PRESERVATION OF SPECIMENS IN PLASTICS 53 



Sometimes, in preparing specimens that consist mainly of thin tis- 

 sues, removal of natural moisture can be carried out advantageously 

 by dehydrating them in alcohol or similar water-absorbing volatile 

 liquids, superior results usually being obtained with tertiary butyl 

 alcohol. This should be carried out through several concentrations, 

 starting with 40- or 50-percent and ending with the 100-percent or 

 water-free liquid, in the manner described in the preceding section. 

 The specimen material should be transferred from one concentration 

 to the next as soon as it becomes uniformly infiltrated with the dehy- 

 drating liquid because there is an active tendency toward pigment 

 extraction and discoloration where any appreciable amount of water is 

 present in the dehydrating liquid. It is furthermore inadvisable to 

 leave the material in the final dehydrating stage for more than a day or 

 two before proceeding with the impregnation or coating step. 



As soon as practicable after dehydration has been completed in this 

 manner, specimens that are suitable for impregnation should be trans- 

 ferred to methacrylate monomer, or partial polymer, and impregnation 

 and embedment carried out as described in the sections that follow. 

 In the case of specimen material that shows a tendency to shrink or 

 collapse when removed from the liquid, transfer should be made directly 

 into the monomer through several successive baths as explained in the 

 preceding section. Specimens that have sufficient dimensional stability 

 to permit removal of the dehydrating liquid by evaporation, and also 

 those that require protective coatings before embedment, should be 

 freed of the dehydrating agent by placing them in a vacuum desiccator 

 or in a moderately warm oven, held between 60° and 75° C. (140° and 

 167° F.) until all traces of the volatile liquid are gone. They should 

 then be stored in a desiccator or other dry container until the impregna- 

 tion or coating step is undertaken. In working with material dehy- 

 drated in this manner, it should be borne in mind that it is usually very 

 brittle and requires considerable care in handling to avoid breakage. 

 It is generally good practice to use forceps or tweezers for this purpose, 

 and each specimen should be kept in a separate box or compartment 

 during storage. 



Various flowers and leaves, together with certain other objects com- 

 posed of thin tissues, can be dehydrated satisfactorily without undue 

 change in shape or color by packing them in a suitable granular or pul- 

 verized medium which absorbs moisture or permits its withdrawal from 

 the specimen. In this method of dehydration, the packing medium em- 

 ployed may be a desiccating substance such as activated silica gel or 

 various ground cereals that have been previously heated for several 

 hours at 80° C. (176° F.) ; or it may be a medium that permits moisture 

 vapor to pass through it, such as sand, talc, starch, sugar, borax, boric 

 acid, salicylic acid, and powdered alum. The use of borax, boric acid, 

 alum, plaster-of-Paris, talc, silica gel, cornstarch, rice flour, and various 

 other substances for maintaining the natural appearance of flowers and 

 leaves during dehydration was reported by Williams (21, 22, 23). Sand or 

 pulverized rock is reported to have been used for this purpose for several 

 centuries. 



Only well-dried, clean packing substances should be used in this pro- 

 cedure, and it is necessary to apply them with special care in order to 

 maintain the natural shape of the dried specimens. Pouring the sand or 

 other packing medium through a long-stemmed funnel has been found to 



