Whether you are largely interested in the history of antique bottles, or you just want to sell them, it is essential to learn the time period from which they develop. Most of this information can be gathered via word of mouth or through history books. However, once a collector becomes more knowledgeable, he or she can decipher their dates of origin through various physical attributes. Some of these distinguishing features include: colors, lettering, and certain types of blemishes, whittle marks, and imprints on the bottom of bottles.
Perhaps one of the most common ways, though, of dating bottles is by looking at their mold seams. As glass bottles went from being free-blown to being mostly mechanically produced, mold seams have gradually extended up the sides of bottles. In his book Antique Trader Bottles Identification and Price Guide Sixth Edition, Michael Polak describes this very clearly, stating that 
“The closer to the top of the bottle the seam extends, the more recent the bottle. On the earliest bottles manufactured before 1860, the mold seams will end low on the neck or at the shoulder. On bottles made between 1860 and 1880, the mold seam stops right below the mouth and makes it easy to detect that the lip was separately formed. Around 1880, the closed mold began to be used. With the closed mold, the neck and lip were mechanically shaped. Then the bottle was severed from the blow-pipe and the ridge on the lip evened off by hand sanding or filing. The closed mold seams usually ends within one-quarter inch from the top of the bottle. After 1900, the seam extends all the way to the top” (Polak 48).
- Polak, Michael. Antique Trader Bottles Identification and Price Guide 6th Edition. Iola, WI: Krause Publications, 2008.
Bottle Bonanza 