Subsurface Laboratory Methods 177 



both the rock slice and the glass slide with an even, thin layer of raw 

 Canada balsam and cook. If the rock slice is porous, apply an excess of 

 balsam. The hot plate should be kept level so that the balsam may flow 

 evenly in all directions. 



The success of mounting the specimen lies in cooking the balsam. If 

 it is insuflficiently done, the rock slice will not adhere to the glass slide; 

 if it is overdone, the balsam will be too brittle and will break away when 

 ground. The object is to cook the balsam to the point that, when it is 

 cooled, one may barely dent it with the thumbnail. While cooking is in 

 progress, the cement is continually tested by taking a small amount on 

 the end of a toothpick and biting it between the front teeth. When balsam 

 is properly cooked, it will stick to the teeth, barely begin to pull, and then 

 abruptly break. In other words, it becomes "tacky." ^^ 



When the cement is properly cooked, place the rock slice on the glass 

 slide with the two balsam-covered sides in contact. Remove the mount 

 from the hot plate to an asbestos pad. With the eraser end of a pencil, 

 work the rock slice around, meanwhile applying considerable pressure 

 in order that the air bubbles and excess balsam will be squeezed from 

 between the slice and the slide. Center the slice and allow the cement to 

 set while pressure is applied. As the mount cools, test the balsam for 

 hardness with the thumbnail. Cooling should be allowed to proceed nor- 

 mally, as sudden changes in temperature tend to pull the rock slice from 

 the glass. The mount is now ready to be ground to the desired thinness. 



Grinding to 0.03 Millimeter 



The final grinding to 0.03 millimeter is carried out in three stages. 

 The mount is ground on the coarse lap with 80- to 100-mesh abrasive 

 until it is about 0.10 millimeter thick. It is then cleansed carefully and 

 transferred to the fine lap wheel with 320-mesh abrasive. During this 

 stage, with proper caution, a well-cemented slide may be ground almost to 

 the desired thinness. The writers ordinarily carry the grinding with 320- 

 mesh abrasive to a point where such colored minerals as hornblende or 

 biotite are fairly transparent when held before a light source. Further 

 grinding by mechanical means becomes hazardous; therefore, the last 

 stage of grinding is done on a glass plate with 600-mesh abrasive in water. 

 The section is moved with a gentle, circular motion on the plate. If the 

 slide has become wedge-shaped, more pressure is applied on the thicker 

 portion to bring the entire rock slice to the same thickness. As the slide 

 is now very thin, constant checking for thickness under the microscope is 

 necessary. Quartz is the mineral commonly used as an index, and the sec- 

 tion has reached 0.03 millimeter when quartz shows a faint, straw-yellow, 

 interference color. 



balBam 



Other cements are available, but the writers have had the most consistent results with raw Canada 



