Cast Steel . 
399: 
r 
more and more heated, which the workmen in this metal 
have ingeniously taken advantage of, as indicating and 
serving to denominate the degree of temper required far 
different' articles. The first perceptible colour is a light 
straw yellow, and this being produced by a small degree 
of heat indicates the highest or hardest temper ; to this 
succeeds a full yellow, then a brown, afterwards a reddish 
i blue, then a light blue, and lastly a full deep blue passing 
I into black, which being the other extremity of the series,, 
denotes the lowest degree of temper, and a hardness only 
a little superior to what the piece of steel would havq ac- 
quired if when heated for the purpose of being hardened it 
had been allowed to cool gradually instead of being plung- 
ed into a cold liquid. The old method of tempering, and 
which indeed is still practised by most manufacturers, is 
to lay the articles on a clear coal fire, or on a hot bar, till 
they exhibit the requisite colour ; but small articles which 
were to be reduced to a blue temper were commonlv 
blazed , that is they were first dipped in oil or melted grease, - 
and then held over a fire till the oil became inflamed, and 
thus evaporated. 
Some particular articles require a nicety of temper 
that is not very easily attained by trusting merely to the 
change of colour, a circumstance that induced Mr. Hart- 
ley, in the year 1789, to take out a patent for a new and 
more accurate method. For this purpose a mercurial 
thermometer graduated as high as 600° is to be immers- 
ed in an iron trough heated by a furnace or lamp placed 
below it, and filled with fusible metal, upon the surface of 
which the steel is to be laid, which may thus be tempered 
with great accuracy at any degree of the thermometer that 
the artist chuses. Oil may be substituted to the fusible 
metal, and the effect will be the same, except that the steel 
being in this case tempered beneath the surface of the li- 
quid, and of course out of the contact of atmospherical air, 
