Recommended reading in the Antique Web:

Understanding Early Glass
by Wayne Mattox

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.   CONT'D