Next to this came a change forced by the introduction of the basic open-hearth process for the manufacture of steel. In this all silica introduced into the open-hearth furnace has to be slagged with lime of something like twice its own amount, which it is very likely to sieze from the bottom of the furnace, and so cause rapid wear of the latter, as well as heavy expense in the consumption of lime and fuel to melt the slag formed.

For this process, therefore, some substitute for sand casting was found to be almost a necessity; this was found in the use of chill molds. These are virtually pig beds of cast iron which replace the ordinary sand beds and are fed from the central sand runner, the same as the original sand bed. These chills are not a modern invention; I have seen one which was made, and presumably used, at a charcoal furnace before the Civil War, but real need for sandless pig iron did not develop until the early nineties, and until then chills never came into general use because they are somewhat troublesome until the details of handling them are learned.

Certain irons, particularly hot irons, low in silicon, and made on a very limy furnace, have the power of attacking and melting away the surface of cold cast iron and so "burning" to it. Obviously if this action took place to any extent the pigs would burn fast in the chills, and it would be impossible to remove them. To prevent this the chills are washed with a solution of clay water soon after the removal of the cast from them, their heat then dries the water and leaves a coating of clay all over the molds. This is virtually infusible and serves to protect the chill from the action of the iron unless the latter is of an unusually cutting nature, when it sometimes burns fast to the chill in spite of the" clay. Sometimes lime is used in place of clay, but clay is more easily held in suspension in water than lime, and is, therefore, more easy to apply uniformly.

These chills are exposed to enormous stresses. The whole bed of iron is obviously the same size as that of the chill at the instant of solidification, but as the iron cools off the chill absorbs its heat so that one gets hotter and the other colder. In order to get the cast out quickly it is customary to water it down with hose nozzles as soon as the iron has solidified, and this of course throws still greater stresses upon the chills, but if the molds in the chill which form the pigs are of the proper shape and properly reinforced, they stand up very well under this treatment. It is necessary to have a considerable thickness of metal below the bottom of the pig, six inches is a minimum and eight is considered better. The bars between the pigs are known as the "cores," and including these the average thickness of the chill is from nine to twelve inches. The width of the chill in the direction of the length of the pig is over four feet and its length may be anything dictated by the size of the cast house and local conditions, but beds eighteen or twenty feet long are not uncommon.

A brief calculation will show that these must be very heavy and in fact they weigh many tons. In some cases the beds are cast in two or three pieces, as this reduces the contraction strains and increases their durability; it also makes the individual pieces much easier to handle, although it is more troublesome to keep them in line with one another when made this way, unless longitudinal bolts are provided for holding them together, this is sometimes done and is good practice.