This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The shape given to castings should be very carefully considered. All changes of form should be gradual. Sharp corners or angles are a source of weakness This is attributed to the manner in which the crystals composing the iron arrange themselves in cooling. They place themselves at right angles to the surfaces forming the corner, so that between the two sets of crystals there is a diagonal line of weakness. All angles, therefore, both external and internal, should be rounded off. There should be no great or abrupt differences in the bulk of the adjacent parts of the same casting, or the smaller portions will cool and contract more quickly than the larger parts. When the different parts of the casting cool at different times, each acts upon the other. The parts which cool first resist the contraction of the others, while those which contract last compress the portions already cool. Thus the casting is under stress before it is called upon to bear any load. The amount of this stress cannot be calculated, and it is therefore a source of danger in using the casting. In some cases it is so great as to fracture the casting before it is loaded at all.
The internal stress, produced by unequal cooling in the different parts of a casting, sometimes causes it to break up spontaneously several days after it has been run. Castings should be covered up and allowed to cool as slowly as possible; they should remain in the sand until cool. If they are removed from the mould in a red-hot state, the metal is liable to injury from too rapid and irregular cooling. The unequal cooling and consequent injury, caused by great and sudden differences in the thickness of parts of a casting, are sometimes avoided by uncovering the thick parts so that they may cool more quickly, or by cooling them with water. It is generally thought that molten cast-iron expands slightly just at the moment when it becomes solid, which causes it to force itself tightly into all the corners of the mould, and take a sharp impression. This, however, has been disputed. Superior castings should never be run direct from the furnace. The iron should be remelted in a cupola. This is called "second melting;" it greatly improves the iron, and gives an opportunity fur mixing different descriptions which improve one another.
Castings required to be turned or bored, and found to be too hard, are softened by being heated for several hours in sand, or in a mixture of coal-dust and bone-ash, and then allowed to cool slowly.