Other diagnostic tools must be used to date these items.Shape is more indicative of function - i.e., what the bottle was used for or contained - but even that has a myriad of exceptions.
Some bottle shapes are indicative of a particular manufacturing era, though many bottle styles/shapes were used for so many years - like the cylinder whiskey "fifth" or square snuff bottle - that the shape itself is not indicative of age.In short, there was (and is) nothing to stop a glassmaker from using an obsolete method in the production of a bottle.3.Some technological changes were expensive and not adopted by glass makers until it became an "adapt or perish" issue and many glass factories just perished.Utilitarian items makes up the bulk of the bottles produced during the 19th century and first half of the 20th century. Bottles intended to be used once to dispense the contained product without much hope of return, though as noted in #4 above, many types of bottles were commonly reused during the 19th and early 20th centuries; or 2.Heavy duty bottles intended to be recycled and reused by the producer or distributor of the contained product (primarily soda, beer & milk bottles); or 3.)Reuse, of course, does not change the manufacturing date of the bottle itself, but care must be exercised when using the known date of one or a few bottles to date other items found from the same context.When a likely or known "older" item is found in a known "newer" site it is referred to as deposition lag.Acceptance often occurred over a period of many years or decades in some cases.This technology lag makes some diagnostic characteristics better than others for dating. As a corollary to #1, consider the following quote: "Treat terminal dates with care.A substantial amount of bottle type specific information must be reviewed by a user to increase the probability of dating accuracy.Additional reference materials outside of this website must often be consulted to narrow down the date of any item as far as is possible and to really get a "feel" for the history of the bottle in question.