[Illustration: Figure 56.--Thermit Crucible Plug.
_A_, Hard burn magnesia stone;
_B_, Magnesia thimble;
_C_, Refractory sand;
_D_, Metal disc;
_E_, Asbestos washer;
_F_, Tapping pin]
_Thermit Metal._--The reaction takes place in a special crucible lined
with magnesia tar, which is baked at a red heat until the tar is driven off
and the magnesia left. This lining should last from twelve to fifteen
reactions. This magnesia lining ends at the bottom of the crucible in a
ring of magnesia stone and this ring carries a magnesia thimble through
which the molten steel passes on its way to the mould. It will usually be
necessary to renew this thimble after each reaction. This lower opening is
closed before filling the crucible with thermit by means of a small disc or
iron carrying a stem, which is called a tapping pin (Figure 56). This pin,
_F_, is placed in the thimble with the stem extending down through the
opening and exposing about two inches. The top of this pin is covered with
an asbestos, washer, _E_, then with another iron disc. _D_, and
finally with a layer of refractory sand. The crucible is tapped by knocking
the stem of the pin upwards with a spade or piece of flat iron about four
feet long.
The charge of thermit is added by placing a few handfuls over the
refractory sand and then pouring in the balance required. The amount of
thermit required is calculated from the wax used.
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