To find the age of an old bottle, observe details such as embossed lettering, vertical seams, shape and symmetry. Use these observations when consulting a reference guide, such as the one at Sha.org.
First, determine whether the bottle is mouth-blown or machine-made. A machine-made bottle typically has a vertical side-mold seam along the full length of the bottle. A mouth-blown bottle, also called a handmade bottle, often has no side seam, is slightly asymmetrical and has concentric rings produced by a turn mold. Manufacturers produced machine-made bottles mostly after 1910.
To find a more precise age estimate of a machine-made bottle, look for other characteristics such as a suction scar on the base, a narrow neck or a ghost seam created by an Owens blow-and-blow machine. Manufacturers produced most narrow-neck bottles with a suction scar after 1910 and most wide mouth bottles before 1900.
Some details to look for on a handmade bottle are embossed information about its contents and the mark of the bottle maker. Some other indications are air venting marks and base seams. Bottle makers rarely placed embossed information on bottles before 1910. The mark of the bottle maker, usually indicated by a name or initials, can lead to narrowing the bottle's age to within a few years.