Series editors' preface - Wood Tools

11.4.3 Acids
Organic acids have the general formula R-
COOH, and the COOH group is called a
carboxyl group. Carboxylic acids usually have
a pKa in the range of 3–5. Simple organic acids
include formic acid, acetic acid and oxalic acid.
O
| |
R – C – OH
Organic acid functional group
O
| |
H – C – OH
Formic acid
O
| |
CH3 – C – OH
Acetic acid
O
| |
C – OH
|
C – OH
| |
O
Oxalic acid
Inorganic acids include hydrochloric acid
(HCl), sulphuric acid (H2SO4) and nitric acid
(HNO3). Water can act as a weak acid or base.
Alcohols are not ionized to an appreciable
degree under normal circumstances but under
certain conditions ionization can occur,
forming an acid or a base:
ROH [ RO – + H + and ROH + H + [ RO + H2 Acids will react with metals and many metal
corrosion products. Pigments that are metal
salts (e.g. cadmium sulphide or lead carbonate)
may be damaged by acidic solutions. Even
though wood itself is weakly acidic, it may be
discoloured by exposure to acids. In the
context of furniture conservation, acids are
used as a component in cleaning solutions, for
example pH buffers, chelating agents and resin
and bile soaps.
11.4.4 Bases
If an organic compound contains an unshared
electron pair it is a potential base. Compounds
such as amines and amides, which have an
unshared electron pair on a nitrogen atom, act
as bases. Alkaline solutions may cause alkaline
hydrolysis of oils and fats. Waxes and resins,
which contain ester bonds, may also be
saponified (ester hydrolysis that converts fats
and oils to water-soluble soaps).
Ammonia (NH 3) is a gas at room temperature
and soluble in both water and alcohol.
When dissolved in water it forms ammonium
hydroxide (NH 4OH). Ammonia will reduce the
surface tension of water and thereby increase
its ability to wet onto surfaces. It is normally
supplied at 28% w/v and diluted for most
uses. It is a relatively weak base (pKa c.9.2).
As a base, it can neutralize weak acids to form
salts that are usually water-soluble.
In paintings conservation a small amount of
ammonia may sometimes be added to deionized
water to remove surface dirt or greasy
accretions from varnished paintings (pH of
8.5–9) because its volatility ensures that it will
not leave a residue and its alkalinity brings
thin films of oil and grease into solution.
Problems may occur if an ammonium hydroxide
solution with an excessively high pH is
used to clean alkaline sensitive surfaces or
materials (e.g. Prussian blue pigment). As the
concentration is increased, ammonia usually
begins to solubilize varnish layers (see
Alkaline reagents in section 16.6.3). It may be
used for this purpose on occasions where it
offers more control than solvents and can be
used in combination or sequentially with
solvents for the removal of varnish layers.
Sodium bicarbonate (NaHCO 3), commonly
known as baking soda, is an alkaline salt. A
0.1% solution has a pH of around 11.
Strong bases, such as sodium hydroxide,
have found occasional use in furniture conservation
for removing pH-sensitive adhesives or
coatings, or as a component in two-part
bleaching systems (see Bleaches in section
13.5.3). They are potentially very damaging
and must be thoroughly rinsed away after any
treatment.
11.5 Aqueous cleaning
Principles of cleaning 529
Aqueous cleaning has a long history in the
cleaning of varnished and decorated surfaces.
Water has many advantages as a cleaning