One of the most pervasive and longest running myths in the world of bottle dating is that the side mold seam can be read like a thermometer to determine the age of a bottle.(See image #1, which is an illustration pointing out the major “parts” of a bottle, including the side mold seam. Illustration from my Historic Bottle Website – a purely educational website.)
The concept is that the higher the side mold seam on the bottle (i.e., the closer to the lip) the later it was made – at least in the era from the early to mid 19th century until the first few decades of the 20th century. This dating tool was first devised by Grace Kendrick in her 1963 book “The Antique Bottle Collector.” This book was pioneering and reprinted many times into the 1970s and is probably the most common and widely quoted bottle book ever written, containing a wealth of generally good information.
This concept was articulated by Kendrick’s in a chapter entitled the “The Applied Lip” which contains an “Age Gauge: Mold Seams of Bottles” chart (Figure 9). Kendrick’s explains in the text (pages 45-47) that:
It is true that the mold seams can be used like a thermometer to determine the approximate age of a bottle. The closer to the top of the bottle the seams extend, the more recent was the production of the bottle.
The chart accompanying this statement notes that bottles made before 1860 have a side mold seam ending on the shoulder or low on the neck, between 1860 and 1880 the seam ends just below the finish (the glassmaker term for a bottle lip), between 1880 and 1900 the seam ends within the finish just below the finish rim (top lip surface), and those made after 1900 have mold seams ending right at the top surface of the finish, i.e., rim (Kendrick 1963).
Dating bottles is complicated
There are examples of bottles having mold seams that fit these date ranges properly. For instance the newest of bottles – those that were machine-made – do have seams ending right at the top (or on top of) the lip or finish. However, the issue of dating bottles is much more complicated than the simple reading of side mold seams. If it were indeed that simple a large chunk of my Historic Bottle Website would be unnecessary!
For example, the mouth-blown process that produces a “tooled” finish frequently erases traces of the side mold seam an inch or more below the base of the finish whereas the typical – and older – “applied” finish has the seam ending higher – right at the base of the finish (Lockhart et. al. 2005e). See image #2 which shows the side mold seam on a “malt tonic” bottle (entire bottle shown in image #3) dating from 1906 to 1916 based on information from business directories, other references, and additional manufacturing related features present on the bottle (the subject of future articles). As a side note, this bottle also has a crown cap accepting lip which was not even invented until 1892. Using the dating “thermometer,” this bottle would presumably date from the 1860 to 1880 period.
The reason I address this issue is that the concept keeps popping up in the literature of bottle dating and identification, ranging from Sellari’s books (Sellari 1970:5) published shortly after Kendrick’s book to as recent as Fike (1998:4) and Heetderk (2002:15). It is also frequently noted by sellers on websites such as eBay® when describing their offerings. For a broader discussion of this subject see the Bottles and Extras magazine article, which I co-authored, entitled Debunking the Myth of the Side Seam Thermometer (Lockhart et al. 2005e). This article is available on the Historic Bottle Website at this link: http://www.sha.org/bottle/pdffiles/Thermometer_BLockhart.pdf
The complicated issue of mold seams and dating is explored in various portions of my Historic Bottle Website (www.sha.org/bottle/index.htm), though in particular on the Bottle Body Characteristics & Mold Seams page (www.sha.org/bottle/moldseams.htm) and the Bottle Bases page (www.sha.org/bottle/bases.htm).
(For more information on the subject of bottle dating and typology – and the terminology used in the above descriptions – please consult my Historic Bottle Website at www.sha.org/bottle/index.htm. The references used in this article can be found listed on my website’s References page at: www.sha.org/bottle/References.htm)