42 Chapter 3
5 Deacidification
It is believed that the water-soluble acidity in paper can be washed out using
water. Alternatively, if water is thought to be too hazardous to use on the paper,
then the acidity may be neutralised while still in the paper, possibly by using
a non-aqueous deacidifying treatment. Not all the acidity can be washed out
of oxidised paper as some of the cellulose will have produced carboxylic acid
groups on the cellulose chain. Many conservators at this stage of a conserva-
tion process prefer to introduce some alkaline material into the paper to neu-
tralise any acids not washed out and which will remain behind to neutralise
any acidity that may develop in the future.
Barrow’s examinations of stable, old papers found that most had a signifi-
cant calcium or magnesium carbonate content. Today, some of the best aque-
ous deacidification treatments introduce these compounds. There are several
popular materials used for aqueous deacidification, one of these is calcium
hydroxide solution (lime water). Calcium hydroxide is not very soluble in
water; a saturated solution contains 1.85 g L^1 in ambient conditions and has
a pH of 12.4 which is quite high. Severely degraded papers are suspected to
undergo alkaline hydrolysis and some inks can bleed at such a high pH.
While the paper is in the solution, the acids are neutralised. When treated
paper is removed from the solution, the imbibed calcium hydroxide reacts
with carbon dioxide in the air to form stable calcium carbonate. If the object
has a very black area, e.g.as in some mezzotint prints, the dried precipitated
carbonate may sometimes be visible as a white bloom, however, usually it is
not seen. The eventual pH of the paper is in the range 8.0–9.0.
If carbon dioxide is bubbled through the calcium hydroxide solution, the
carbonate is precipitated at first but later it dissolves again with the formation
of calcium bicarbonate solution. This solution varies in pH depending on the
quantity of carbon dioxide in solution but the pH is generally about 7. It can
be used as a deacidifying treatment when a high pH must be avoided.
A process called the Barrow two-stage treatment deposits twice as much
CaCO 3 by first immersing in calcium hydroxide, then calcium bicarbonate:
Ca(OH) 2 Ca(HCO 3 ) 2 →2CaCO 3 2H 2 OBarrow’s method is seldom used now, as the quantity of CaCO 3 deposited
with the single step processes is considered adequate.
Magnesium hydrogen carbonate may be prepared by bubbling CO 2 through
a suspension of magnesium carbonate (light) in water. Many conservators
make this by placing the suspension in a soda siphon and screwing on a bulb
of compressed CO 2. This has a neutral, if not slightly acidic, pH. The eventual
product, on drying, is a form of basic magnesium carbonate.