If after
this the piece of Steel be held in some convenient heat, till by degrees
certain colours appear upon the surface of the brightned metal, the very
hard and brittle tone of the metal, by degrees relaxes and becomes much
more tough and soft; namely, the action of the heat does by degrees loosen
the parts of the Steel that were before streached or set _atilt_ as it
were, and stayed open by each other, whereby they become relaxed and set at
liberty, whence some of the more brittle interjacent parts are thrust out
and melted into a thin skin on the surface of the Steel, which from no
colour increases to a deep Purple, and so onward by these _gradations_ or
consecutions, _White, Yellow, Orange, Minium, Scarlet, Purple, Blew,
Watchet,_ &c. and the parts within are more conveniently, and
proportionately mixt; and so they gradually subside into a texture which is
much better proportion'd and closer joyn'd, whence that rigidnesse of parts
ceases, and the parts begin to acquire their former _ductilness_.
Now, that 'tis nothing but the vitrify'd metal that sticks upon the surface
of the colour'd body, is evident from this, that if by any means it be
scraped and rubb'd off, the metal underneath it is white and clear; and if
it be kept longer in the fire, so as to increase to a considerable
thickness, it may, by blows, be beaten off in flakes.
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