Taxidermy Freeze Dry
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Because of the diversity of specimens being freeze-dried for taxidermy purposes (from Brahma bull head mounts to minnows used for fishing bait), trial and error has often been the method used to determine how different taxidermy specimens should be pre-frozen and freeze dried. The success or failure often directly relates to the proper pre- freezing of the taxidermy specimen, and the temperature at which the specimen chamber is held during the drying stage.
The Eutectic Temperature
When frozen, various taxidermy specimens will totally solidify at different temperatures. The temperature of complete solidification is the taxidermy specimens eutectic (pronounced yu-tek-tic) temperature. The American Heritage Dictionary, 1969, defines eutectic as, "the lowest possible temperature of solidification for any mixture of specified constituents"
An example of the variance found in freezing temperatures is that of water - not all water freezes at 32 degrees F. When water is contaminated with other elements , such as salt, nitrogen, mercury, industrial waste, etc., it's temperature of complete solidification will change. Since one of the basic premises if freeze dry taxidermy is animals are made up of about 70% water by weight, the importance of eutectic temperature is readily evident.
When determining eutectic temperatures, taxidermy specimens may be divided into two general groups: those that contain large amounts of body fats and oils (most fish, certain species of ducks, amphibians, etc.) and those that do not ( the majority of mammals, reptiles, game birds, delicate flowers, etc.) Because of the chemical makeup, taxidermy specimens in the group (greasy specimens) require lower eutectic temperatures than the second group (non-greasy specimens).
Pre - Freezing
Because natural salts are usually found in combination with water in taxidermy specimens, pre-freezing should be done in the shortest time possible. Slow freezing times lead to salt concentration in the specimen resulting in lower eutectic temperatures and increased chance for shrinkage. Rapid freezing reduces salt concentrations and also results in smaller ice crystals, which create less tissue distortion.
Rapid freezing may be obtained by freezing taxidermy specimens in commercial freezers with temperatures of less than -13 degrees F. (-25 degrees C.) it is generally more economical to pre-freeze the specimens in a commercial deep freezer, than use the specimen chamber as a freezer. Freeze dry machines are most efficient when "drying" frozen material, rather than initially freezing the material, then drying it. The important factor in pre-freezing any type of specimen is that the specimen is completely frozen throughout.
The Drying Stage
Once the specimen has been prepared for preservation, mounted, posed, and completely frozen, it is ready for the drying stage. the important factor in the drying stage is that the eutectic temperature is maintained. Because of the lower eutectic temperature of greasy specimens, a low processing temperature in the specimen chamber is needed to prevent shrinkage. Generally this temperature should be as 0 degrees F. and then raised even higher after a period of time. Using these figures as guidelines, the taxidermist using the freeze dry machine should determine at what temperature his work is most successful. Some factors to consider when analyzing proper processing temperatures include:
- Working in batch loads (all the same type specimen) may be the most economical way to work, depending on your own situation. Batch loads allow easy raising of temperatures as the process progresses.
- When working with mixed loads (greasy & non-greasy specimens) one initial temperature should be decided upon (e.g. -5 degrees F. or 0 Degrees F).
- Some shrinkage may be traded off for speed. Mammals may be run at slightly higher temperatures allowing slightly faster drying times. The animals fur will cover most of the shrinking that takes place.
- Not only does a variety of chemical make-ups exist between different species, it may also exist between members of the same species due to different diets, and physical environments.
- Because if the lack of air to conduct heat within the specimen chamber, some differences of temperature will exist inside the specimen chamber itself. This will require "rotating of the stock".
- The greatest water loss will occur early in the drying stage when the dried shell is the thinnest and offers the least resistance to water vapor movement.
The Economics Of Different Temperatures
The rate of freeze drying is directly related to vapor pressure. the higher the specimen chamber temperature is, the higher the vapor pressure is at a given vacuum, and the faster the drying is achieved. The rate of moisture migration from the specimen chamber, to the ice bank, is a also related to vapor pressure. The greater the temperature difference between the specimen chamber, and the ice bank. for example, a unit with the specimen chamber set at 5 deg. F. and the Ice bank running at -55 deg. F., resulting in a temperature difference of 60 deg. Will move moisture faster than the same machine with the specimen chamber set a -5 deg. F., resulting in a temperature difference of 50 deg. F.
Dealing With Greasy Specimens
Certain steps may be taken to raise the low eutectic temperatures of greasy specimens. This will allow the taxidermist to operate the freeze dry unit more economically at higher temperatures. the first step is to eliminate the extreme amounts of fats and oils that cause the low eutectic temperatures. In the preparation stage, a 20 minute bath in a mixture of one cup of bicarbonate of soda per gallon of water will assist in neutralizing fats and oils. Also, injections of grease tallowfiers, such as antioxidant will help in preparing greasy and oily specimens for more efficient freeze drying.
Some of the questions that should be addressed when considering the right temperatures to process a particular specimen are:
- What is the Physiology of the specimen?
- What is the Chemistry of the specimen?
- What solid mass of the specimen can be removed to speed up the process, and not effect shrinkage?
If you analyze specimens with these questions in the initial planning stages, problems can be eliminated. there is nothing, to our knowledge, that can not be successfully freeze dried without shrinkage, as long as it is freeze dried at the proper temperature.