MOULD MAKING
Most
displays are composed of specimens that have been replicated. It is
rare to find the original fossil on display. Should the fossil happen
to break it is irreplaceable. To reduce this risk museums create
replicas. The replicas are made of light weight material making the
mounts easier to dismantle and to travel with. This section describes
the processes involved in creating a mount for display.
Mold Making
The first step
in making a display is to create a replica. Specimens from the
collection are chosen, based on preservation and stability. The molds
created for casting may destroy the original fossil, making the
selection process of well preserved specimens highly favourable.
Based on the
specimen chosen for replication, there are three types of molds that
can be made here at the Canadian Fossil Discovery Centre. The first compound,
PMC-121/30 A and B, is used for smaller, more stable specimens, the
brush-on 40 compound is used for larger specimens producing greater
flexibility and the last type of rubber compound used for mold making
is a latex rubber C1204.
All three
rubber compounds are moisture sensitive. These products must be stored
and worked with in a low humidity environment. When mixing the
products, even a wooden mixer may hold moisture. You must work
quickly with the compounds or they will be of little use after a given
time as it will harden very quickly.
PMC-121/30
PMC-121/30 is a
urethane liquid rubber. Part A and Part B are combined in a mixing
container and mixed for approximately 3 minutes. The average pot life
(time to work with the compound in the container) is approximately 30
minutes, requiring a cure time of 16 hours.
A plaster base
must be created with the specimen situated in it, acting as the bottom
mold. Grooves and a pouring spout are added to the base to ensure the
mould will seal and create a spout to pour the casting material into.
The base is placed in an enclosure making sure all sides are sealed to
prevent leaking. The rubber is poured into the enclosure and allowed
to cure.
Once cured the
first part of the mold is released from the base. This new rubber mold
acts as the base for the second portion of the mold. The specimen is
placed in the new rubber base and once again sealed in the enclosure.
A second batch of liquid rubber is made and poured over the new base
to make a perfect fit.
This rubber is
amber in colour and can easily be identified. It is the strongest
mould in the museum, but is soft enough not to harm stable fossils.
Brush-on 40
Brush-on 40 is
a rubber compound composed of two parts, A and B. Part A is a liquid
and B is similar in texture to plaster or putty. They are mixed
together in equal parts and brushed onto the specimen in layers.
Approximately 5 to 10 layers are applied. Each layer requires 20 minutes to
set before the next layer can be applied. Once all the layers are
added the mold must cure for 16 hours before it can be released from
the specimen. Each layer must be mixed independently, due to a short
pot life.
This rubber
mold is the softest the museum uses to create larger replicas, such
as skulls.
Latex Rubber
C1204
This rubber
compound is commonly referred to as the cheesecloth mold. This mold is similar to the brush-on in application, but there
is no mixing of compounds. The rubber from the container can be
directly applied to the specimen. Cheesecloth is added for additional
support to the mold. Strips of gauze or cheesecloth are placed on the
surface of the specimen. The latex rubber is brushed onto the entire
surface, filling in every depression and feature of the specimen.
Multiple layers are applied with an approximate 20 minute wait between
layers until a desired strength is achieved.
This type of
mold is very durable but not as flexible as the brush-on 40. The
cheesecloth provides extra stability. This rubber compound can be
taken into the field to make molds of footprints and other well
preserved specimens.
