Understanding how it was made; molten glass was blown, as one would blow a balloon, then shaped with tools or molds by a "gaffer," is the first step toward identifying valuable antique glass in your own back yard.  And, it does show up, frequently.  One of my mom's friends, Shelly, bought a miniature "blown" glass bottle with an "applied" handle at a local tag sale last spring.  Despite a small "heat-check" crack, a crack that happened in making when the red hot gooey handle was applied to the cooler bottle, the insignificant appearing two-inch tall amber demijohn was sold for four-hundred dollars-fair profit on a nickel investment!  Shelly 's expertise is toys, not glass.  She encounters hundreds of inexpensively priced glass pieces every week when she's making her antique hunting rounds..  She took a shot on the bottle because it was a low risk gamble, and it looked interesting.  Here's a few tips that may rouse your interest if you happen to cross paths with an old piece of glass.

Inspect the base for a "potil mark."  Gaffers held the red-hot glass gobs they fashioned with an iron "pontil rod" which was snapped off when the article was finished.  This hand-crafted glass making technique leaves a trail for antique detectives; a round jagged scar in the middle of the base.  Sometimes, especially in finer pieces made after 1790, this pontil scar was removed by polishing, resulting in a smooth bowl-like indentation called a "polished pontil."  A pontil mark identifies "blown" glass, and opens the door to the possibility of age and value.

Inspect the base for wear.  Remember the "U" rule of thumb in the "ABC's of Antique Collecting:  Unless wear is where it belongs, it does not belong."  Old glass objects will have tiny flat spots on those small areas of the base where the object would come into contact with a table, etc..  Note, that an uneven gaffer-fashioned base seldom presents a flat surface, and that a base which has been "worn" in non-contact areas is quite probably a fake.

• Examine "molded" glass.  Figural flasks, decanters, embossed bottles, decorative "lacy" glass, pattern glass, and many other old glassware items were shaped in molds.  Unfortunately, fakes and reproductions of early molded glass far exceeds original items today.  Authentic pieces are weighty and sharply-edged compared to contemporary examples. Identifying irregularities like "spill-over," where too much glass was poured into the mold, and "annealing lines," hair-like inconsistencies on the surface, can also be of aid in ferreting-out old glass.
• Look for a few bubbles in the glass.  Air bubbles were evidence of shoddy quality to older day glassmakers, nevertheless, some did appear.  Bubbly glass, should purchased by only those intent on building a collection of Mexican souvenirs.
• Listen for a "ping." When tapped, early "flint" glass emits a bell-like tone. Not all old glass rings, however. "Lime" or "soda" glass and closed-neck pieces like bottles and decanters seldom "ping."
• "Extrinsic decoration;" decoration introduced to glass after it was fashioned and allowed to cool, like cutting, engraving, etching, enameling, and gilding, can add considerable value to glass. This is especially true if the work is well-executed and gives an indication as to its history.  A recently discovered bottle with "Amelung" style engraving is expected to bring in excess of $20,000 at a Rhode Island auction this month.

Early glass in color can be quite valuable. Authentic glass can often be recognized by the vibrancy of its color. Recognition can only be learned by experience.  Visit a museum, like the Corning Glass Museum, in Corning NY, to study old glass.  It 's a beautiful window into yesterday.

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