Ewymore gave the basic info for the time-period of the D. Evans Company, which was one of "top four" biggest manufacturers of military buttons during the civil war. The problem for button-diggers and collectors is that the company continued doing that for 80 years after the civil war's end. We relic-diggers have found 1902-to-World-War-Two US Army "Great Seal" buttons with the D. Evans backmark.
That company's very long history of manufacturing metal buttons is why I urge diggers and collectors to buy a copy of "the backmark book." Its title is American Military Button Makers And Dealers; Their Backmarks & Dates (by William F. McGuinn and Bruce S. Bazelon. It costs about $25, and you'll be glad you spent that money. Very importantly, this book contains many photos showing the various "evolutions" of the major button-making companies' backmarks. For example, when a Scovill backmark has the abbreviation "Co" (note the small letter o) instead of "CO", the button is post-civil-war era.