Cast SteeL 
421 
tier a plate of steel may be hardened so as to be perfectly 
brittle, and yet continue so sound as to ring like a bell ; 
an effect which I never could produce in any other way* 
Mr. Stodart has lately made trial of this method, and con- 
siders it to be a great acquisition to the art, as in fact I 
found it. 
The letting down, or tempering of hard steel, is consider- 
ed as absolutely necessary for the production of a fine and 
durable edge. It has been usual to do this by heating the 
hardened steel, till its bright surface exhibits some known 
colour by oxidation. The first colour is a very faint 
straw colour, becoming deeper and deeper by increase of 
heat, to a fine deep golden yellow, which changes irregu- 
larly to purple, then to an uniform blue, succeeded by 
white and several successive faint repetitions of these se- 
ries. It is well known, that the hardest state of tempered 
instruments, such as razors and surgeons instruments, is 
indicated by this straw colour ; that a deeper colour is re- 
quired for leather cutter’s knives, and other tools that re- 
quire the edge to be turned o 1 one side; that the blue 
which indicates a good temper for springs, is almost too 
soft for any cutting instrument except saws, and such 
tools as are sharpened with a file ; and that the lower states 
of hardness are not at all adapted to this use. But it is 
of considerable importance, that the letting down or tem- 
pering, as well as the hardening, should be effected by 
heat equally applied, and that the temperatures, especial- 
ly at the lower heats, where greater hardness is to be left, 
should be more precisely ascertained than can be done by 
the different shades of oxidation. Mr. Hartley first practis- 
ed the method of immersing hard steel in heated oil, or the 
fusible compound of lead five parts, tin three, and bis- 
muth eight. The temperature of either of these fluids 
may be ascertained in the usual manner, when it does not 
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