Roberta--I would check Noel Hume first--he explains it clearly, but there are
plenty of other sources (Lyle Stone's Fort Michilimackinac report, for
instance). Roenke and Chance and Chance have developed regression formula
based dating methods (like pipe stem dating) which appear to be useful on 19th
century glass, but the authors argue that they are only sure that they work in
their particular study area. Carl Steen