Also in 1875 Carpenter was defense counsel for officials of the Miners’ National Association (MNA) in an important labor case that sought to apply the legal concept of conspiracy to union picketing and organization efforts. The case grew out of a coal miners' strike in Clearfield County, Pennsylvania which was supported by the MNA, then numerically one of the largest American trade unions. First over fifty of the striking workers were tried upon a charge of criminal conspiracy and thirty-six of them were convicted with some sentenced to jail, although they appeared to have been guilty of no more than peaceful picketing. Then John Siney, the president of the MNA, and Xingo Parks, one of the union’s best organizers, were tried on the same charge. Siney was acquitted, but Parks convicted, although pardoned shortly thereafter.