Although the stripped ingot may be at a glowing red heat as it comes from the mould, it cannot be rolled because the interior is yet liquid and the solid exterior would merely disrupt and allow the liquid interior to squirt out under the pressure of the rolls. The heat throughout the mass of the ingot must be nearly equalized in order that each part may be affected about alike under the rolls, although an ingot rolls best when hotter at the center than on the surfaces. To equalize external and internal heat, i. e, to stop the loss of heat from the exterior surface and allow it to become heated from the inside while, at the same time, the inside metal is solidifying, the ingot is placed in the soaking pit. The original soaking pit was merely a hole in dry ground lined with fire brick and provided with a thick fire-brick cover. The ingot radiated its heat to the walls of this pit and the non-conducting walls retained the heat until the ingot was of nearly equal temperature throughout.

Fig. 38. - Soaking Pit for Steel Ingots.

It frequently happened that ingots became too cold before they reached the pit, due to unavoidable delays, to be hot enough for rolling after they were "soaked." This condition necessitated some way of supplying external heat to the pits. The necessity of heating soaking-pits has gradually developed the soaking-pit shown in Fig. 38. This pit is heated by the regenerative system described with the open-hearth furnace.

As soon as ingots are stripped, they are hauled to the pits, weighed, lifted by a specially fitted crane and lowered into the pits. From

Fig. 39. - View over Tops of Soaking Pits.

6 to 8 ingots are placed in one pit, and they are kept in there, with the pit covered, for an hour or more, until they are at or above a bright red heat. No gas need be turned on if the ingot is stripped very hot, and mere "soaking" is all that is needed.

Fig. 39 shows a number of soaking pits. The crane is in the act of lowering an ingot into an open pit. The pit covers are slid horizontally by means of hydraulic cylinders, marked C, in the figure.