Here are some tidbits on collecting antique bottles.
Foremost the rarer the bottle the more it's worth
Bottles value are based on:
the glass color: green, brown, white, and clear
the geographical location: western, northern (were was it manufactured)
the rarity: how many of them are available
the crudity: glass uneveness shows individual characteristics of being handmade..
Rarity of a bottle is rated at 7 levels (based on my research).
Common: well known about 500 available
Scarce: don't see them all the time, about 100 or more available
Very scarce: difficult to find about 50 available
Rare: about 21 to 50 available
Very rare: 20 to 25 available
Extremely rare: 5 to 10 available/exists
Unique: only one known.
Be careful when cleaning antiques. I have damaged several pieces trying to clean them, some have even shattered and crazed by just putting them in water. Cleaning bottles can decrease it value. Cleaning can remove more than dirt and stains. Some of the characteristics of the bottle can be altered.
Some identifying characteristics of antique bottles
Bottles made 1805 and earlier have tops that were separately applied, after the body was made. They have a small drop of molten glass descending from the top. Bottles made after 1885 have tops that are part of the original bottle. The entire bottle was molded and when the bottle came out of the mold a turn tool was used to form the rim of the top.
Pontil, two types
Open pontil, a circular shard of glass on the base of the bottle. This is made when the glass is held by a pontil rod which is stuck in the base of the glass when it is taken out of the mold. The glass makers snaps the pontil rod from the bottom of the glass leaving a mark.
Graphite pontil, also know as an iron or improved pontil. The glassmaker uses a bare rod that leaves a grayish black mark on the base of the bottle.
Types of bottles
There are many types of bottles. You have whiskey, soda, beer, milk, and medicine. Those are not the only ones. Collecting bottles is a popular craft. Foremost though you want to collect the unique one of a kind bottle. There are many links available for pictures of bottles and additional information.