Wicked Weathering of Glass Artifacts

Hi Fort Followers,

It’s been over half a semester here at WMU, where we’ve been hard at work analyzing artifacts in the Fort St. Joseph archaeology lab. Tasked with creating a research project, my fellow student Ian and I landed on glass artifacts as our main focus of study. We both had many questions, like how did people of the fort use glass containers? Is there an area on site where certain types of glass are more concentrated than others? What does the color and thickness tell us about life at the fort? In order to better understand some of our research questions, we took a step back and did some background research. Then, we began sorting the artifact collection of glass into categories of non 17th/18th century, window glass, and container glass. In doing so, we’ve been exposed to countless examples of glass artifacts found at Fort St. Joseph. I began to wonder, why are some pieces of glass patinated and some are not? How does glass even look like that?

    Patination refers to the weathering of glass. “Patina” is a crust formed on the surface of glass after having been weathered in soil. The site is exposed to lots of water, being situated directly next to the river. This causes the chemical composition of the glass to be altered with time. I was amazed when I saw just how interesting patinated glass could look. In general, the samples of patinated glass found at FSJ have an iridescent, flaky appearance. Sometimes, the layer of crust on the glass can even flake off. It is important to note that there is a lot of variation in the appearance of patinated glass. Some, you can see the glass’s original color with a very thin layer of iridescence overtop. Others are more severe, with many layers of cloudy, flaky material which obscure the whole entire shard. These pieces can easily be mistaken for ceramics because the layers of patina cover the glass completely, making it appear a creamy tan color. Interestingly, some pieces look completely unaffected by weathering. Before handling these artifacts, I never knew it was possible that glass could be weathered in this way. I wondered how it was possible for this incredible change to occur. 

    After doing some research on the process of patination, I found out that it is too difficult a process to generalize. Understanding the formation of patination is complex because it includes many variables, such as the chemical composition of the parent glass, patina, the sediment in which it was buried, and of the water that it was exposed to. In addition, the outer crust is often formed in combination with elements outside of its parent structure (Clifford & Semeniuk 2020). Information that can be gleaned from this is that, in general, the glass at FSJ which shows iridescent patination has a certain chemical composition which allows for this type of weathering to occur. I am no chemist, but I still found it interesting that it is possible for glass to be chemically altered through time. It took a journey from the 17th/18th century and ended up in an archaeology lab with eager students scrambling for data. That, in and of itself, is something worth note.

Adyn

References

Clifford, P., & Semenuik, V. (2020). Patina on Historic Glass: A Case Study from Cossack, Western Australia. Cambridge Scholars Publishing. https://www.cambridgescholars.com/resources/pdfs/978-1-5275-5940-0-sample.pdf