Dating antique bottles requires knowledge of the evolution of bottle technology and the ability to research manufacturers and bottling companies. Although glass bottles have been made for a few thousand years, it was not until the 19th century that bottle use became common, coinciding with the industrial revolution. By the mid-19th century, embossed lettering and marking on bottle bodies and bases, denoting manufacturers and products, made more precise dating possible. In addition to technology, products and manufacturers, certain types of glass colours will also aid in dating.
Examine the bottle's symmetry. Asymmetry is an indication of a handblown bottle.
Look for mould seams. The earliest bottles were handblown by a glassblower with a blowpipe and lack seams.
Is the bottle highly symmetrical, but lacking mould seams? This type of bottle was probably dip-moulded and dates after circa 1820.
Is the base indented with an irregular to round pontil scar? This, and no mould seams, is another indication of a handblown bottle. A pontil rod held the nearly molten bottle during the final stages of manufacture. The scar was left when the pontil was detached from the bottle.
Look for side mould seams. If the seams disappear in the neck, the bottle was probably "blown-in-mould" and dates circa 1820 to early 1900s.
Is a pontil mark present along with disappearing side mould seams? This bottle probably dates circa 1820 to 1860.
Do the mould seams disappear in the neck, but the bottle lacks a pontil mark? Blown-in-mould bottles without pontil marks date circa 1860s to 1910s.
Do the mould seams go all the way from the base to the lip? This is a machine-made bottle and dates after 1910.
Look for a "suction" scar on the base. This will be shallower, wider and more perfectly circular than a pontil mark. Bottles with suction scars were made in an Owens Automatic Bottle Machine and date after 1903. The Owens machine revolutionised the bottle industry.
Check for lack of bubbles and uniform glass thickness. This is another indication of a machine-made bottle.
Is embossed lettering present? Most bottles with embossed lettering date from the late 19th century and later. Lettering on the body will give clues to the contents and lettering on the base will give clues to the manufacturer. Both require more specific research to refine the bottle's date beyond observation of technology.
Is the lettering applied and in colour? These bottles date after 1940.
Is the bottle machine made and aqua- or light green in colour? Aqua glass typically dates from the late 19th century to the 1920s.
Does the bottle appear light purple in colour? Bottles with a purplish or amethyst hue are made from manganese dioxide glass and date circa 1880 to 1920. The glass turns purple when exposed to UV light.
Is the bottle colourless? Machine-made colourless bottles date after 1905.
Keep in mind that older technology often persisted and some bottles date later than you might think. For example, glassblowers still make handblown bottles today. Also keep in mind that bottles could have been used and refilled multiple times. Recycling was very common until the mid-20th century. The lettering "Federal Law Forbids Sale or Reuse of This Bottle" indicates a liquor bottle made from 1935 to the 1960s.
Do not estimate a bottle's age by its shape. Many identical shapes persisted through time and others were brought back for nostalgia and made common again.