Biological Castings

The preservation and display of biological specimens have historically been limited to the use of liquid solutions or dehydration. With the advent of thermosetting resins and proper technique, it is now possible to preserve specimens in a clear, odorless unit that is not subject to breakage. The advantage over conventional museum jars is that there is no odor or deterioration of the specimen, you can achieve excellent clarity and it is relatively simple to do.

Materials needed:

Casting Resin

Catalyst

Molds for casting (poly molds, re-usable glass or homemade mold)

Chemicals:

Formaldehyde

Ethylene or propylene glycol

Preserving and relaxing chemicals. ( see section under) “collecting and Narcotization”

Polyethylene stainless steel, or enameled pans for treating and preserving specimens.

Directions:

Basic handling of Casting Resin for Biological Uses:

Clear Casting Resin has been widely used for these applications; However, as in any chemical process to obtain optimum results, it is essential that the directions be followed closely. After a degree of experience is obtained, many collectors will be capable of deviating from these directions to obtain special effects.

Collecting and Narcotization:

Specimens must be treated properly to ensure life-like , relaxed specimens for casting. The following chemicals have been used by biological collectors and have been proven successful in relaxing specimens prior to preservation:

Insects- 70% isopropyl alcohol.

Marine invertebrates- Magnesium chloride in collecting water.

Reptiles and amphibians- Ether or chloroform.

Dehydration and Preserving:

These specimens which can be air-dried without deterioration or shrinkage can be embedded directly into the casting resin without preserving techniques. Such specimens as snail shells,small starfish, nuts, seeds, small insects, most flat botanical specimens, cones, some fungi, and bone and skeletal parts, can be simply air-dried and embedded. Such specimens must be completely dry . If there is any doubt as to the complete dryness, specimens should be placed in a desiccator with silica gel.

A 10% formaldehyde solution is suggested as the basic preserving or fixing solution. Specimens should be covered with at least one inch of preserving liquid and left in the formalin for approximately one day for each inch of body mass. Small mammals, reptiles, and amphibians should be eviscerated and the body cavity packed with cotton prior to fixing in formalin.

Those specimens which cannot be air-dried will have to be cast “wet”. This is contrary to our basic rules, but it may be successfully accomplished by using a compatible liquid rather than water. Ethylene or propylene glycol are inexpensive reagents and can be used with excellent results to replace the water after preserving in formalin solution. A polyethylene, stainless steel or enameled pan may be used or glycol solutions.   

Small specimens should be soaked in the glycol solution for a minimum of 24 hours. Reptiles, amphibians, and small mammals, should be soaked up to one week and the solution changed at least once during the soaking period. Any water left in the specimen will tend to cloud the casting resin.

On removing specimens from the glycol solution they should be drained or blotted. Place in a cup of uncatalyzed resin for 24 hours. Let specimen drain before continuing the casting procedure.

With most small animals, some care should be taken during preserving and dehydration to ensure that proper and attractive positioning of the specimen is obtained. The use of pins or thread will generally hold the specimens during the soaking periods.

Casting Procedure:

All molds, except those made from polyethylene, should be coated with a mold release prior to pouring casting resin. Resin should be stored and used at room temperature for optimum results. Most specimens will require at least three pours to completely encapsulate them.

Pour the first, or bottom, layer and allow the resin to cure to a soft, jelly-like consistency. Then place the specimen into the mold on top of this layer. Pour the second layer over the specimen to secure it in place, and allow it to gel. Do not pour the second layer so deep that the specimen tends to float. It is not necessary to completely cover the specimen on the second pour.

In complex specimens there may be a problem of air bubbles clinging to concave or undercut areas when embedded in the resin. This may be eliminated by dipping the specimen in resin prior to placing it in the mold.

The third layer may be poured once the second layer has gelled.

In order to prevent heat build-up, it is best to keep the catalyst level as low as possible. Until experience is gained, three drops of catalyst per ounce of casting resin is suggested and making pours of approximately 3/8 of an inch in depth. Because of the inhibiting action of the air on the resin surface, the exposed surface of all castings tends to be slightly tacky.

To ensure a hard, glossy top surface, one of the following is suggested:

(1). Spray with acrylic spray after the surface has cured as much as possible.

(2). A thin layer of Perma Glaze can be brushed on.

(3). Cut a piece of Mylar film the same size as casting. Place directly on the final resin layer immediately after pouring. Carefully press out any air bubbles. Mylar may be easily stripped off as soon as resin is cured. This provides a barrier to the air and will give the best finish.

Cleaning of tools and working area is easily accomplished with acetone.