When the right temperature is reached, the whole is withdrawn from the furnace; the die is lifted out of the box and plunged into the water immediately. There must be no delay at this point whatever; a few moments’ exposure of the hot die to the air will result in oxidation and scaling of the engraving.
In heating such a die a furnace should be used. It can be done in a smith’s fire, but it is a hazardous plan, and gives many chances for a failure.
A furnace with an even bed of incandescent coke is good, and such a furnace is very useful for many other purposes.
Where many dies are to be hardened, the handiest appliance is a little furnace with brick floor and lining, and heated by petroleum or gas, so arranged that the flames will not impinge upon the piece to be heated.
Such furnaces are now made to work so perfectly that illuminating-gas is found to be an economical fuel.
For quenching there should be plenty of water. For small dies that can be handled easily by one man a large tub or tank of water will answer if the operator will keep the die in rapid motion in the water.
Running water is the best. A handy plan is to have the inlet-pipe project vertically a short distance through the bottom of the tank, producing a strong upward current which will strike directly against the face of the submerged die.
Some prefer a downward stream; others a side stream; others, again, prefer a shower-bath; and, again, some use side jets.
A very efficient tank has a partition running from a few inches from the bottom to within a few inches of the surface of the water, and so placed as to separate, say, nine tenths of the tank from one tenth. In the smaller compartment there is an Archimedean screw driven at a speed of 200 to 300 revolutions; this drives the water under the partition and out over the top in a violent current. The steel is quenched in the larger space. Where water is an item of expense, this plan is economical, and it is certainly efficient.
An excellent way of quenching large faces, such as anvils, is to have a tank raised twelve to fifteen feet from the floor. In the bottom of the tank is a pipe with a valve, to be operated by a lever. The whole is enclosed in a sort of closet with a door in one side. When the piece is hot, it is placed immediately under the pipe, the door is closed, the valve is opened, and a great body of water is dashed down upon the face that is to be hardened.