While excavating at one of my project properties this week I found these lovely pieces of amethyst glass. They appear to be parts of a wide-mouth jar or bottle.

Amethyst glass is not only pretty, but is very useful to archaeologists. When found, this glass, as well as many types of glass, can be helpful for figuring out the age of an archaeological site. Glass making processes have changed over time, and color and marks on glass can indicate the time-range of manufacture.

The jar parts I found started life as colorless glass, due to the addition of the neutralizing agent manganese in the glass ingredients. Over time the jar was exposed to ultraviolet light, turning this manganese-infused glass from colorless to amethyst. While glass makers may have been using manganese in glass for several centuries it’s use was most common from the 1880s to about World War I, with some still manufactured into the 1930s. So when archaeologists find amethyst glass they can say it is likely not older than 1880.

In addition to color, the method of manufacture can be used to tell how old a glass item might be. Glass-making methods have evolved over time and have generally progressed from free-blown, to blown-into-mold, to machine-made. These manufacturing processes leave specific diagnostic marks on the glass. The fragments of glass I found appear to be from a machine-made vessel. According to the Society for Historical Archaeology’s website on historic glass bottle identification (https://sha.org/bottle/) machine-made amethyst glass bottles date from 1905 to the 1920s or 1930s. This potentially relates the glass I found to the early history of the home built in 1930, or perhaps more likely to the history of the Washington State School for the Deaf which is in close vicinity to the property.

While these artifacts do not date to the time of the first Fort Vancouver, they are still fascinating pieces of the history of the area.