Dating bottles by their tops and bases
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.
For more information on the subject of glassmaker markings see the Makers Marks page, which is a sub-page linked to the Glassmaking & Glassmakers page.
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.
However, the rectangular and square bottles also have distinctly beveled edges and the octagonal bottle is actual round right at the heel of the bottle - all of which complicate describing the base profiles a bit.This potential for datable features is very useful since bottles are more often than not lacking embossing - an attribute which can often enhance dating opportunities.Also, shape is extremely variable and usually only loosely connected with age and function.This commonly encountered symbol (at least in bottles found on Western historic sites) was intended either as a cryptic form of makers mark or simply an artistic flare of a specific mold cutter or engraver at the SF&PGW (or located in the Bay Area).Although the marking cannot be irrefutably attributed to SF&PGW, it presence is still a datable feature as empirical evidence indicate that bottles with this marking (primarily liquor and food bottles) date between the mid-1870s and about 1890.