How do I date an old glass bottle by the seams and bottom?

The mold seams are one of the best clues. On many bottles made before about 1900, the side seams stop short of the lip, because the top was finished by hand. Once automatic bottle machines spread in the early 1900s, seams usually run all the way up through the top of the lip. Bottles that are older still may have no side seams at all if they were free-blown.

Check the bottom. A rough or round scar called a pontil mark generally points to an earlier bottle. Many later bottles have maker's marks, plant numbers or date codes embossed on the base, and bottle reference guides list what those marks mean.

Other clues include the color of the glass, bubbles or a wavy surface, and the shape of the top. Clear glass that has turned light purple in sunlight often contains manganese, which was common in a certain period. Combine several clues rather than relying on one, and a bottle club or reference guide can help confirm the date.