You may never know. The color of the metal itself can vary from
glasshouse to glasshouse. Have you found fragments of the bases of
case/case/gin bottles on that site? You could compare the color of the
metal with the thinnest area of the base piece.  How about the quantity
of pieces found? Were there windows in the structure? A determination
may not always be easy. That is why the word "possible" can be used in
historical archaeology.

maryjaneshaw