Lynn -- sounds  like a South type 18  -- a flat cu alloy disc with a stamped backmark. I can't say I have seen "Imperial London" but there are numerous similar marks. Recent examples I have seen are
marked "Best Orange London" "Gilt" "Treble Gilt"  and various makers names. These would not be iunexpected in a site dating to the period you described.

This is by no means an exhaustive list of handy references:

South, Stanley
1964    Analysis of the Buttons from Brunswick Town and Fort Fisher             Florida Anthropologist          XVII    2       113     133

see also:

Hinks, Stephen
1988    A Structural and Functional Analysis of Eighteenth Century Buttons      MA Thesis, Anthropology, College of William and Mary    Williamsburg, Va.

best of luck, Carl Steen



3/4/2002 1:47:11 PM, Lynn Evans <[log in to unmask]> wrote:

>We found a plain brass (copper alloy) button at a c.1790-1840
>domestic/farm/mill site.  The backmark is LONDON*IMPERIAL or
>IMPERIAL*LONDON.  I have not found it in any of our (mostly military)
>sources.  Is anyone familiar with this mark, or can you suggest good
>sources for non-military buttons?  Thanks in advance.  Lynn
>
>
>
>Lynn L.M. Evans, Ph.D.
>Curator of Archaeology
>Mackinac State Historic Parks
>P.O. Box 873
>Mackinaw City, MI 49701
>231-436-4100
>[log in to unmask]
>
Carl Steen
Archaeologist
The Diachronic Research Foundation
PO Box 50394
Columbia, SC 29250
Web Site: http://diachronicresearch.com