Series editors' preface - Wood Tools

554 Conservation of Furniture
mixed with water. It is free from crystalline
silica impurities and is environmentally inert.
A 2–5% concentration of the fine white
powder in water will produce a clear gel. The
gel is made by slowly adding Laponite to
water whilst stirring and then leaving the mix
for several hours to stand and thicken. The pH
of the gel should be checked and adjusted as
necessary as a 2% suspension has a pH around
9.8. If a water/solvent gel is required, Laponite
should be added to the water and left to sit
for approximately 20 minutes. At this point,
miscible solvents such as alcohol or acetone
may be added, up to around one-third of the
total volume of liquid, to produce a water/
solvent gel.
When treatment is complete, the bulk of the
gel can be removed with swabs and/or a soft
bristle brush, followed by a water rinse.
Complete removal of residues from porous
surfaces may be very difficult. The addition of
cellulose fibre (e.g. Arbocel) to a Laponite gel
(1:1) will aid in residue removal (Navarro,
2000). The gel is usually applied and removed
wet, as dried residues can be particularly
problematic. A moisture permeable barrier
layer can be used between the gel and a
porous or vulnerable surface to prevent the
risk of residues remaining on the surface after
treatment. The use of a barrier layer will
reduce absorption of material from the surface
into the gel.
Sepiolite is a naturally occurring clay. It has
a fine particle size, is grey–white in colour and
contains hydrated magnesium silicate. It will
absorb water or solvent to form a thick paste.
As with Laponite, complete removal of
residues from porous surfaces may be very
difficult. As it is a natural material, it may
contain some impurities. Attapulgite clays are
also naturally occurring and are composed
primarily of hydrated magnesium aluminium
silicate. They have a three-dimensional chain
structure that enables them to absorb large
quantities of water. Sepiolite and attapulgite
gels are not transparent and are generally used
on ceramic or stone surfaces.
Cellulose ethers
Cellulose ethers are a large family of polymeric
materials derived from the etherification of
wood pulp (cellulose) resulting in the substitution
of hydroxyl groups with alkyl or
hydroxyalkyl groups. There are a wide range
of products that are fundamentally similar,
differing in the number and type of groups
added to the glucose monomer. Methyl cellulose
(MC) and hydroxypropyl cellulose (HPC)
may be used to gel aqueous cleaning
solutions. Gel viscosity is dependent on the
polymer chain length of the selected cellulose
ether. The viscosity of methyl cellulose
products supplied by Sigma, for example,
ranges from 15 to 4000 cps/sec for a 2%
aqueous solution at 20 °C. Several terms are
needed to describe the chemical and physical
properties of cellulose ethers. The degree of
substitution refers to the number of alkyl
groups added to each glucose unit of the
polymer chain. A low degree of substitution
will indicate a material that is water-soluble; a
high degree of substitution indicates an
alcohol-soluble material. Molar substitution
refers to the number of alkene oxide units
added per glucose unit.
It is essential to remove all gelled material
from the surface after cleaning. The clearance
procedure required is dependent on the
solubility of the particular cellulose ether used.
Gel residues may form a film that will effect
the optical properties of the surface and will
support mould growth. This propensity for
fungal growth means that only fresh solutions
should be used and sterile dark conditions are
required for storage.
Methyl cellulose is non-ionic, water-soluble
(below 90 °C) and produces a thick gel at
relatively low concentrations. Methyl cellulose
is used in some commercial cleaning products
because it holds dirt in suspension and helps
prevent redeposition on the surface. Methyl
cellulose gels tend to be less viscous than
those made with a colloidal clay. A gel can be
made by slowly adding the powder whilst
continuously stirring the solution. Agitation
should be continued for a further 30 minutes.
Clumps may form initially but will be broken
down by continued stirring. Methyl cellulose
does not hydrate immediately but with continuous
stirring a uniform solution will form that
will gradually become more viscous until
hydration is complete. Methyl cellulose is pH
tolerant and is stable between pH 3 to 11,
although gels will take significantly longer to
form in acidic solutions. Concentration may
vary between 0.5 and 5% or more depending