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.
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Pontiled base fragments could also be from later produced "specialty" bottles which are described below.5.
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.
We can always have some indication of a starting date for a technique if we can find who first put the idea into practice.
But any technique, once developed, can be used right up to the present - as many collectors know who have been so unfortunate as to rely too heavily on a popular termination date as sure evidence of true antiquity..." (Toulouse 1969b).
See the Bottle Typing/Diagnostic Shapes page for more information on the subject..