Knowledge Feature – X-Ray Fluorescence

Pic: The extra-large beads from Sexwamin

If you follow me on Twitter or read this blog on a regular basis, you have definitely heard me talking about blown-glass (B-G) beads.  If you’re new to my blog and find yourself wondering what I’m talking about, I’ll summarize briefly: I uncovered some super rare B-G beads on an archaeological site in BC that I studied for my MA thesis.  Based on specific manufacturing characteristics on these beads I determined they were most likely manufactured in Bohemia (Czech Republic) after 1876. For more info, take a look at my previous blog posts talking about my research (Part 1 and Part 2).

In archaeology, we never give anything 100% certainty.  We draw conclusions based on the data and evidence we have at hand, conclusions which we deem to be “likely”, or “most likely” if you’re feeling really confident.  In the case of glass beads, using manufacturing characteristics is one line of macroscopic evidence and data that we can follow to help us form our conclusions.  To help support (or possibly negate) those conclusions, microscopic forms of data can be studied, including x-ray fluorescence (XRF).

XRF analysis can be used to study the elemental (Remember the periodic table of elements?) composition of different materials, in a non-destructive manner.  The basic

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One example of a portable XRF machine (this one is manufactured by Olympus)
idea is that an x-ray is shot at an item, which makes the atoms of the item’s material(s) super excited.  As the atoms bounce around and party, they release a burst of energy (the fluorescence).  This energy burst is characteristic of individual elements and, as it flashes back to the XRF machine, the analysis of that burst can tell us which elements are present in the material (i.e. lead, potassium, etc.)

In the world of glass beads, XRF analysis can tell us a) what kind of glass “recipe”, or composition, was used to make the beads, and b) what “recipe” was used to colour the glass (if it’s coloured glass).  By “recipe” I’m referring to the different elements used to create the glass.  Why would we want to know this?  Because different geographic regions used different recipes for their glass beads, and learning these differences can give us insight into where glass beads were made.  Understanding regional differences in glass recipes holds potential for better understanding the movement and trade of glass beads.  Sometimes different recipes were unintentional (i.e. different types of trees used to fuel the glass furnaces would result in different compositions of the glass).  Other times they were intentional, especially when it came to colouring glass.

Ruby Red St Catherines Beads
Bohemian ruby-red beads discovered on St. Catherine’s Island, GA (Blair et al., 2009: 299)

For example, through my MA research I learned that for a while in the 17th century there was a fierce battle over ruby-red coloured glass.  Venetian bead-makers had a secret recipe that Bohemian bead-makers were desperate to know.  When they couldn’t learn the Venetian secret, Bohemian bead-makers figured out their own recipe using lead and gold to create a ruby-red colour that became even more popular than the Venetian red glass they had been so desperate to copy!

Why am I telling you about XRF analysis?  For starters, it’s a non-destructive method of analysis that archaeologists should be aware of.  More excitingly, I’ve been setting up a follow-up project on the B-G beads from Sexwamin that involves XRF analysis, which I’m now able to say is coming to fruition!  I’ve been given permission from the shishalh Nation to attempt XRF analysis on the B-G beads we uncovered.  Once my current field project is over I’ll be spending time at the University of Toronto working with a friend who is qualified to perform XRF testing and is also going to teach me how to do it.

I had to do a bit of preliminary work to help with my analysis.  Because B-G beads are so rare in archaeological contexts, it was important to know whether or not XRF analysis had been performed on B-G beads.  Specifically, B-G beads attributed to Bohemian origins.  I need to know what kind of glass composition I’m going to be looking for in the Sexwamin beads in order to support/negate my hypothesis that they were manufactured in Bohemia.  If XRF analysis has never been done on Bohemian B-G beads, than all the information I’m able to gather from my own XRF analysis would be brand new.  Which is still pretty cool and important to record, but knowing the glass composition would be preferred (for comparative purposes).

What I’ve found out is that XRF analysis has been performed on Bohemian B-G beads (Burgess and Dussubieux, 2007).  In 1934 over 56,000 glass trade beads were recovered from a mortuary site (Sullivans Island) off the coast of Washington state, including a very small collection of B-G beads.  By small collection I mean one intact bead and one bead fragment.  Neither of these beads matched the style of the Sexwamin beads (they were decorated with a grooved pattern), but they were attributed to Bohemian origins, and were blown in a mold.  While not an exact match to the Sexwamin beads, the analysis from the Sullivans Island beads still yielded important information I can use in my own XRF work – the composition of the B-G beads.

Burgess Bohemia Bead Values
These are the elemental values I’ll be looking for in the B-G beads from Sexwamin (higher potassium (K) and calcium (Ca) concentrations were detected in Bohemian B-G beads) (Burgess and Dussubieux, 2007: 61)
Burgess Bohemia Bead Colour values.png
Some of the elements used in colouring glass beads, which I’ll also be keeping my eyes open for (Burgess and Dussubieux, 2007: 61)

From the work that Burgess and Dussubieux (2007) did, I now have something to compare my XRF results to.  I’ve somewhat randomly selected a sample size of 35 (of the 244 intact beads) beads – 5 random extra-large orange, 5 random extra-large yellow, 1 medium blue, and the remaining 24 will be of miscellaneous colours of the gold, silver, and indeterminately coloured medium beads.  If the Sexwamin beads match the composition of the Sullivans Island beads, than that will support to my conclusions that the Sexwamin beads were manufactured in Bohemia.  If the compositions don’t match, than we have 2 options to consider:

  1. The beads from Sexwamin were not manufactured in Bohemia.
  2. The beads were manufactured in Bohemia, but more intricately styled B-G beads required different compositions than simple, unadorned beads.

There’s no guarantee that the XRF analysis will work at all.  Glass can be tricky to analyze as some elements are too light to be easily detected.  In other cases, different elements can result in similar x-rays being bounced back to the machine and difficulty in differentiating between them.  XRF analysis, especially portable, non-destructive XRF analysis might simply not be precise enough for detailed readings on the glass composition of certain beads (See Bonneau et al., 2014 for a nice break down on the potentials and limitations of different analytical techniques for glass beads).  What I’m trying to say is that before I worry myself about different outcomes of the analysis, I have to wait and see if the analysis works at all.  There’s really only one way to find out – to just do it.  And if it works, then I can start getting excited.  Even if the results aren’t exactly what I’m hoping for (definite Bohemian values), it’s still information that we didn’t have before, information that the shishalh Nation can include in their history at Sexwamin.  And in the case of blown-glass beads, as far as I know this will be the first time a collection of this sample size will be subjected to XRF analysis.  It’s definitely the first time B-G beads found in BC will be subjected to XRF analysis (which is no surprise given these are the only known blown-glass beads in BC to date), and the first time B-G beads found in Canada will be XRF analyzed.  So really, any news is interesting news.  I certainly can’t wait to give XRF a try and hopefully learn something new about the B-G beads from Sexwamin!

References:

Bonneau, A., J.-F. Moreau, R.G.V. Hancock, and K. Karklins. (2014). Archaeometrical Analysis of Glass Beads: Potential, Limitations, and Results. Beads, 26: 35-46.

Burgess, L.E., and L. Dussubieux. (2007). Chemical Composition of Late 18th and 19th-Century Glass Beads from Western North America: Clues to Sourcing Beads. Beads, 19: 58-73.

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