made from wood pulp. The type of
pulp and how it is processed determines the quality of the
mountboard
Alpha cellulose indicates a very pure high quality woodpulp
Nowadays the term acidfree has become devalued as an indicator
of quality in mountboard and more specific information is
needed to ascertain the quality;
Lignin-free is now generally considered a better indicator.
Lignin is the binder of wood cellulose and is very acidic.
Removing lignin from wood pulp reduces the quantity of pulp
produced from a tree by about 60%. Thus in commercial manufacture
there are good economic reasons for wanting to use it but
it is probably the most common cause of damage and deterioration
in paper and board.
pH is the measure used to indicate the level of acidity or
alkalinity in a substance. The scale runs from 1 to 14 with
7 being neutral and 1 being highly acidic. Usually you will
find that the pH given for mountboards is slightly higher
than 7 making them slightly alkali. This is to allow for the
absorption of acidic pollutants from the atmosphere or adjacent
materials which will bring down the pH level as the board
ages.
Most boards are buffered with calcium carbonate (chalk, to
you and me) to assist in absorbing and neutralising acids
and atmospheric pollutants. Some photographs and other items
are happier in an environment without this buffering and we
can offer unbuffered Museum Board. In our framing department
we also use a range of boards which use a “molecular
trap” designed to catch any pollutants trying to sneak
past the buffering agents (ask Roy if you really want to know
more about this).
Colour lightfastness. Most mountboards are coloured with
dyes and their stability varies considerably. As a rough rule
of thumb Level 1 boards are much less lightfast than Level
2 & 3 but manufacturers are gradually beginning to publish
information about the colour stability of their boards.
The Blue Wool Scale is the traditional indicator of lightfastness
for dyes, a figure of 6 or higher being considered acceptable
for mountboards. In some cases this is being superceded by
a “Delta E” rating.
The Silver Tarnish Test and Photographic Activity Test are
both used as indicators that a board is safe for use with
photographs. |
