To identify an old blue glass bottle, look at the age and the typology. In some cases, this information appears on the bottle itself. Otherwise, it's necessary to look at defining characteristics to identify it. Understanding the differences between bottle shapes, dates and typology is necessary for identifying an old blue glass bottle's purpose.
When the date is not available on the bottle's neck or base, look at its neck for a side mold seam. Side mold seams on the neck indicate early 19th to early 20th century production. In the absence of side mold seams, it is likely the bottle has a later manufacturing date. Side mold seams indicate that production took place between 1820 and 1910. An absence of side mold seams suggests an 1880 to 1950 production date.
According to the Historic Glass Bottle Identification and Information website, typology is not always a reliable way of determining a bottle's use. Because bottles were repurposed for new functions, the markings on them may not be accurate. It is sometimes possible to identify a bottle according to shape. For example, a bottle that is round and features heavy glass possibly held a carbonated beverage. Similarly, a bottle that is round at the cross section indicates a beer bottle.