4-8 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



carried out in desiccator or oven as explained in the preceding section. 

 The coloring matter, however, requires protection from actual contact 

 with the resin during embedment in order to prevent either discoloration 

 of the plastic through extraction of soluble pigments or alteration of 

 specimen color as the result of bleaching by the catalyst or modifying 

 action by the resin while in the liquid state. 



Protection can in most cases be obtained by coating the specimen 

 with a water-dispersible colloidal substance such as acacia gum (gum 

 arabic), gelatin, egg albumin (egg white), casein, algin, polyvinyl alco- 

 hol, methyl cellulose, or carboxy methyl cellulose. Various prepared 

 glues and pastes may also be used. To prevent peeling or cracking of 

 the coating when dry, it is generally advisable to plasticize the coating 

 substance with glycerine, sorbitol, or polyethylene glycol. The amount 

 of plasticizing liquid needed in any given case should be determined 

 experimentally. The coating material should be applied as an aqueous 

 solution (or dispersion) having a moderately low viscosity, and best 

 results can be obtained by applying several successive layers and allow- 

 ing ample drying time for each. When gelatin or glue is used, treat- 

 ment with 5- to 10-percent formalin to render each layer insoluble may 

 in some instances prove to be helpful in building up a suitably impervious 

 coating. In the application of protective coatings, the specimens are 

 ordinarily dipped into the solution; but painting or spraying may some- 

 times prove to be preferable. 



Specimens having pigments that do not permit the use of water- 

 dispersible colloidal substances can in some instances be protected more 

 or less satisfactorily by coating them with cellulose acetate, or with a 

 transparent plastic such as vinyl acetate-chloride copolymer dispersed 

 in a suitable organic volatile solvent and applied by dipping or spraying. 

 Several layers are usually necessary, and complete evaporation of sol- 

 vent from each coat is necessary before the next is put on. 



Nonrigid durably pigmented material. — This class comprises two 

 types of material: (1) nonrigid specimens that retain their color satis- 

 factorily when dehydrated in alcohol or in similar moisture-removing 

 volatile liquids, or when subjected to freeze-drying; and (2) plant 

 material made up mainly of thin tissues having low moisture content 

 that can be air-dried without undesirable change in appearance when 

 packed in suitable dry granular dehydrating substances. 



The first type in this class includes soft-bodied insects and other 

 zoological specimens that have more or less stable pigmentation, and 

 also a limited variety of fleshy plant material. Specimens of this type 

 usually can be prepared by dehydration in suitable liquids without pre- 

 liminary preservation. Ethyl alcohol is the most generally employed 

 dehydrating agent for such material, but isopropyl alcohol, tertiary 

 butyl alcohol, acetone, and dioxane can also be used, and in certain 

 cases may yield superior results. Because of the volatility of these 

 liquids, all containers in which they are placed should be kept covered 

 and should not be brought near open flames. In carrying out the dehy- 

 dration of nonrigid material in water-absorbing volatile liquids it is 

 usually necessary to do so gradually through a number of stages in order 

 to prevent the shrinkage and distortion which tend to occur when the 

 entire water content is removed in one operation. 



The first step in dehydration consists of immersing the specimen in a 

 bath of alcohol (or other suitable dehydrating liquid) which has been 



