Series editors' preface - Wood Tools

488 Conservation of Furniture
Table 10.5 Comparative properties of four silicone rubbers
T20 T28 RTV-11 RTV-101
Colour Grey Grey White Red
Grade Medium soft Standard Firm Hard
Specific gravity 1.25 1.29 1.18 1.5
Consistency Pourable Pourable Very pourable Pourable
Viscosity cps/sec 26 000 ± 4000 27 000 ± 4000 11 000 25 000
Shore A hardness 16 ± 3 25 ± 2 41 55
Elongation at break % 650 ± 100 350 ± 100 186 150
Linear shrinkage 0.60% 0.60% 0.60% 0.20%
Tear strength N/mm 21 20 3 4.5 MPa
Catalyst T6 catalyst 5% T6 catalyst 5% Dibutyl tin dilaurate 0.5% Dibutyl tin dilaurate 0.5%
Pot life 100 minutes 100 minutes 1.5 hours 48 minutes
Demould 24 hours 24 hours 24 hours 5.5 hours
These values are a guide only and are not specifications.
Source: Information courtesy of Alec Tiranti Ltd (UK)
napthenate, which in some cases is premixed
into the supplied resin. Resins usually cure best
at around 20 °C (68 °F). Temperatures above or
below this will speed or slow curing respectively.
The cure of polyesters is inhibited by
many metallic elements such as cobalt and
cadmium, found in some pigments (e.g. cobalt
blue and cadmium red) or as a component in
metallo-organic dyes (e.g. Orasol dyes).
Shrinkage of up to 12% is common, though this
can be reduced by up to half if fillers are used.
Polyester catalyst can cause blindness by even
brief eye contact. Fume extraction or personal
respirators, gloves and goggles are recommended
while mixing components and extraction
may be needed until the resin has cured.
Epoxy resins are often used by conservators
as a casting material in conjunction with a silicone
rubber mould. They have the advantages
of minimal shrinkage (0.5%), low odour and
set at room temperature. Pattern-makers often
use bulked or pigmented epoxies for casting.
Air bubbles can be avoided by pre-wetting
with unbulked adhesive. A small puddle of
unbulked resin may be poured into the deepest
part of the mould or pattern and then the
bulked resin poured slowly and evenly into the
puddle. The unbulked resin will pre-wet and
avoid the formation of air bubbles. Unbulked
resin will rise to the surface of the cast where
it will provide the cast with a smooth finish.
Low viscosity epoxies can be used in conjunction
with silk ribbon or glass cloth to laminate
repairs, build up or reinforce curved areas. The
appearance and surface properties of epoxy
casts can be manipulated by the addition of
pigments and fillers. A variety of formulations
of epoxy resins are available and it may be
useful to contact the manufacturer (e.g. Ciba
Geigy) to match a product to a given requirement.
Epoxies bulked with phenolic microballoons
and foaming epoxies that generate
bubbles as they cure both result in low weight
casts with the general density of wood. As with
polyester resins, the hardener or catalyst
presents a health and safety hazard.
Other materials used for moulding and casting
include latex, polyacrylate resin (Larsen,
1981), dental impression compounds (Chase
and Zycherman, 1981) and Vinamold (Alec
Tiranti Ltd, 1990). Small items may also be cast
from materials such as Paraloid resins or
PVACs with fillers such as whiting or microballoons,
although such putties will exhibit
shrinkage that is proportionate to the solvent
carrier content.
10.5.3 Release agents
The nature and material of the original will
define what preparation is needed before a
mould can be taken. Most moulding and casting
materials require a compatible release
agent to allow clean separation from object or
mould. Release agents should be compatible
with, and removable from, the original and
should not damage it or its surface coating.
Porous surfaces may need to be sealed before
a release agent is applied. Release agents must
have a lower surface energy than the mould