THE VAPOUR PHASE DEACIDIFICATION OF BOOKS AND DOCUMENTS
Most of the methods used for deacidification of books and documents suggested during the last few
years have been based on solutions of alkaline or basic substances dissolved usually in water and
sometimes in non-aqueous solvents. All these methods tend to be messy and not well adapted to the
needs and resources of document repairers and librarians. More recently, preliminary experiments
have been made to exploit the obvious convenience of gaseous processes of deacidification.
Many gaseous alkalis are known to the chemist, so many in fact that at first sight there might appear
to be an embarras de richesses. Screening tests however soon showed that the choice of a
convenient, effective and permanent deacidifying volatile alkali would be limited to a
comparatively small number of substances. These early tests eliminated the commonest of the
volatile alkalis, viz. ammonia, on the ground that it was too weak a base completely to neutralize a
strong acid such as sulphuric—the commonest of the acids known to shorten the life of stored
paper. This is unfortunate since ammonia is cheap and readily available, but acid paper treated with
excess of ammonia and exposed to the air becomes slightly acid again—probably too acid for
permanence under normal storage conditions.
Certain organic derivatives of ammonia—the amines—are much more alkaline than ammonia and
could therefore be relied upon to reduce the acidity of acid paper to a degree believed to be
satisfactory for long storage. These bases are so numerous that it was some time before a few were
found having a wide range of properties besides alkalinity which would make them suitable for
practical application by librarians and archivists. Finally one of these—cyclohexylamine carbonate
(C.H.C.)—showed so much promise that it was examined in greater detail and now forms the basis
of a new and attractive vapour phase deacidification (V.P.D.) process.
The VPD process is versatile and lends itself to a wide range of techniques which may be adapted to
suit
Íthe diverse needs of practical document repair for example, the active material CHC may be
absorbed on sheets of absorbent paper which can then be used as interleaving in books. By this
method a few sheets of VPD paper, e.g. one sheet to every 50-100 pages of the book, may be evenly
spaced throughout the book which is then closed and replaced on its shelf and left to itself for a
week or two. The interleaving process fakes only a few minutes and the book should be evenly and
completely deacidified with no appreciable change in its appearance except perhaps for books made
from cheap newsprint which may become yellowish—a change which would take place anyway
after a few years of storage.
A cheaper method well suited to boxed papers and files is to use the CHC in powder form enclosed
in small paper envelopes. These packets are simply placed in the boxes with the loose papers and
put aside until needed again for reference. The vapour from the CHC should permeate the whole
box in a few weeks time and give uniform and satisfactory deacidification. Naturally the boxes
should be sound and free from ventilation holes.
These two techniques appear to be the ones most likely to meet practical requirements though
others come readily to mind. It may be possible to mark the impregnated papers with a sensitive dye
which will change colour to indicate when they are exhausted.
In practice it will be necessary to check from time to time that the deacidification is complete, and
this can be done either by marking selected pages or papers with indicator ink—brom cresol
green—using the special fountain pens and ink now available. If a special rubber stamp is used for
this purpose instead of the pen the mark made, would be free from most of the objections raised by
archivists to anything suggesting falsification. If any marking of the pages whatever is objected to,
loose pieces of paper carrying the indicator mark adjusted to give the acid colour can be interleaved
in strategic positions or alternatively pieces of red litmus paper may be used. The chief objection to
this alternative is that the paper to be deacidified may be much more acid than the test paper and
may therefore remain more acid than the test paper suggests.
The permanence of the dcacidification has been tested by heating the deacidifiied paper samples to
15O°C. (300°F.) without increasing the acidity. The vapour from CHC has an odour resembling
ammonia but is not so pungent. It has been very thoroughly examined by the Air Ministry at Porton
for toxicity and found to be safe enough for general use by military personel as an anti-rust
preparation. The vapour is naturally strongly alkaline otherwise it would be no use for VPD; in
large doses it could cause some irritation of the membranes of nose and throat but usually the smell
gives ample warning of any undue concentration in the atmosphere.
The preliminary tests have been so far very encouraging and efforts are being made to prepare
impregnated tissues and ready weighed and packaged powder on a commercial scale for use by
archivists and librarians.
W. H. LANGWELL, F.R.I.C.