94 BUULETESr 18 4, UlSriTED STATES NATIOiNlAL MUSEUM 



and varied structures, surfaces of a few square inches are desirable; 

 but very small samples of hexahedrites and ataxites are often sufficient. 



The most convenient form is a slice, which preferably should be 

 of uniform thickness so that the surface may be level on the micro- 

 scope stage. For use with a metallographic camera where the speci- 

 men lies on the stage with its polished face downward, the shape is 

 immaterial. 



Macroexamination. — While the greater part of metallographic 

 study is done with the microscope and camera, at moderate to high 

 magnification, macroexamination — by the eye, with a hand glass, or 

 with a microscope at low power — is of great importance. 



Only the coarser structures are thus observable, but these should 

 be carefully studied as a preliminary to the more minute examina- 

 tion. Thus the presence and character of visible inclusions should 

 be noted. Most of them are soon recognized easily, but it may be 

 necessary to verify their nature by their hardness, by sulphur print- 

 ing, or other tests. 



In octahedrites the characteristic structure often can be seen only 

 with the unaided eye and on a fairly large surface. When the struc- 

 ture is coarse and faint it might escape observation under the micro- 

 scope. The sheen of kamacite also, even when strong to the eye, 

 often is not observable on the very lightly etched surfaces employed 

 at high magnification. 



Preparation jor macroexamination. — The preparation of a speci- 

 men for macroexamination involves the preliminary steps in pre- 

 paring it for micro examination and photogi'aphy. These are as 

 follows : 



1. Obtaining a flat surface: This may be done with a hacksaw (auto- 

 matic if possible) or, if available, by a power-driven steel band fed 

 with emery and water such as some museums and laboratories pos- 

 sess. An emery disk may be used for cutting when the iron contains 

 silicate inclusions. When a specimen is to be examined without 

 removing a slice, a flat area of suitable size may be produced on a 

 carborundum wheel, care being taken that it is not unduly heated. 



2. Polishing: The plane surface is conveniently smoothed on 

 coarse emery paper or cloth, or on the side of a carborundum wheel. 

 A round piece of emery paper may be applied (without adhesive) to 

 a disk of wood or metal mounted on a grinding machine. The coarse 

 emery paper is followed with finer grades up to No. or No. 00 metal- 

 lographic emery paper, or its equivalent, each time care being taken 

 to remove all scratches left by the preceding coarser paper. The 

 polish is now sufficient for macroetching, although under high mag- 

 nification the surface would appear so rough that fine structures 

 could not be seen. 



