ETCH METHODS Acid etching is a simple and rapid method 



applicable to the study of the texture; grain 

 size; ratio of calcite to dolomite; and the type, distribution, and approximate 

 amount of insoluble material or fossil content of limestones and dolomites. 

 Acid etching does not replace, but only supplements, thin-section and insoluble- 

 residue studies. Etching often reveals impurities and three-dimensional grain 

 relationships. The distribution of insoluble materials and their relationship to 

 bedding also are brought out clearly by etching. 



Very little has been published on acid etch tests. Probably the most 

 complete discussions are given by Lamar (1950) and Ives (1955). 



The necessary equipment for etch tests are a rock saw, polishing abrasive, 

 flat-bottomed glass dishes or beakers, acid (hydrochloric and acetic) , and a 

 binocular microscope. The size of the sample used depends upon the material 

 available and the information desired. Ives (1955) used slabs about 6 inches 

 long, 15/g inches wide, and approximately ]/ 2 inch thick. The author has used 

 slabs about 2 inches square and y 4 to i/^> inch thick. Larger or smaller pieces 

 may be used, but at least one side should be flat and polished. The specimen is 

 placed in a flat-bottomed dish with the polished side up. The polished surface 

 is leveled up on a water surface and held in place with modeling clay if neces- 

 sary. Inclined surface may be channeled by rising streams of bubbles, and 

 this channeling may be confused with significant etching results. 



The most commonly used acids are dilute hydrochloric and acetic. Lamar 

 (1950) recommends 23 cubic centimeters of C.P. glacial acetic acid in 100 

 cubic centimeters of water or 8 cubic centimeters of Concentrated hydrochloric 

 acid in 100 cubic centimeters of water. He also recommends etching for 20 

 minutes in acetic acid and 5 minutes in hydrochloric. It is essential that the 

 acid reaction be slow; otherwise, the delicate features etched out may be de- 

 stroyed. Since limestones and dolomites may differ in their acid reaction, the 

 author recommends covering the specimen with about a half inch of water and 

 then adding sufficient acid to start mild effervescence. After the polished surface 

 has been thoroughly etched, the specimen is removed from the acid and gently 

 washed. It is best to immerse the specimen two or three times in a beaker of 

 water instead of washing it under a faucet as there is less danger of destroying 

 the features brought out by the etching. 



Acetic acid is less uniform in its etching action and usually produces a 

 rough surface. Hydrochloric acid usually produces a smoother surface and 

 often gives a polished appearance to the surface. In general both acids should 

 be tested because it is sometimes difficult to predict which one gives the best 

 results. Figures 6-5, 6-6, 6-7, and 6-8 are photographs of etched surfaces pre- 

 pared by Lamar (1950). 



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