Currently the calcium-based treatments are preferred over those containing
magnesium as the former seem to induce less yellowing on ageing. Alkaline
compounds of Group I have never found general favour as deacidifying treat-
ments as they cause yellowing, are too alkaline or are hygroscopic, however,
a few workers have recommended slightly alkaline sodium salts, e.g.borates,
as buffers.
If a paper becomes extremely fragile when immersed in water or has dyes
or image materials that are water-soluble or otherwise fugitive, immersion in
water cannot be used for deacidification. One option is to use one from a range
of non-aqueous treatments. Calcium and magnesium are both in Group II of
the Periodic Table. The next heaviest element in the group is barium, which has
a hydroxide, and unlike those for Ca and Mg, is soluble in methanol. Ba(OH) 2
solution was used for deacidification in the period 1970–1985 (approximately),
but fell out of use as health and safety concerns about the toxicity of Ba com-
pounds became more important.
Historically, the next successful agent to be used was magnesium methox-
ide, Mg(OMe) 2 , which can be also be dissolved in methanol. The solution
could be diluted in fluorocarbon solvents that have the advantage of being poor
solvents, thus reducing the tendency for dyes and inks to bleed from the paper
that might have occurred if further methanol had been used as a diluent.
However, the solution proved too water-sensitive for many purposes as the
Mg(OMe) 2 tends to react readily with water and precipitates out as Mg(OH) 2.
This problem was overcome by saturating the solution with CO 2. The result is
now a popular product called methyl magnesium carbonate, although its precise
composition is uncertain. The fluorocarbon diluent has now been replaced by
environmentally safer solvents.
A different approach is to use an inert organic solvent that contains micro-
particles of an alkaline material such as calcium hydroxide or magnesium
oxide. These treatments are suitable for single sheets where the suspension can
be sprayed on or the sheet immersed. Another process uses micro-particles
directed at the paper in a stream of air. Until recently, it was thought that this
type of treatment could not work, as without water the base and acid do not
have an easy time making contact to bring about a neutralisation reaction.
However, the processes perform well in accelerated ageing tests and we can
only assume that paper contains sufficient water in ambient conditions for
acid to move through fibres until they make contact with one of the neutral-
ising particles. Several other ingenious methods for deacidifying paper are
presently available.
Many libraries have large holdings of books printed on acid paper.
Deacidifying books individually is a very time-consuming and expensive
process. A process in which large numbers of books could be deacidified at the
same time (mass deacidification) is a desirable invention. The problem has
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