In the 1870s, a written agreement containing a pledge not to join a union was commonly referred to as the "Infamous Document." This strengthens the belief that American employers in their resort to individual contracts were consciously following English precedents. This anti-union pledge was also called an "iron clad document," and from this time until the close of the 19th century "iron-clad" was the customary name for the non-union promise. Beginning with New York in 1887, sixteen states wrote on their statute books declarations making it a criminal act to force employees to agree not to join unions. The Congress of the United States incorporated in the Erdman Act of 1898 a provision relating to carriers engaged in interstate commerce.
During the last decade of the 19th century and the opening years of the 20th, the individual, anti-union promise declined in importance as an instrument in labor warfare. Its novelty had worn off; workers no longer felt themselves morally bound to live up to it and union organizers, of course, wholly disregarded it. In the early 20th century, the individual, anti-union promise was resorted to frequently in coal mining and in the metal trades. And it was not membership in a union that was usually prohibited, but participation in those essential activities without which membership is valueless.
In 1910, the International United Brotherhood of Leather Workers on Horse Goods, following an unsuccessful conference with the National Saddlery Manufacturers' Association, called a national strike in the saddlery industry for the 8-hour day. The strike proved a failure, and a large number of employers required verbal or written promises to abandon and remain out of the organization as a condition of re-employment.
