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Most machine-made bottles have mold seams about the thickness of a hair while most visible mouth-blown mold seams tend to be several times as thick, higher, but more rounded.
(Mold seam thickness and how high it protrudes [height] is of only moderate use in telling a machine-made bottle from a mouth-blown bottle, though if a bottle fragment has a hair fine mold seam, it is highly likely to be from a machine-made bottle.) 3.
The statement about machine-made bottles may seem contradictory (finer but more visually distinct) but is a function of the higher machine blowing pressure.
Earlier machine-made bottles (1905-1920s) tend to have somewhat thicker/higher mold seams than later machine-made bottles due to the increasing precision in mold machining and machinery in general as time progressed.
The ghost seams are caused by the parison mold parts and if visible enough will be "attached" to the vertical seams in the finish.
Click on ghost seam to view a close-up explanatory picture of this attribute.
Feature #7 describes a couple glass related features that are quite consistent in machine-made bottles, but not diagnostic, i.e., mouth-blown bottles may sometimes have few/no bubbles in the glass and even thickness."Ghost" seams are usually present on the neck, shoulder, and/or body of the bottle if made by a blow-and-blow machine (like the Owens Automatic Bottle Machine).These are faint, somewhat wandering, hairline seams which if present (usually) are sporadically visible on the sides of machine-made bottles.The machine is operated much as all pressing machines are..." (National Glass Budget 1910; Lockhart pers. This little bottle has a moderately narrow neck and a distinct valve or ejection mark on the base indicating press-and-blow machine manufacture. The 1908 photo below is from the Lewis Hine collection (Library of Congress) and shows an early, probably O'Neill (Barrett 2011) semi-automatic press-and-blow 4 mold milk bottle (which have relatively wide mouths) machine which came with the following caption: "Machine that blows 4 milk bottles at a time. Blowing air would have been supplied by the hose visible at the top of the set of blow molds to the left, where the final "blow" part of the cycle took place.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)."gob feeders") though a similar type mark without the feathering is induced by the parison/blank mold of most other blow-and-blow machines - including up to the present day.Press-and-blow machines usually have a round valve mark on the base but lack either the suction or parison scars.(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.There are at least two additional finish related mold seams - one at the top of the finish which encircles either the bore or sometimes the outside of the upper lip portion of the finish (sometimes of these seams are present) and a horizontal seam immediately below the finish which circles the extreme upper neck (called a "neck ring parting line").Click on the picture to the left to view an illustration which shows both of these seams or click machine-made finish to view an image which shows well the seam below the finish.