Zoology Publications from Victoria University of Wellington—Nos. 42 to 46
[Introduction]
The value of plastic embedded specimens as a teaching aid is now well recognised. It was to take advantage of these benefits, especially as a replacement for wet-mount bulky specimens, that a programme of embedding was initiated in the Zoology Department, Victoria University. From instruction leaflets, etc., the technique appeared to be a simple one, with little or no problems to trap the unwary. This we found to be true to a limited extent for the range of teaching material required in the department.
The teaching material we wished to embed covered a wide range both in surface and bulk texture and in size. The material varied from hard, barely porous shells and teeth to the open meshwork of the Porifera, through bone and cartilage to chitin-covered arthropods or coelenterates, and to delicate jellyfish or the early stages of the chick blastodisc. The largest specimen we have so far embedded is an eviscerated mammalian embryo 15 × 5 × 3.5 cm in which the skeleton had been stained with Alizarin Red S to distinguish the ossified tissue. The unpolished plastic block for this specimen measured 23 × 5 × 10 cm. The smallest individual specimens embedded were unstained copepods, 3-4 mm in length.
In the course of overcoming our difficulties with the embedding of such material, we also tried and found successful other procedures for sectioning the embedded material, and for block shaping and polishing. Comments on these procedures and on the embedding techniques we finally adopted we felt might be of interest and assistance to other workers contemplating plastic embedding of a similar range of zoological material. The quantities of material and the timing given for the embedding processes apply to polyester resin Au 8018C, styrine monomer and catalyst M.E.P.K., as manufactured by A. C. Hatrick (N.Z.) Ltd., Tawa, Wellington.
Figs. 1 & 2 Formica faced L-pieces placed to form a mould.
Fig. 3 Sectional view of mould to show the approximate degree of "tackiness of the resin surface prior to the pouring of the next layer.
Fig. 4 Metal-sided, 4 in one mould showing also the glass cover plate.
Flexibility of usage from a single "box" mould for large projects (Figure 4) is obtained by the provision of metal inserts, giving a one, two, three, or four-in-one box. Ease of removal of the blocks from this type of mould comes from the ability to open out the sides. The glass base and top plate give a surface to the hardened block that needs very little polishing. Virtually no presurfacing of the glass or metal with a releaser compound such as silicone wax is necessary as the plastic shrinks away from the mould during hardening.
Basically, the whole process of embedding can be summarized in four words: dehydrate, clear, embed the specimen and polish the plastic block. But before proceeding with a project, if you wish to admire your handiwork as a plastic block which has a high surface gloss, is almost crystal clear, and without bubbles, it is necessary to possess considerable patience and be aware of some of the problems that may be encountered in carrying out the four basic techniques noted above.