Bottle-making technology in 1800 was the same as it had been for more than 100 years. Bottles made circa 1800 were all handmade or "mouth blown" by a glass blower. He would use a blowpipe and a heat source to form bottles. With the help of an assistant to remove the bottles at the end of the process, the glass blower might work 12 hours a day. The demand for bottles was great during this time. Bottles were used for medicines, spirits, essential oils, mineral waters and other purposes. These bottles were sometimes refilled and used many times.
Search for a pontil mark on the bottom, typical of bottles circa 1800. This is the spot where the bottle was broken free from the blowpipe during manufacturing. It will look like a dimple or a bump and is likely to be irregularly shaped. These bottles can be asymmetrical, or unevenly shaped and there may be bubbles in the glass.
Inspect for embossing. Bottles from this time period sometimes had labels but not always. In the case of many of the patent medicines, the bottles came wrapped in the broadside, or pamphlet, that described the medicine.
Measure the size of the bottle, which can sometimes suggest what it was used for. Small bottles were used to hold medicines and essential oils. Larger bottles held mineral water or spirits such as whiskey. The oddly shaped bottles called torpedo bottles had to be laid on their sides since they did not have flat bottoms. This was to keep the cork stopper damp so it did not dry out and cause carbonation to escape.
Determine the thickness and colour of the glass. A bottle that was made of heavy glass indicated it was to be refilled many times. The glass may be clear, amber coloured, deep olive green or other colours.
Look for a lip or bottle opening that is oddly shaped. It may have a flange that sticks out or it may be rounded. Bottles from around 1800 never had screw-on lids, so any indication of this means the bottle is of later production.
Buy bottles cheaply at flea markets or online. Pricing for bottles circa 1800 varies greatly. Bottles that are scarce or unusual may be expensive. In the bottle collecting world, it's not uncommon for prices to rise dramatically due to demand. Always research the bottle's value before selling it, using trusted websites that deal in this type of antique. Such sites as eBay.com, Tias.com and RubyLane.com, as well as bottle-specific collector sites, provide up-to-date information.
Good condition usually means the bottle is worth more. Bypass bottles that are chipped or cracked.
Bubbles in glass do not necessarily mean the glass is old. Never take someone's word that a bottle is old. Do your own research.