BOTTLE MOLDS AND SEAM DATING:
At the height of the industrial revolution, glass manufacturers quickly adapted mass production techniques -- instead of molding or shaping the whole bottle by hand in the open air, as was originally done. It was found that by blowing the molten glass into prepackaged molds greatly simplified the process.
However, the technology to mass-produce the whole bottle did not appear at once. It came in small steps. At first molds could only be used to create the lower parts of the bottle. The neck and lid would then have to be done (just as in the old days) by hand. But as time when on and the molding technology would increase and allow for this.
The figures, (below) give a timeline for these changes. Antique collectors can also use these seams as a dating technique for their bottles.
At one time almost all bottles were completely hand blown (in the open air) without the aid of molds of any kind. Such work required skilled craftsmen and a great deal of manufacturing time. Small wonder then that in many cases the bottle was more than half the cost of the product.
Fortunately by the 1860's, or around the time of the American Civil War, glass technology had progressed enough to allow manufactures to make extensive use of molds. This greatly simplified the process of bottle making, as well as the amount of time it took to manufacture each one. Instead of making each part by hand, the mold allowed a worker to simply insert a hollow pipe or reed (with a glob of molten glass) into the mold and literally inject the glass against the mold.
However, mold technology did not spring up all at once -- it evolved over a period of time. At first, only the lower parts of the bottle could be injection molded, the rest still had to be hand made or blown. However, as time went on, more and more of the bottle was injection-molded and less and less hand blown. With each increment the seam kept going up and up.
I have found the reliability of this method of dating to be almost absolute. While it is always possible that some manufactures could have kept and used some of their old molds well past their technical life, for the most part, they were jumping over themselves, trying to obtain the latest and most economical means of production.
This miniature toy mold (made of clay instead of iron), shows why/how the bottle seams (running down on either side of the bottle) come into being.
No matter how snug or tight the fit between the two sections (and in some cases three) of the mold, there's always a rough edge where they make contact--thus creating a seam.
It should be remembered that glass manufactures were always interested in, and kept an eye out for, new ways to increase productivity. Due to the inexpensiveness of molds and the equipment they required, as soon as a new technique was created it was quickly copied by the rest of the industry. It was a simple matter of staying economically competitive.