Virtual Kingdom A&S 2020: Chalcedony/Striking Glass

After my last project which was focused on just successfully making a glass, I began trying to refine my batch composition to eliminate lead so I would be comfortable using it in projects that do not stay with me. Shortly after accomplishing this, I got distracted by the range of effects that can be produced by including silver in glass. One effect I focused on is called striking, which is where the glass shifts through a rainbow of colors when reheated.

I approached this under the impression that striking glass was a more recent creation, but in the middle of the project, I came across extant pieces with similar coloration that were referred to as chalcedony glass. I eventually found a recipe dated to 1443 for this glass, and it did include striking as the last step of the process. This recipe, however, called for mercury and lead in its components. I had just managed to get lead out of my batch, and I was definitely not going to introduce mercury, so this project focused more on comparing the materials and final results than directly recreating the historical glass.
 

To use striking glass, it goes through a process of resetting, cooling, then striking. The glass is reset by getting it hot enough that the silver crystals which currently exist are dissolved into the glass and it turns transparent. Next, the glass is cooled, either by just removing it from the flame, using a marver, or if making vessels, blowing the bubble. When it cools, some of the silver is reduced out of the structure of the glass and forms particles of metal throughout. Finally, when it is reheated to strike, those silver particles serve as nucleation sites for the silver still dissolved in the glass and they begin to grow. The smallest crystals produce a yellow color. As they grow, the color shifts to an amber, then through purple, blue, green, and finally tan. If the color strikes more than desired, the whole process can be repeated by resetting the glass again.
 

The beads seen here are an assortment from all the batches of glass I made, with the most recent in the bottom row. Some of them have a core of commercial black or white glass to lower the amount of special glass used per bead.

    The bottles are all from my last batch of glass, although the three on the left have had commercial white glass blended in to try to enhance the banding. The fourth bottle is all striking glass and has some variegation visible still, although it is less pronounced than the historical examples. This is likely because I blended all my batch ingredients together then melted them, instead of melting the base glass then blending in the metals later like the period instructions call for. You can see the effect that rate of cooling has on the final colors by how the beads take on a rainbow of colors while the bottles stay more on the amber side of the scale.

    Here is the documentation submitted with the project, which goes more into the composition of the glass.

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