3o8 HOMER G. TURNER AND H. R. RANDALL 



The polished specimen is finally placed in a drying oven over 

 a Bunsen burner and heated from room temperature to about 220 

 degrees Centigrade, to remove moisture, and to obviate splitting 

 of the surface due to sudden heating during the etching process. 

 After this temperature has been maintained for about an hour, the 

 specimen is removed with the forceps and the polished surface 

 immediately brought to a red heat by means of the oxidizing blow- 

 pipe flame. A differential oxidation is thus produced which reveals 

 the structure in relief without destroying the polish to any great 

 extent. The surface should appear only slightly foggy owing to an 

 almost imperceptible film of ash. 



The duller layers in the coal etch more rapidly than the bright 

 ones, making it necessary at times to warm and etch repeatedly 

 in order to bring out the greatest detail. Those varieties which 

 show little lamination and are made up almost wholly of jetty 

 bright coal must be etched in a little different way to overcome 

 splitting of the surface. Instead of the oven a sand bath is used. 

 The coal is immersed in clean dry sand with only the polished 

 surface exposed. The bath is placed over a battery of four Bunsen 

 burners and brought to a temperature of 300 degrees Centigrade. 

 This temperature is maintained for about half an hour when the 

 polished surface is heated in place to a bright red heat with a blast 

 lamp, using a rather large blue flame which is played slowly back 

 and forth over the surface. A little experience will demonstrate 

 the advisability of re-etching to further define partially hidden 

 structure. 



Various attempts were also made to utilize oxygen in etching. 

 A stream of cold oxygen was directed on the surface of a specimen 

 heated to about 400° Centigrade, but only served to keep the surface 

 cool and prevent etching. Warm oxygen used in the same way gave 

 no better results. Oxygen at very high temperatures was not used 

 in this way. Etching is produced, however, by heating pohshed 

 specimens in an atmosphere of hot oxygen under slight pressure. 

 In the latter case the coal is placed in a piece of glass combustion 

 tubing about two feet long. Oxygen is passed through water in 

 a wash bottle and into the combustion tube through a rubber stopper 



idfi 



