1922 



Journal of Applied Microscopy 



The applications should be frequent rather than in greater quantities at longer 

 intervals. For the sake of cleanliness the water supply arranged to trickle con- 

 stantly on the disks of most grinding machines should be abandoned and the 

 pastes applied wet enough not to need additional water. 



To bring out the micro-structure of iron or steel it is generally necessary to 

 apply an " etching " solution, although this is not always needed. Fig. 1 is a 

 micro-photograph of a section of pig iron as it appears at the completion of the 

 polishing process ; no etching having been done. This is styled polishing in 

 " relief. " Fig. 2 is a micro-photograph of a piece of steel before etching, and 

 Fig. o is the same piece after the application of a 25 per cent, solution of tincture 

 of iodine in alcohol as an " etch ". 



The reagents used for etching iron and steel are various, depending mainly 



on the carbon-content of the 



steel and its heat-treatment. 

 For mild and untreated steel, 

 tincture of iodine, full strength, 

 and in a 25 per cent, solution 

 in alcohol, is used ; for hard- 

 ened steels 5 per cent., 10 per 

 cent., and 20 per cent, solu- 

 tions of nitric acid in alcohol 

 are very satisfactory. 



The polished surface of 

 the micro-section should be 

 freed from all grease and dirt 

 by washing in alcohol and care- 

 fully dried. If the etching 

 fluid is iodine, it is best applied 

 with the tip of the finger or a 

 ' camel's hair brush, keeping 

 Fig. 4.— Steel heated to l:iOl)° C. f^^^^^ ^j. ^^^^^ lightly rubbing 



over the surface being etched and carefully watching the process, to prevent 

 too deep an etching. The time necessary to properly etch a specimen cannot 

 be definitely stated, as it varies greatly, ranging from a few seconds to a minute ; 

 depending upon the nature of the steel and the etching solution employed. A 

 beginner will find it best to err on the safe side of stopping the process before 

 the etching is complete ; for should the examination show the structure in- 

 sufficiently developed, the etching may be repeated until a desired result is 

 obtained. A rule (with exceptions, of course) is to stop the action of the etch 

 when specular appearance of the specimen has just disappeared and before the 

 surface becomes dull and uniformly grey. 



Having decided that the section is sufficiently etched, quickly hold the spec- 

 imen under a tap of iiowing water for a few moments, then remove all traces of 

 moisture by the application of a few drops of alcohol, and finally dry it in a blast 

 of air or by careful friction with very clean, old, and washed-out linen or cotton 

 cloth. 



