Identifying the age of a bottle is not always as easy as it may first seem. While bluish and dark-tinted glass may indicate a piece older than a clear glass bottle, the many reproductions that exist today rule this out as a conclusive way to date a bottle. Instead, the bottle must be examined for signs of how it was produced and markings that indicate when and where it was made. A true vintage bottle will have several telltale indicators of its age.
Examine the bottle for a mold seam leftover from the forming of the bottle. If the seam runs to the top of the lip, this is indicative of a machine-made bottle from 1910 to the present. A seam that runs to the top of the neck but stops short of the lip is a feature of bottles produced between 1880 and 1910. If the seam extends only part way up the neck, the bottle was most likely formed between the late 1700s and the early 1800s. No seam indicates a handblown glass bottle that predates the use of molds in bottle making. This is usually a clear identifier of an antique bottle.
Check the bottom of the bottle for what is known as a pontil mark. This will be a ring or area of rough glass from where a supporting rod held the bottle during glass blowing and definitively dates the glass as being vintage.
Compare marks and numberings found on the bottom of the bottle to those in a resource such as "Bottle Makers and Their Marks" by Toulouse. There are several common marks that indicate older, vintage glass. The Owens bottle ring, an embossed circle with date and mold numbers, indicates an automated molding process employed between 1910 and 1960. "W.T. Co." represents Whitall Tatum, a maker of glass from around the turn of the century.
Look for marks that may indicate a newer or reproduction bottle. These bottles may look vintage in appearance, though they are actually skilled remakes designed to appear to be older bottles. "Wheaton" stamped on the base represents Wheaton Glassworks, which produced many reproductions throughout the 20th century. Likewise, "CB" for Clevenger Brothers is another maker of reproduction glassware. The stamping "Federal Law Prohibits Sale or Reuse of this Bottle" indicates a post-Prohibition bottle from 1930 or later.
Examine the mouth and lip of the bottle, as this is a good indicator of age. Bottles with screw tops were made no earlier than 1910, while cork-topped bottles ceased production largely at the beginning of the 20th century. Handblown bottles will have a lip that has been applied as a secondary process and not formed in the original blowing of the bottle. Molded bottles have the lip molded as part of the entire bottle. An applied lip usually means a bottle made before 1890, as this is around the time that mass-production of handblown bottles ceased.
If at all unsure of the age of your bottle, consult an expert, as many high-quality reproductions exist on the market. These reproductions can easily fool the novice glass collector.