Series editors' preface - Wood Tools

effective in breaking down coatings quickly
but may also attack the substrate wood. It is
rarely necessary to use such strong reagents.
Although alkaline pH may be useful in removing
some coatings, the potential for severe
damage of sensitive substrate materials generally
makes the use of such materials undesirable.
In addition, unless residues of alkaline
strippers are meticulously removed, there is a
danger that subsequent coating materials will
be damaged. It may be necessary to rinse the
surface with a dilute acetic acid solution to
neutralize any residues.
11.4 Chemical cleaning
Chemical cleaning of decorative surfaces
involves the use of reagents, which are chemicals
that break primary molecular bonds,
converting dirt, varnish or other unwanted
material to a different form in order to remove
it from the surface. Unlike solvent cleaning,
original material cannot be recovered in the
same form that was present on the object.
After cleaning is complete, any non-volatile
reagents must be rinsed from a surface.
Common reagents used in furniture conservation
include acids and bases, discussed here,
and chelating agents and enzymes, discussed
in the context of aqueous cleaning. Oxidizing
and reducing agents are discussed in the
context of bleaching in section 13.5.3.
11.4.1 Introduction to acids and bases
Acid–base interactions are a complex area of
organic chemistry. Acids and bases are important
in cleaning decorative surfaces because
acidity and alkalinity (or basicity) may be used
to solve problems for which common organic
solvents are ineffective. Brønsted and Lowry
defined an acid as a proton donor and a base
as a proton acceptor. A proton is the same as
a positively charged hydrogen ion (H + ).
Acid–base reactions are equilibrium reactions.
In equilibrium reactions the concentration of
reactants and products remains the same if
external conditions remain unchanged. The
reactions do not stop, however, but the
forward reaction is equal to the reverse
reaction and so the concentrations remain
unchanged.
Principles of cleaning 527
An acid is a substance that releases hydrogen
ions in solution, dissociating to form a
negatively charged anion and a positively
charged hydrogen ion. Acids will turn litmus
paper red. In practice hydrogen ions do not
exist in solution and are more likely to attach
to a water molecule, forming a hydronium
(H 3O + ) ion. The equation for the ionization of
hydrochloric acid is:
HCl(aq) + H 2O(l) [ H 3O + (aq) + Cl – (aq)
Thus hydrochloric acid is an acid because it
loses a H + ion to form Cl – . In this reaction
water is a base because it accepts the H + ion
to form H 3O + .
Generally speaking, inorganic acids are
strong and organic acids are weak. The
concept of strength (degree of ionization) is
different to that of concentration. An acid is
said to be strong if a high percentage dissociates
to form ions. Thus if solutions of nitric
acid (a strong acid) and acetic acid (a weak
acid) of the same molar concentration were
compared, all of the nitric acid would have
become ionized whilst only a very small
proportion (about 1/1000) of the acetic acid
would have dissociated. It should be remembered
that both strong and weak acids can
produce highly acidic solutions with a low pH.
Formic acid, for example, is a weak organic
acid but can be supplied at an 88% concentration
with a pH between 0 and 0.5.
A base is a substance that liberates OH –
(hydroxide) ions in solution (that may be nonaqueous),
and reacts with an acid to form a
salt. Bases will turn litmus paper blue. A base
that already contains hydroxide ions, such as
sodium hydroxide (NaOH [ Na + + OH – ), is
considered a strong base. A base that reacts
to form hydroxide ions, such as ammonia, is
considered a weak base (NH 3 + H 2O [ NH 4 +
+ OH – : note that in this reaction water is acting
as an acid). An alkali is a base that produces
hydroxide ions in aqueous solutions.
Concentrated weak bases and dilute strong
bases can both produce solutions with an
extremely high pH. pH measures the hydronium
ion concentration in an aqueous solution
(i.e. measures solution acidity or alkalinity),
not the strength of an acid or base.
Acids and bases can play a role in both
aqueous and non-aqueous systems. They are