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RUBBER MOLDS AND PLASTER CASTS

IN THE PALEONTOLOGICAL LABORATORY

BY

JAMES H. QUINN

ASSISTANT IN PALEONTOLOGY

1HE LIBRARY OF THE

MAY 2 91940

UNIVERSITY OF ILLINOIS

TECHNIQUE SERIES

FIELD MUSEUM OF NATURAL HISTORY

NUMBER 6

APRIL 27, 1940

RUBBER MOLDS AND PLASTER CASTS
IN THE PALEONTOLOGICAL LABORATORY

BY

JAMES H. QUINN

ASSISTANT IN PALEONTOLOGY

THE LIBHAKY OF THE

MAY 2 91940

UNIVERSITY OF ILLINOIS

TECHNIQUE SERIES

FIELD MUSEUM OF NATURAL HISTORY

NUMBER 6

APRIL 27, 1940

PRINTED IN THE UNITED STATES OF AMERICA
BY FIELD MUSEUM PK&SS

RUBBER MOLDS AND PLASTER CASTS
IN THE PALEONTOLOGICAL LABORATORY

By James H. Quinn

INTRODUCTION

Accurate plaster casts of fossils are of recognized value in paleon-
tology. They serve as useful substitutes for rare specimens and
types, making copies of such fossils available to various museums
for purposes of comparison and exhibition.

Such casts are ordinarily made by the use of plaster piece-molds
or of flexible glue or gelatine molds. Both techniques are intricate
and they present many difficulties, especially when complicated
objects such as skulls are cast. The risk of damage to fragile fossils is
considerable when either of these casting methods is used. Further-
more, the production of good casts depends to a great extent on the
skill and experience of the molder. Therefore, any method that
is calculated to simplify the casting of fossils should be of interest
to paleontologists and also to plaster molders, as such a process is
applicable to other fields in which plaster casting is used.

Liquid rubber was first introduced into Field Museum's labora-
tories in 1934 as a casting medium for the preparation of brain
casts, and a brief account of the process was published by Patterson
(Field Mus. Nat. Hist., Geol. Ser., 6, pp. 273-274, 1937). Several
brain casts were prepared by means of the technique there described.
The facility with which this otherwise difficult operation was per-
formed indicated that liquid rubber might well prove to be a reliable
material for the construction of molds from which a high grade of
plaster casts could be produced with a minimum of labor, and with
little risk of damage to fragile fossils.

A method of constructing flexible rubber molds was not as easily
developed as the brain cast procedure had led us to expect. Since
the interiors of the skulls from which the brain casts were taken
were somewhat inaccessible to measurement, no suspicion of the
possibility of shrinkage or distortion was entertained. This serious
fault of the material became apparent as soon as an attempt was
made to construct molds of rubber. It was also found that the color
in certain of the liquid rubber compounds stained the models,' and

1 The term "model" as used in this paper indicates the specimen of which
casts are being made.

4 Field Museum of Natural History— Technique, No. 6

that the water in the rubber tended to loosen joints when mended
bones were used as models. Except for these faults, rubber molds
produced casts of such attractive appearance that persistent efforts
were made to find a means of eliminating the objectionable features.
After much experimentation, simple techniques for constructing
rubber molds were perfected.

This paper is intended to describe the technique of using liquid
rubber in plaster-casting in so far as it is applicable to making casts
of fossils, especially of fossil bones. It is beyond the scope of the
present account to discuss the uses of liquid rubber in other branches
of museum preparation, a field in which numerous possibilities are
being explored.

I wish to express my thanks to Mr. Elmer S. Riggs for aid and
encouragement in both the experimental work and in the writing of
this paper. Thanks are due to Mr. John R. Millar, Mr. Karl P.
Schmidt, and Mr. Bryan Patterson for advice and criticism. The
drawings are the work of Mr. John Conrad Hansen.

LIQUID RUBBER

Collection and preparation. — Rubber is derived principally from
the sap of trees of the genus Hevea. The sap, or "milk," as it is
commonly called, is a thin fluid, white in color and alkaline in re-
action. After standing a few hours it ferments, becomes acid in
reaction and coagulates. Owing to this characteristic it was formerly
impracticable to transport liquid rubber. Since it was collected by
natives, over widely separated areas, it was always coagulated,
dried, and brought to market in lumps. The development of rubber
plantations has made it feasible to process and prepare the rubber
for export in the liquid state, so that it is now available in quantity
for use in the manufacture of numerous articles formerly made from
dried rubber.

On the plantation, the sap is collected into tanks and a 5 per cent
solution of ammonia is added as a preservative. It is then con-
centrated by removing about 30 per cent of the water content.
The concentration is accomplished in much the same manner as
that employed in separating butter fat from milk. After standing
a few hours in a vat the rubber rises to the top of the solution,
just as cream rises on milk, and is drawn off. A second method
of separating the rubber and water is by centrifuging, as in the
mechanical method of skimming milk by the cream separator. Both
processes are known as "creaming." After the solution has been

Rubber Molds and Plaster Casts 5

creamed it may be preserved for considerable periods in air-tight
containers without danger of spoiHng.

Latex may be vulcanized while in the liquid state, so that on
coagulation it displays the characteristics of processed, vulcanized
rubber. Some latex compounds manufactured for use in molding
or for other commercial purposes, contain vulcanizing agents and
accelerators so combined that vulcanization takes place after the
compound has coagulated and dried. Such compounds behave
like unvulcanized rubber for a considerable period of time, and may
be considered as such in so far as the making and curing^ of molds is
concerned. The degree of vulcanization, the time of coagulation, and
the content of water in rubber solutions play an important part in
the processing of rubber molds. The cause and effect of these
properties are interesting and some knowledge of them will prove
of value in selecting or preparing a suitable molding material.

Properties of liquid rubber. — Liquid rubber coagulates on losing
a small amount of its water content, and acquires the characteristics
of solid rubber. It becomes elastic and cohesive, and begins to
develop resistance to alteration of form. Thus, most of the adjust-
ment in the volume of a film of liquid rubber spread on a model
must take place after it has lost the property of fluidity. Since
the water, which ranges from 25 to 40 per cent of the total original
volume, is nearly all lost in drying, considerable adjustment neces-
sarily takes place. As water is lost, the rubber tends to reduce
its volume. Since the model resists any reduction in dimension, the
film can shrink only in thickness. A tension is thus set up in the
rubber to compensate for its inability to reduce its area. The major
difficulties encountered in making rubber molds arise from this
tendency of rubber to build up tension while drying, a characteristic
that causes the film to drav/ away from the surface of the model in
depressions, and to contract on removal from the model.

The amount of tension that may be acquired by the solid film
depends on its thickness and on the rapidity of drying. The rate
of accumulation of tension depends directly on the rapidity of drying.

Since rubber is not perfectly elastic, it also has the ability to
lose tension. The loss of tension in the rubber mold is a slow process
not directly connected with the rate at which it has been accumulated,
and depends, to a great extent, on the degree of vulcanization of
the rubber.

1 The term "curing" designates the period of time allowed for the rubber
film to lose its tension.

6 Field Museum of Natural History— Technique, No. 6

Vulcanizing makes rubber more perfectly elastic, but also makes
it progressively harder and likewise less susceptible to alteration
of form by any applied force. The degree of vulcanization may
range from that in slightly vulcanized products, such as rubber
bands, to that in highly vulcanized objects, such as hard-rubber
pipe stems.

It may readily be seen that, given an equal application of force,
the form of an ordinary rubber band can be altered much more
quickly and to a greater degree than can that of a similar band of
more highly vulcanized rubber; for example, a rubber band that is
stretched as far as possible without being broken will not regain its
original form when released, but will be appreciably thinner and
longer. The same effect would be obtained by allowing it to remain
in a less stretched condition over a period of time. A more highly
vulcanized piece of rubber would require more force to stretch it,
would not stretch as far, and would more nearly regain its original
form after relaxation of the force. The greater the degree of vulcani-
zation the more extreme these tendencies become. In practice,
molds made of vulcanized rubber must be dried more slowly than
those made of unvulcanized rubber so as to prevent or reduce the
danger of pulling away in depressions. A long period of time must
be allowed for loss of tension in order to minimize the possibility
of contraction (approximately a month should be allowed for curing
a mold having a thickness of one-eighth of an inch). A further
defect produced by vulcanization in molding compounds lies in the
reduction of their adhesive qualities. Vulcanized rubber does not
adhere to the model as closely as unvulcanized. The rubber must
be applied in successive layers when making a mold, and there is a
tendency towards separation of the layers when vulcanized rubber
is used.

The unvulcanized latex is a much more satisfactory medium to
work with than vulcanized latex, since the tension is lost more
rapidly and more nearly keeps pace with its accumulation. Molds
of unvulcanized rubber dry quickly, do not pull away in depressions,
and usually lose all tension in about two weeks. There is less danger
of separation of the successive layers, since the unvulcanized rubber
is more adhesive. Molds may be built up and reinforced with cloth
with greater facility and with more satisfactory results, when used
in conjunction with an unvulcanized compound.

Unvulcanized compounds containing vulcanizing agents and
accelerators so compounded that vulcanization takes place after

Rubber Molds and Plaster Casts 7

the mold has cured, are the most satisfactory media for molding,
since they have all the advantages and none of the disadvantages
of either of the other types. Such rubber molding material produces
accurate replicas with a minimum of failure or damage to the model.
The chief advantage of this type of compound over ordinary unvul-
canized latex is that it may be applied more easily and smoothly.

Nevertheless, it should be noted that ordinary unvulcanized
latex is capable of producing molds equal in every way to those
made of prepared molding compounds of the above type, except that
the rubber may deteriorate more quickly and the mold may be
more easily distorted by excessive stretching. These faults are not
serious, since all the casts desired can be made long before the
rubber begins to break down and excessive stretching of the rubber
can be avoided by proper construction of the mold. For these
reasons, wherever a suitable prepared compound is not available,
unvulcanized latex, which is obtainable almost everj^where, may
be substituted and will, with only a little more care, yield entirely
satisfactory results.

MAKING RUBBER MOLDS

General procedure. — Basically, the process of constructing a rubber
mold consists of the following simple steps:

(1) The model is prepared by cleaning, repairing cracks, and
hardening the surface with shellac or with any other substance
commonly used for the purpose.

(2) The liquid rubber molding material is applied to form a
reasonably uniform film over the entire surface of the model. Ordi-
narily, the rubber is applied with a brush in successive layers, and
reinforced with cloth wherever necessary.

(3) The mold thus formed is allowed to dry and cure.

(4) The mold is opened, the model removed, and an orifice
arranged of sufficient size and in a suitable position for the purpose
of pouring plaster into the mold when casting.

In practice, the method of applying the rubber varies but little.
Provision for opening and closing the mold and for handling the
model during construction of the mold, however, imposes the
necessity of varying the method to suit the need. In accordance
with the type of construction used, and for the sake of convenience,
molds may be grouped in two general types: (a) The one-piece
or continuous mold, and (6) the two-piece mold, constructed in
two separate sections, each covering roughly half of the model.

8 Field Museum of Natural History— Technique, No. 6

Before a mold can be made, it is necessary to decide on the tj^De of
mold to be used and the manner in which it shall be arranged, so
that the completed mold can be removed without breakage from
the model and from the casts to be formed in it, and in order that
the joints or seams left by the edges of the mold may be as incon-
spicuous as possible. For these reasons the types of rubber molds
and their application are discussed below in considerable detail.

Continuous molds. — The simplest type of rubber mold is prepared
by applying the molding material over the entire model in a con-
tinuous film so that it completely encloses the model. The opening
and closing of such a mold presents a major problem in its construe^
tion. Ordinarily, the opening is provided by cutting the mold (after]
it has cured) along a line that will be as inconspicuous as possible
on the finished cast. The most feasible method of closing this type
of mold for casting is accomplished by cementing the edges with
rubber cement. Since the rubber mold is usually quite thin (one-
eighth inch or less) there is little contact surface, and the joint
thus made will not withstand the pressure of a large quantity of
plaster. Furthermore, dried rubber cement accumulates on the edges
of the mold (since it must be applied each time a cast is made) with
the result that with continued use they become rough, do not fit
closely, and cause undesirably conspicuous seams on the casts.
This method is suitable for small molds intended to yield only a
limited number of casts. On larger models, where other types of
molds cannot be used, the continuous mold may be employed in
conjunction with a device that will increase the contact area and
thus strengthen the joint.

Two methods devised for increasing the contact area in con-
tinuous molds may be described as follows:

(1) A strip of thin celluloid (set at a right angle to the surface) is
attached to the model with celluloid cement so that the strip follows
the line of the desired opening. The edges of the mold are built
up to any required width on the face thus provided. Joints of this
type have considerable strength even when cemented with a very
thin solution of rubber cement. A thin rubber cement solution may
be used to better advantage than a thick one, since it accumulates
more slowly on the contact faces of the mold.

(2) A flap arrangement may be constructed. After half the
thickness of the rubber film has been built up, a strip of paper one-
half inch wide is laid on, one edge coinciding with the line along which
the mold is to be opened. The paper acts as a separator, so that

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after the mold has been completed the outer half may be cut along
one edge of the paper strip and the inner half along the other edge,
thus forming a "lap" joint.

In order that the entire surface of the model may be cov-
ered with rubber in a single operation, a rigid holder or "handle"
should be attached so that the molder may hold or turn the

Fig. 2
hatched).

Diagram of skull, showing areas blocked off with plasteline (cross-
X-X denotes line of section shown in fig. 3.

model in any position desired, without touching any part of the
mold. The handle may be attached by cementing a rod in a natural
opening, on a broken surface, or into a hole drilled in a restored area.
Models that are too large to be treated in this manner, or that
offer no opportunity to attach a handle, should be enclosed in a two-
piece mold.

Two-piece molds. — Two-piece molds require more time and care
in construction than continuous molds. For this reason they are
not generally used in cases where only a few casts are to be made.
They become necessary when, due to the nature of the model, a
simpler sort of mold is unsuitable, or when a large number of casts
is desired from a mold. It is unnecessary to provide the model with
a handle since only one half of the mold is made at a time. The
contact edges of the mold can readily be constructed with guides
or keys to produce an accurate register. The molds may be removed
from the model or casts with greater facility, since no cutting is
necessary.

Mold-making may vary considerably in detail according to the
features presented by the model and the individual preference of
the molder. For this reason an illustrated account of the prepara-
tion of the molds of a representative fossil appears to be the most

Rubber Molds and Plaster Casts

11

useful and lucid method of describing the process. For this purpose
a skull of the phororhacoid bird Mesemhriornis has been chosen.
This specimen is fairly well preserved, with the bone hard and
dense, except for a section of the occipital region and the interorbital
septum which has been cracked and softened by weathering (fig. 1).

Preparation of the model. — The fossil was given a coat of thin
white shellac (cut to 1 lb. per gallon of alcohol) to harden and
protect the surface. The various openings such as the internal
nares, that portion of the orbital area not divided by the septum,
the space behind the lachrymals, and all other openings bridged
over by bone, were blocked off with plasteline (fig. 2). Since the
mold was to be made in two sections, the skull was set up on a board
and blocked with plasteline (fig. 3). A wall of this material, about
five-eighths of an inch wide, was built up around the skull. The
flat upper surface of the wall was made to follow the median line of
the skull as closely as possible, dividing it longitudinally. A
plasteline dam, one-fourth of an inch thick and three-fourths of an
inch wide, was then attached to the outer surface of the wall to form

Fig. 3. Longitudinal horizontal section of skull, upper half covered with
rubber. Solid black line, cross section of rubber; widely spaced diagonal hachures,
cross section of bone; closely spaced diagonal hachures, cross section of plasteline
units A (wall) and B (dam); cross hachures, internal view of plasteline wall, A.

one side of a trough, the skull forming the other. Cone-shaped
depressions were punched in the floor of the trough to act as guides
in assembling the completed mold (a continuous V-shaped groove
might also have been added for the same purpose). All the plasteline
surfaces intended to come in contact with the rubber were shellacked
and allowed to dry.

Process of making the mold. — The American Anode Company's
Formula No. 10099A was the molding compound used. The first

12 Field Museum of Natural History— Technique, No. 6

coat was applied with a small brush, care being taken to work the
rubber into all depressions in order to eliminate bubbles and air
pockets. This application was followed immediately by a second
one to insure complete covering of the surface. The thin film thus
made was allowed to dry over night. Since a thin film of rubber
dries quickly and contains only a small amount of water, it does not
wet the bone appreciably, most of the water being given off into the
air. When dry, it is fairly waterproof, serving to resist the tendency
of the heavier following coat to drive its contained water inward
as it dries and contracts.

Pre-shrunk cheesecloth was cut into strips from two to three
inches wide and about six inches in length. These were laid on

B

Fig. 4. Detail section through beak region showing second half of the mold
and the plasteline dam in place. B, plasteline dam.

over a fresh coating of rubber and brushed down so that they adhered
closely to the surface of the mold. About four layers of cloth were
successively applied, except in areas where some stretching of the
rubber would be required to remove the mold. The cloth does not
reduce the flexibility of the mold, but it does prevent the rubber
from stretching.

In areas where cloth was not applied, the rubber was built up
to a somewhat greater thickness than on the parts of the mold
containing cloth. The mold was then allowed to dry for six days.
At the end of this period the skull was turned over and the wall
of plasteline removed, leaving the dam in place and exposing the
edge of the first section of the mold (fig. 4). All the rubber surfaces
intended to come in contact with the new section were shellacked,
the shellac acting as a separator, and allowed to dry. The second
half of the mold was then made in the same manner, so that the
entire skull was covered with the molding material. Two weeks were
allowed thereafter for the completed mold to dry and cure.

This mold was made as thin-walled as possible in order to reduce
the danger of damage to the skull which, in spite of its size, is fragile
due to the many slender processes and thin sections of bone compris-
ing much of its posterior area. A somewhat heavier mold (six or

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seven layers of cloth and rubber) would otherwise have been more
satisfactory and easier to work with in casting.

A plaster matrix might also have been constructed as an additional
support for the mold. In this case a matrix was not made, since it
was thought inadvisable to subject the skull to any treatment not
absolutely necessary in obtaining accurate casts. On larger models
it is often advisable to use a plaster matrix in conjunction with the
rubber mold.

THE plaster matrix

The plaster matrix serves the three-fold purpose of acting as a
guide for orienting the parts of the mold, as an agent for closing the
mold without the use of cement, and as a rigid reinforcement for the
flexible rubber mold. It is ordinarily employed over large molds
that are required to yield a considerable number of casts. The
matrix may be constructed after the mold is completed, or each half
may be made after the completion of the corresponding half of the
rubber mold. In either case, it is constructed before the model
has been removed from the mold.

After the first half of the mold has dried thoroughly, the plaste-
line dam (fig. 6) around the edges of the mold is removed and a
second wall is set up in such a way that its upper surface falls about
one-half inch below the first wall that forms the edge of the rubber
mold. All undercuts on the mold are filled in with plasteline so that
the completed matrix can be lifted off. The rubber surfaces
intended to come in contact with the plaster should be covered with
tissue paper, which acts as a separator, so that the matrix will be
easily removable. Strips of burlap or coarse cheesecloth are then
soaked in plaster and water and laid on over the mold until a thick-
ness of about one-fourth inch is reached. In very large molds, the
matrix should be somewhat thicker. After the plaster has hardened,
the work may be turned over and the plasteline wall removed.
The rubber surfaces intended to form part of the joint in the
rubber mold are then coated with shellac to act as a separator,
and the second half of the rubber mold applied. The projecting
edge of the plaster matrix will be found to act as a dam for the
second half of the mold (fig. 7). After the second half of the
mold has dried, the matrix may be completed. The rim of the first
half of the matrix should project about one-fourth of an inch
above the edges of the mold. Its inner surface should be trimmed
so that it slopes outward in order that the second half of the matrix,
which fits inside the rim of the first half, can be loosened easily.

Rubber Molds and Plaster Casts

15

The plaster surfaces intended to come in contact with the new
section of the matrix may be coated with vaseUne or some other
separator, care being taken to keep the separating agent (if it is
oily) away from the rubber (p. 16).

The rubber surfaces intended to come in contact with the matrix
are treated as they were in the first operation, and the plaster jacket

Fig. 6. Detail section through beak region showing suggested arrange-
ment for first half of plaster matrix. Vertical hachures, cross section of plaster
matrix. A, plasteline wall; C, secondary plasteline wall.

is applied in the same manner as the first section. After the plaster
has hardened, the entire matrix may be removed and allowed to
dry (very large matrices should be dried on the mold to prevent
warping). The inner surfaces of the matrix are then treated with
several coats of shellac to harden them and to facilitate the removal
of any plaster that may leak out of the mold in the process of casting.

In certain cases it may be necessary to construct the matrix in
more than two sections in order to remove it from the mold. It
is generally better to avoid increase in the number of pieces, if it
is at all possible, since the danger of misfitting the parts of the

Fig. 7. Detail section through beak region, showing completed plaster
matrix. Vertical hachures, cross section of matrix.

matrix and mold is thus increased. Many variations of the process
described are of course possible. However the matrix is designed
and constructed, it should hold the mold so that its parts are
properly oriented, and should bear on the projecting edges of
the mold in such a way as to reduce the possibility of leakage,

16 Field Museum of Natural History— Technique, No. 6

thus relieving the molder of the necessity of cementing the edges of
the mold. These two functions of the matrix are of greater import-
ance than that of support for the mold, and they should be given first
consideration in designing the matrix.

CARE OF rubber MOLDS

Casts should be removed from the mold as soon as possible
after they have hardened. If allowed to remain in the mold for too
long a time they may acquire a chalky surface, and leave a thin
coating of plaster on the inside of the mold. This plaster residue
may be removed by washing the mold with a weak solution of
hydrochloric acid (5 per cent), followed by a water rinse.

It is better not to use new molds until they have been allowed
to stand open for a few hours after removal from the model. If
the molder wishes to use them immediately, he should employ the
acid wash and water-rinse just mentioned or he may again be con-
fronted with a chalky surface and a plaster coating on the mold.

"separating agents

The use of separating agents in rubber mold making is limited
to the contact areas between the halves of a two-piece mold, and to
the cases in which a rubber mold is formed over a rubber model.
White shellac serves effectually for this purpose. Oils or greases
cannot be applied directly to rubber as they cause it to soften
and swell.

Sometimes the internal surfaces of a new mold stick together if
they come into contact. This may be prevented by dusting talc
or dry plaster into the mold as it is removed from the model. After
a cast has been made in the mold the tendency to stick disappears.
The necessity of putting tissue paper on the outer surface of a mold
over which a matrix is to be constructed has been mentioned above.

BRUSHES

Brushes used in applying rubber compounds should be of rather
coarse texture. They must be kept clean, should be dampened with
water before they are used, and must be washed out with clean,
cold water immediately afterwards. Any rubber that may coagulate
in the brush can be removed by soaking the brush in benzine for a
short time.

thinning the compound

Rubber compounds that are too thick for brushing onto the
model may be thinned by the addition of a very little clear, cold

Rubber Molds and Plaster Casts 17

water. If a considerable amount of the ammonia has evaporated
from the compound, it may be thinned by using a 5 per cent solution
of ammonia to insure against coagulation.

PLASTELINE MODELS

Rubber molds are often made over plasteline models. The
rubber and plasteline usually adhere so that the mold is coated
internally with plasteline on removal. It may be cleaned by scraping
and by washing the plasteline off with alcohol. This difficulty can
be entirely avoided by thoroughly chilling the work before removing
the mold. If possible, it should be frozen. The mold will then be
found to separate cleanly from the plasteline.

PLASTER-CASTING

Essentially, the process of making a cast consists of mixing
plaster and water together, pouring the mixture into a mold, and
allowing it to set. Most of the defects occurring in casts (given an
accurate mold) are caused by bubbles and pockets of air trapped
against the surface of the mold.

In order to reduce the number of air bubbles to a minimum, it
is important that the plaster mixture be prepared as free from air
as possible. It should also be smooth and creamy (free from lumps),
so that it will flow into constricted areas. The process of mixing
plaster in water is generally accomplished in the following manner:

A quantity of clean water (about two-thirds the volume of "mix"
desired) is placed in a clean, suitable container, usually an enameled
pan or rubber plaster bowl. The plaster is sifted in so that it settles
into the water almost as rapidly as it falls. The mixture is rich
enough for most purposes when a thin scum of plaster begins to
form on the surface, or when no more plaster will settle freely into
the water. The mixture is allowed to stand until the surface scum
has become saturated. The bowl is then jarred to cause the surface
plaster to settle into the mixture. The mix may then be stirred
slowly until it has a creamy appearance and is free from lumps.
It is then ready to be poured into the mold.

In pouring the plaster, the mold should be tilted so that the
plaster may flow gently down the inside rather than falling directly
to the bottom.

The time required for plaster to set varies somewhat, according
to the temperature of the water, the richness of the mixture, and the
grade of plaster used. Molding plaster sets in fifteen or twenty
minutes. Dental plaster sets more quickly. Higher temperature

18 Field Museum of Natural History — Technique, No. 6

of the water promotes quicker setting. Dirty tools or water, or tools
coated with set plaster usually quicken the setting time. Salt acts
as an accelerator. Glue or alcohol acts as a retarder, slowing the
setting of the plaster according to the amount added to the mixture.

Casting in rubber molds. — The process of making hollow casts
is exactly suited to the characteristics of the rubber mold, and by
making use of this process the casting may be done successfully
with or without a rigid matrix. Rubber molds although flexible
are elastic and have considerable ability to retain the shape in which
they are formed. For this reason thin rubber molds are capable
of producing accurate casts without being entirely supported by a
matrix. It is often impracticable to construct a matrix so that it
completely supports the mold, since in the case of complicated
models the matrix would need to be made in almost as many sections
as a plaster piece-mold in order to be removable.

To obtain hollow casts of uniform thickness, the plaster is
poured into the mold in successive batches. The first application
should form a thin shell approximately one-sixteenth of an inch
thick inside the mold, all excess plaster being drained off. The
second application clings to the freshly set plaster in the mold with
greater facility, and forms a considerably thicker shell. If the process
is repeated a third time, the shell attains a thickness of one-fourth
inch or more, which will be found sufficiently strong for all practical
purposes.

The process is adaptable to the molding technique here described
because the rubber is sufficiently rigid to support the weight of the
small quantity of plaster required for the initial coating without
distorting the mold. On setting, it adds to the rigidity of the mold,
and serves as a sort of internal matrix. When pouring the second
application care needs to be taken in handling the mold so as to
prevent damage to the thin first coating. The plaster should be
poured in small quantities, rotating the mold each time in order to
distribute the weight of the fresh plaster over the mold. Allowing
too much plaster to accumulate in the lowest portion of the mold
is likely to crack the initial coating. After the second batch of
plaster has set, the final application may be made in a sufficient
quantity to produce a cast of suitable strength. If desired, the
mold may be completely filled.

The hollow-cast technique provides a highly successful means
of reducing the danger of imperfections caused by air traps in
complicated molds. The act of turning or rotating the mold in

Rubber Molds and Plaster Casts 19

every direction aids the plaster to flow into every part of the mold.
In this connection it is advisable to use a rather thin mixture for
the second coating in casts where the passages between relatively
large areas are very constricted; otherwise, the second coating may
fail to reach some areas, since it loses its ability to flow rather
quickly on contact with set plaster as the latter quickly absorbs
water from the fresh mixture.

Treatment of hollow casts. — Hollow casts have the advantage of
being light to handle and they require less plaster in the making.
They are sufficiently strong and serviceable for most purposes. They
have the disadvantage of being more easily damaged than solid
casts. However, certain precautions may be taken to increase their
durability.

There are now on the market several brands of extra hard
plaster of which HydrocaP is an example. These hard plasters
function very well in casting, and may be used for one or all the
batches in the hollow cast. The strength of ordinary plaster may
be greatly increased by mixing it in water near the boiling point.
Treated in this manner it sets about 20 per cent faster than plaster
in cold water.

A small amount of dextrine solution'^ may be mixed with the
plaster intended for the secondary coatings. It acts as a bonding
agent between the layers. It is not advisable to use dextrine in the
first coat, since it increases the surface tension and viscosity of the
plaster, thus increasing the danger of air bubbles.

The casts may be impregnated with a thin solution of white
shellac, after they have been thoroughly dried. Since the walls
of the cast are thin the shellac has an opportunity to penetrate the
plaster completely, provided the casts are submerged in it and allowed
to remain a few hours.

Large and small casts. — Molds too large to manipulate in distrib-
uting the plaster in the casting process require a special technique.
The mold is made in two sections, each section being reinforced by
a matrix constructed so that it supports as much of the mold as
possible. In casting, each half of the mold is coated with an even
thickness of plaster which may be either poured or brushed on.
Care should be taken not to apply more plaster than the mold will
bear in areas that are not reinforced by the matrix.

1 United States Gypsum Company. Hydrocal cement plaster.

2 Dextrine, Globe 160, Corn Products Refining Company, one part to five
parts water, stirred cold, heated to about 190° F. and allowed to cool.

20 Field Museum of Natural History — Technique, No. 6

After the initial coating has set, it is reinforced with plaster and
tow, or other fiber, so that the two sections will be light enough to
handle and strong enough to be durable. The edges of the sections
of the cast should correspond very closely with the edges of the
molds. After the plaster has hardened, the molds are removed,
and the two sections of the cast fitted together and cemented with
plaster. If the work is carefully done the joints will fit closely and
be hardly noticeable. At any rate, in such a case the advantage
of having a light cast outweighs the disadvantage of possible slight
mutilation along the joints.

Minute fossils, such as jaws and maxillaries of very small animals,
present a special set of obstacles to be overcome in casting. The
major one arises from the difficulty encountered in filling the in-
terstices of the mold with plaster. Use of the separating agents
required by most molding compounds almost invariably obscures
detail. Also, it may be difficult to apply the molding compound
to the model in a satisfactory manner. The rubber technique
surmounts these obstacles so well that it is actually easier to cast
very small jaws and maxillaries than it is to cast larger ones. For
convenience in handling, a slender needle may be cemented to the
model with celluloid cement. A thin solution of rubber is carefully
brushed on to form the first coating. As soon as this coating has
been applied, the surplus should be blown off as far as possible.
This action will expose any uncovered areas or air bubbles. Such
areas should be carefully filled and the entire model again coated.
The mold is then built up to the required thickness and allowed
to dry. After drying, the mold is cut open along the line of the
needle, and far enough to permit easy removal of the model. In
casting, the mold is turned inside out, thus exposing and opening
the minute interstices which can then be readily coated with plaster.
It is then returned to its normal form and the filling is completed.
The excess plaster, if any, is drained off until the edges of the mold
meet naturally. It may then be set aside until the plaster has
hardened. Since no separating agent is used when making rub-
ber molds, the surface detail of the model is so perfectly reproduced
that the casts compare favorably with the model when examined
under the microscope.

CONCLUSION

The technique herein outlined is not a solution to all the prob-
lems that may be encountered in making plaster casts. Nevertheless,
rubber has proved of such usefulness that it may safely

Rubber Molds and Plaster Casts 21

be said that no other material offers a comparable combination
of advantages and of freedom from faults. The most outstanding
advantages of liquid rubber may be enumerated as follows: simple
technique of construction; adaptability; superior durability of molds;
excellent reproduction of minute detail; and minimum damage to
the object being cast.

The procedure of making rubber molds is adaptable to a labora-
tory routine in the hands of a single person. The work of constructing
a mold may be carried on along with other projects on hand since
the operations involved need but little time and attention to com-
plete any one step. Furthermore, mold-making may be continued
or interrupted at any stage; it can be done at odd moments, and
need not represent a time-consuming burden. By the use of this
technique it is feasible for any institution to produce useful and
accurate casts without the necessity of obtaining the services of a
skilled molder, or of maintaining a special casting department.

IHE LIBRARY OF THE

MAY 2 91940

UNIVERSITY OF ILLINOIS

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UNIVERStTY OF ILLINOIS-URBANA

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