10.9.
REPLICATION FROM SOLUTION OR MELT
341
A promising, relatively simple method for a thick first-stage replica of
a flat surface starts with a thin sheet of polymerized methacrylate which
can be purchased commercially. This is soaked in methacrylate monomer
until its surface is softened. It is then laid over the surface to be replicated, covered with a piece of glass and squeezed moderately. Apparently
there remains sufficient catalyst in the original sheet of plastic to polymerize the new surface layer in a period of a few hours, or overnight,
without resorting to heat.
Other replicating substances which can be deposited from organic
solution include resins such as "Zapon," and the nitrocelluloses and
related compounds. Water soluble substances include polyvinyl alcohol,
methyl cellulose, and gelatin. If the specimen can withstand moderate
temperatures (from 45° to 80°C), replicas can be made using melts of
many resins and even waxes. The metallurgists, who ordinarily are not
concerned with temperature limitations, like to use thermo-setting plastics
such as polystyrene with applied pressure as well as heat. In that case,
a block or sheet of polystyrene is softened at about 165°C, and held
pressed against the object to be replicated until it sets as it cools. Ordinarily it then would be shadowed, a layer of carbon evaporated upon it,
and the polystyrene finally dissolved with ethylene dichloride.
Under some circumstances plastic replicas can be isolated and cleaned
by dissolving or digesting the specimen. If this is to be done nowadays,
however, evaporated carbon (discussed in the next section) more likely is
the material of choice for the replica. Plastic replicas are ordinarily used
when they are to be floated off the specimen, or are to be mechanically
stripped with force.
The character of the specimen will determine whether the relatively
delicate flotation method is possible. If so, it generally will be necessary to
tease the edges of the replica to get it started. Breathing on the replica to
coat its surface with condensed moisture sometimes helps. There will be
many specimens, however, which prove completely refractory to all such
tricks.
Mechanical stripping will require little or much force depending upon
the nature of the specimen. This, in turn, will determine in part how
thin a replicating film can be obtained. Stripping may be aided by combining it with flotation by running a drop of water between film and
specimen as they are pulled apart. Sometimes a preliminary period of
mild oven drying helps; so may chilling in a refrigerator! The bonding
between specimen and replica may also be altered by time, so what is
impossible immediately after casting may prove possible the next day.
REPLICATION FROM SOLUTION OR MELT
341
A promising, relatively simple method for a thick first-stage replica of
a flat surface starts with a thin sheet of polymerized methacrylate which
can be purchased commercially. This is soaked in methacrylate monomer
until its surface is softened. It is then laid over the surface to be replicated, covered with a piece of glass and squeezed moderately. Apparently
there remains sufficient catalyst in the original sheet of plastic to polymerize the new surface layer in a period of a few hours, or overnight,
without resorting to heat.
Other replicating substances which can be deposited from organic
solution include resins such as "Zapon," and the nitrocelluloses and
related compounds. Water soluble substances include polyvinyl alcohol,
methyl cellulose, and gelatin. If the specimen can withstand moderate
temperatures (from 45° to 80°C), replicas can be made using melts of
many resins and even waxes. The metallurgists, who ordinarily are not
concerned with temperature limitations, like to use thermo-setting plastics
such as polystyrene with applied pressure as well as heat. In that case,
a block or sheet of polystyrene is softened at about 165°C, and held
pressed against the object to be replicated until it sets as it cools. Ordinarily it then would be shadowed, a layer of carbon evaporated upon it,
and the polystyrene finally dissolved with ethylene dichloride.
Under some circumstances plastic replicas can be isolated and cleaned
by dissolving or digesting the specimen. If this is to be done nowadays,
however, evaporated carbon (discussed in the next section) more likely is
the material of choice for the replica. Plastic replicas are ordinarily used
when they are to be floated off the specimen, or are to be mechanically
stripped with force.
The character of the specimen will determine whether the relatively
delicate flotation method is possible. If so, it generally will be necessary to
tease the edges of the replica to get it started. Breathing on the replica to
coat its surface with condensed moisture sometimes helps. There will be
many specimens, however, which prove completely refractory to all such
tricks.
Mechanical stripping will require little or much force depending upon
the nature of the specimen. This, in turn, will determine in part how
thin a replicating film can be obtained. Stripping may be aided by combining it with flotation by running a drop of water between film and
specimen as they are pulled apart. Sometimes a preliminary period of
mild oven drying helps; so may chilling in a refrigerator! The bonding
between specimen and replica may also be altered by time, so what is
impossible immediately after casting may prove possible the next day.