Absolute dating numerical age method
Both seams are quite diagnostic of machine manufacture and are usually visible, though the seam at the top of the finish can be hard to see on some bottles - especially if the finish was fire polished.In the glassmaking trade, these seams along with the side mold seams within the finish or just below are referred to as "neck ring" or "neckring" seams since they were formed by the separate neck ring portion of a machine mold (Tooley 1953).
Added evidence to this theory is that an identical shape and size (2 oz.) "Round Shoe Polish" bottle is shown in the "Machine Made Ware" section of Cumberland's 1911 catalog (Cumberland Glass 1911). Very few narrow neck bottles made on the Owens machines will pre-date that time also. 2007d].) It is thought that probably all pre-1905 semi-automatic bottle machine production in the U. was relegated to wide-mouth bottles/jars due to limitations of the press-and-blow machines at that time (Toulouse 1967; Miller & Sullivan 1981; Jones & Sullivan 1989; Cable 1999; Miller & Mc Nichol 2002; Lockhart pers. This image (click to enlarge) also shows that on many early press-and-blow machines the parison mold was one-piece (note absence of mold hinges) as the narrow non-inflated parison could be removed from either the base or finish mold end (depending on the type machine) whereas the blow mold had to be two-piece (hinges obvious) to remove the expanded and finished bottle if there was any narrowing from the body to the neck/finish (like a typical milk bottle would have).
It was noted in 1910 that the Cumberland Glass Company (Bridgeton, NJ) had "...succeeded in perfecting a machine that will satisfactorily produce narrow neck bottles, such as catsups, beer bottles, etc., at a big saving over the hand method." The method used was unusual and may have been unique in bottle-making history: "The machine differs from all others, and in getting the neck upon the bottle, the vessel is made in two sections, the neck being put upon the bowl with a second operation. Manager says machines are fast coming into play in bottle industry, plans eventually to have machines in place of "carrying in boys." Location: Clarksburg, West Virginia" (Library of Congress).
This is accomplished so that there is no perceptible mark upon the bottle showing the joint, and the bottle stands every possible test as to strength. Although products of this machine are not conclusively known a bottle such as the one at this link - offset seams shoe polish bottle - may well be a product of the described machine as there is a distinct and abrupt interface edge at the shoulder where the mold seams for both the body and neck end and are offset. This two table semi-automatic machine would have been hand fed with glass (furnace likely to the right) and does have the two different mold sets with the parison molds (where the first "press" part of the cycle took place) the set on the right.
(Note: the term "parison mold" and "blank mold" are synonymous for the first mold in the two mold machine process.): (see the Note box below point #3 for an exception) run up to the highest point of the finish and often onto the extreme top finish surface (i.e., onto the rim or lip).
On many early (very early 1900s into the 1920s) and occasional later (1930s and later) machine-made bottles the vertical body/neck and finish mold seams are discontinuous and offset from each other; click offset seams for a picture of this attribute.
(Note: It is likely that other types of suction based automatic bottle machines made in Europe in the 1920s - and possibly later - also produced a suction scar on the base of their products [Pearson 1928]. The presence of a circular valve mark on the base of a bottle (typically a wide mouth bottle or jar) is sure evidence of machine-made manufacture by a press-and-blow machine. This is especially true of later machine made bottles, i.e. (Note: The presence or absence of bubbles in the glass and relatively even distribution of the glass throughout the characteristic is not a primary feature of either machine-made or mouth-blown bottles, though there are strong trends.