Series editors' preface - Wood Tools

keep the contact time of both cleaning and
rinse solutions to a minimum. Water is a
relatively slow evaporator and it may be
necessary to remove any excess by blotting or
gently swabbing the surface dry.
11.5.1 pH and aqueous cleaning
pH is a measure, in moles per litre, of the
hydronium ion (H3O + ) concentration in an
aqueous solution and is a shorthand way of
describing acidity or alkalinity. pH has no
meaning for non-aqueous solutions such as
organic solvents. The higher the concentration
of hydronium ions (H3O + ) in relation to
hydroxide ions (OH – ), the more acidic the
solution. pH is measured on a negative
logarithmic scale that runs from 0 to 14. A pH
of 7 indicates a neutral solution (an equal ratio
of hydronium and hydroxide ions), pH below
7 indicates increasing acidity (a higher concentration
of hydronium ions) and pH above 7
indicates increasing alkalinity (a higher
concentration of hydroxide ions). Because the
scale is logarithmic, a change of one number
on the scale indicates a tenfold difference in
concentration. A solution of pH 10 contains
one-tenth of the number of hydronium ions
and ten times as many hydroxide ions as the
same volume of solution of pH 9. This means
that a solution with a pH of 10 is ten times
more alkaline than one with a pH of 9, and
one hundred times more alkaline than a
solution with a pH of 8.
The pH of an aqueous cleaning solution is
critical to its performance. Disregarding pH
may cause irreversible damage to a coating or
a decorated substrate. A pH between 5.5 and
8.5 has been recommended for oil based films
such as oil paint or oil gilding (Wolbers et al.,
1990). Below 5.5 there is a risk of acidic attack
of pigments such as lead and copper carbonates.
Some pigments, such as gamboge,
Prussian blue, cochineal, alizarin red, logwood
(Haemotoxylon campechianum), brazilwood
(Caesalpinia crista) and indigo are sensitive
to, and may be damaged by, alkaline conditions
(Daniels, 1982). Above a pH of 8.5 there
is an increasing risk of saponifying the oil
binder, resulting in a partial breakdown of the
polymer matrix and the production of wholly
or partially soluble soaps. These may either be
removed by the cleaning solution or soften the
Principles of cleaning 531
oil film if retained within it. Proteins are very
sensitive to changes in pH and may be
damaged by both acidic and alkaline conditions.
Natural resin varnishes are sensitive to,
and may be damaged by, elevated pH. Whilst
the effects of acidic and alkaline conditions
may be unwelcome in a cleaning treatment,
they may occasionally be exploited for the
removal of unwanted adhesives or coatings,
for example the use of elevated pH to remove
unformulized casein (Lang, 1999).
Raising pH slightly (c.7.5–8.5) will help
bring weakly acidic dirt or other materials into
solution, prevent precipitates such as insoluble
metal salts from forming, and assist surfactants
in picking up greasy materials. pH may have
an effect on additives in aqueous cleaning
solutions. Elevated pH will result in increased
deposition of detergent residues, for example,
and the activity of enzymes is pH-specific
(Wolbers et al., 1990).
The pH of a cleaning solution can be
lowered by the addition of an acid or raised
by the addition of a base. A basic rule of
chemistry states that an acid plus a base will
give salt plus water. It is essential for cleaning
that any salt thus formed should be soluble,
otherwise it will remain as an unwanted residual
precipitate on the cleaned surface.
A Note on calibrating pH meters pH meters
must be calibrated at regular intervals,
sometimes daily, to ensure accurate measurements.
The first step is usually to condition the
pH meter by soaking the electrode tip of the
meter in water for 10–30 minutes. Once the
meter is conditioned, it can be calibrated. pH
meters usually require calibration at two points
on the pH scale. If the solution to be tested
is known to be acidic, the pH meter should
be calibrated at two points in the acidic range
(vice versa for known alkaline solutions). If
the pH of the solution is unknown, or if the
pH meter has only one calibrating screw, the
pH meter should be calibrated at 7 (neutral)
and a measurement taken of the pH of the
solution. If this reading is acidic, the meter
should be recalibrated within the acidic pH
range (vice versa for alkaline readings). After
this second calibration, the meter is ready to
accurately measure the pH of the unknown
solution. The meter should be rinsed in
distilled or deionized water between calibrations
and after use.