Courtesy of Hal Gelfius
ACGA Ethics & Authenticity Committee Chairman

FluorescenceWhen exposed to a 15-watt black fluorescent light bulb in a dark room, the piece should reflect a lime-green color.

Signature:  If the piece is signed, the signature should be sharp, clear and distinct — without smudges or inconsistencies.

Blank:  Take the piece to a window with good daylight and examine clear, uncut sections of the glass.  The blank should:
— be perfectly clear
— have a shape that was known to be used by the maker
— have consistent thickness
— not have thin spots or a high spot in the center of the blank

Weight:  Cut glass is typically heavy for its size and thickness due to a high content of lead.  A piece that is lightweight should be carefully evaluated as possibly counterfeit or foreign cut glass.

Wear Marks:  Most old pieces have been used and should show random scratches on the bottom and the inside of the piece, or perhaps even small nicks and abrasions.  A piece with no wear marks is rather suspect of not being authentic.

Diamond Wheel Cutting:  Diamond-wheel cutting is often used on contemporary cut glass and leaves small grooves in the glass which can be detected if you look closely, especially when the piece is acid polished rather than wood polished.  Errors in the cutting of the pattern can also expose the piece as not authentic.