Series editors' preface - Wood Tools

616 Conservation of Furniture
by an aqueous rinse to remove as much acetic
acid and hydrogen peroxide as possible. Both
these materials are quite tenacious, however,
and a small residue will remain. The hydrogen
peroxide used in part B is chemically
unstable and commercial solutions are often
slightly acidic to slow down decomposition.
Supplies should be purchased in small quantities
as required.
Bennett (1990) recommends a bleaching
procedure that produces a less radical change
than would be produced by the use of a two
part bleach according to the manufacturer’s
instructions. Part A is applied as usual but part
B is left on the surface for only a third to half
the application time of part A. At the end of
this time, any excess liquid on the surface is
wiped away along the grain with a clean rag
and a concentrated solution of oxalic acid is
applied. The oxalic acid reacts very quickly
and is then removed with a damp rag before
a 33% solution of acetic acid is applied. The
surface is then rinsed two or three times with
water and allowed to air dry thoroughly.
Hydrogen peroxide is sometimes sold under
the trade name ‘Superbleach’, which consists
of an aqueous solution of hydrogen peroxide.
Eight parts of this hydrogen peroxide (35%) is
mixed with 1 part ammonia (35%/880) in an
open container. Hydrogen peroxide produces
up to thirty times its own volume of oxygen
as it reacts with the ammonia and the pressure
of this gas in a closed container may cause it
to explode. The solution is applied immediately
to the wood and left there for up to
fifteen minutes. Treatment may be repeated if
necessary before the surface is rinsed with
water. Bleached wood should be left for at
least seventy-two hours before the application
of other surface coatings or colorants. Since
both components are volatile, other methods
of neutralization are unnecessary.
Hydrogen peroxide alone may be used, but
the absence of an alkali means the hydrogen
peroxide is not protonated and the bleaching
reaction will therefore be limited. It can be
obtained in a variety of concentrations (3%,
35%, 50%, 70% and 90%) in water, but the 35%
solution is usually used for bleaching timber.
It may be applied to the surface of the timber
and left until the desired colour change has
taken place. It is neutralized by an aqueous
rinse.
It should be noted that hydrogen peroxide
will decompose explosively in the presence of
acids, platinum, silver, copper, chromium,
iron, zinc, lead or manganese, especially in
conditions of alkaline pH, and that highly
concentrated solutions of hydrogen peroxide
(65%) may ignite combustible materials.
Hydrogen peroxide is unstable and will
decompose if exposed to heat or light.
Addition of colour to wood repairs:
pigments, lakes and stains
Pigments Pigments are insoluble in organic
solvents and water and vary in their lightfastness.
Pigment properties are listed in Table
5.3. They may be classified into three types –
inorganic, synthetic inorganic and organic
pigments. Inorganic or mineral pigments
include the native earths, such as ochre,
umber and sienna, and calcined native earths
including burnt umber, burnt sienna etc.
Synthetic inorganic pigments include cadmium
yellow, zinc oxide, titanium oxide etc. Organic
pigments may be derived from vegetable
sources, such as gamboge, indigo and madder,
animal sources such as Indian yellow and
cochineal or may be manufactured synthetics.
Some pigments present a health hazard and
the supplier’s or manufacturer’s material safety
data sheet should be consulted. Some traditional
colorants for wood are described in
Table 5.4. In a few cases chemical reactions
with atmospheric pollution may cause
pigments to change colour, for example lead
white may blacken if exposed to atmospheric
hydrogen sulphide.
Lakes Lakes were made by precipitating an
organic colouring material or a dye onto an
insoluble and inert base. Traditionally the base
was aluminium hydrate or calcium sulphate
but later a range of materials, including
barytes, tin oxide and zinc oxide were used.
Stains Stains (and dyes) are colouring materials
that may be dissolved in a solvent (e.g.
water or alcohol) and impart a transparent
colour that does not obscure the substrate
material. Furniture restorers have traditionally
classified stains according to their solubility
and, given the complexity of modern dye
chemistry, this convention will be used below.
The solubility of a stain was considered as part