The bases of dating
The bottom of a bottle is usually the thickest part, retaining more temperature throughout the production line.
Because the bottom is hotter, it is also more fluid and has a tendency to sag, forming a shape like a spinning top which makes it unstable on flat surfaces.
These are usually some type of embossed letters, symbols, and/or numbers which were molded onto the base of the bottle for a variety of glassmaker determined reasons.
Click Owens-Illinois bottle base to view a picture of the relatively elaborate bottle base mold markings used by the Owens-Illinois Glass Company during the mid-20th century.
Giving a bottle an arched shape at the bottom means that if it does sag, it can do so without touching the bottom.
(British Glass 2004) In conjunction with the finish (lip), the various attributes and features found on the base of a bottle allows for some of the better opportunities for the manufacturing based dating of a bottle.
This page is divided into two sections based on the primary methods by which bottles were manufactured - mouth-blown (hand-made) or machine-made (both semi and fully automatic) - since base features on these types of bottles generally differ significantly.This is because there is little dating information to be gleaned from simply the base profile, though the profile can assist at times with the typing of a bottle.Simply put, this is the shape of the bottle base when viewed straight on and is usually the cross-section shape of the bottle body, though there are exceptions to the latter.The base of a bottle typically only has one primary function (besides helping hold the bottle together) and that is to provide a flat surface for the bottle to stand upright.
Of course there are exceptions to that "rule", most notable being the rounded base mineral & soda water bottles whose purpose was to make sure the bottle did Bottle bottoms aren't flat because they need an arched structure to allow them to be stable on a flat surface.For additional information, click Dating Page to go to the pertinent section of that describes the differences between these bottles.