We know from the ledgers that there were sconce and looking glasses at
Marlborough. Archeological refuse supplies us with confirmation in
pieces of clear lead glass with slight surviving evidence of the tinfoil
and mercury with which the backs originally were coated. One piece (USNM
59.1693) has a beveled edge 7/8 inch wide, characteristic of plate-glass
wall mirrors of the colonial period. A curved groove on this piece,
along which the fracture occurred, is probable evidence of engraved
decoration.
Window glass is of two principal types. One has a pale-olive cast. A few
fragments of this type have finished edges, indicating that they are
from the perimeters of sheets of crown glass and that Mercer purchased
whole crown sheets and had them cut up. It may be assumed that this
greenish glass is the oldest, perhaps surviving from Mercer's early
period.
The other type is the more familiar aquamarine window glass still to be
found in 18th-century houses. A large corner of a rectangular pane has
the slightly bent contour of crown glass, which is the English type of
window glass made by blowing great bubbles of glass which were spun to
form huge discs. The discs sometimes were cut up into panes of stock
sizes and then shipped to America, or else were sent in whole sheets, to
be cut up by storekeepers here or to be sold directly to planters and
other users of window glass in quantity.
The centers of these sheets increased in thickness and bore large scars
where the massive pontil rods which had held the sheets during their
manipulation were broken off. The center portions also were cut into
panes, which were used in transom lights and windows where light was
needed but a view was not. Hence they served not only to utilize an
otherwise useless part of the crown-glass sheets, but also to impart a
decorative quality to the window. They are still known to us as
"bullseyes." A piece of a bullseye pane of aquamarine glass occurs in
the Marlborough finds. The pontil scar itself is missing, but the thick
curving section leaves little doubt as to its original appearance. A
similar fragment was found at Rosewell.