Below comes an excerpt from Practical Blacksmithing Vol. 4 by Milton Richardson ©1889
* * * The hammer is generally known as a rude instrument, but as a matter of fact it is in some of its uses a very refined one, requiring great care and skill in its use. * * *
Time forbids that I should refer to more than a few prominent forms of hammers. The carpenter's mallet has a large rectangular head, because, as his tools are held in wooden handles, he must not use a hard substance to drive them with, or he will split the handles. Wood being light, he must have a large head to the mallet in order to give it weight enough.
The author than goes on to explain a stone mason's hammer, a machinist's hammer.
USES OF THE HAMMER.
In whatever form we find the hammer, it is used for three purposes only, namely, to crush, to drive and to stretch. And the most interesting of these operations are stretching and driving. The goldbeater, the blacksmith, the sawmaker, the plate straightener and the machinist, as well as many others, employ the hammer to stretch ; while the carpenter, the machinist, and others too numerous to mention, use the hammer to drive. Among the stretching operations there are many quite interesting ones. Here in Fig. 3, for example, is a piece of iron, two inches wide, and an inch thick, bent to the shape of the letter u. This piece of wire is, you observe, too short to fit between the jaws, and I will now bend the piece and close the jaws by simply hammering the outside of the curved end with a tack hammer. The proof that the blows have bent the piece is evident, because the piece of wire now fits tightly instead of being loose, as before the hammering. The principle involved in this operation is that the blows have stretched the outer surface, or outside curve, making it longer and forcing the jaws together. If we perform a similar operation upon a straight piece" of metal, the side receiving the blows will actually rise up, becoming convex and making the other side concave, giving us the seeming anomaly of the metal moving in the opposite direction to that in which the blows tend to force it. This process is termed pening, because, usually, the pene of the hammer is used to perform it. It is sometimes resorted to in order to straighten the frame-work of machines, and even to refit work that has worn loose.