Chains made of iron links were known to the first smiths or workers in iron. And though chain is now replaced in many instances by lighter and stronger cables, it is one of the staple iron and steel products. One writer1 estimates that in 1905 there were in the United States thirty chain mills, having a total yearly output of 50,000 tons of chain.

1 "The Manufacture of Chain," L. B. Powell, Iron Age, Jan. 5, 1905.

The early chain makers often welded their links at the end, and even made circular links. But modern practice is to make the link an oval and to weld the stock on one side of the link. The stock used varies in diameter from approximately 1/8 inch to over 3 inches, and the links are from 1/2 inch to a foot or more long. Safe practice limits the diameter of the stock from approximately 3/8 inch to 2 inches, as sizes smaller are apt to burn when heated and sizes larger are apt to weld poorly and are unworkable.

Chain is still largely made by hand, and requires skill and care for every link. Speed and a uniform product, have been developed by special machines in late years. The links are now cut from a spring-spiral of the iron, are heated in a gas oven, are welded and swaged by a hydraulic press with die of suitable shape. Small chain can now be made on an automatic electric welder.

As with welded pipe, the chain iron should not be of high tensile strength, about 50,000 pounds per square inch, and should be quite pure. The ultimate strength of a chain depends on its design and the perfection of the weld. And as life and property are constantly at the mercy of just one poor weld, the different nations and insurance companies prescribe tests that are approximately double what the chain will be allowed to undergo in practice. In this country the U. S. Testing Board and in Great Britain the British Admiralty Board and Lloyd's restrict the load to 50 per cent, of the ultimate strength of the weakest link.