I predominantly consider myself a social archaeologist. I kindof slipped and fell into Experimental Archaeology and I now find the same thing happening with material science and the overall topic of Archaeometallurgy, so I thought I’d lay out some of the things I’m encountering, mostly to formulate my findings to coherently, but also to encourage anyone reading this to point out if I’m on the right track or completely wrong.
I’ve surprised even myself in my own level of interest in this field, but it’s amazing how much one can learn about the breakage and deformation of bronze by simply understanding what’s happening at a microstructural level.
After putting a call out on the Experimental Historical Bronze Casting Facebook page for any thoughts and advice, I was presented with the terminology “hot-shorts”. This sparked in my brain that I’d actually come across this term before and never followed it up.
Now, in my naivety I merrily googled: “Hot shorts”, which of course were not the results I was looking for. Similarly, “metal-working hot shorts” presented some very interesting images.
As is her habit, my wife (the self-proclaimed Google-Queen) came to the rescue and helped me to avoid looking at unwanted pictures of hot-shorts directing me to the right metallurgical pages.
Hot-shortness refers to a process of metal fragmentation that often occurs in steels and forged iron when minor elements within the composition (such as copper) liquefy and cause inherent weakness at higher temperatures. The metal then splits along the grains. Dr. Maikel Kuijpers explored similar instances in his thesis on Early Bronze Age axes, when high levels of antimony or bismuth were encountered in bronze pieces, which likely caused hot-shorts.
Where am I going with this, I hear you ask? Well, much of the deliberate destruction of swords, spears, axes etc. seems to represent intentional hot-shorts, caused when the objects have been heated up and become brittle at the higher temperatures, breaking upon impact. I have found subjecting objects to this thermal stress to be the most effective way for breaking them down, recreating replica fragments.
Of course, in these instances, it is not high levels of antimony or bismuth causing brittleness and fragmentation, but most likely the lead inclusions. While in the right conditions tin dissolves into the copper, establishing a strong matrix, that increasing the overall hardness of bronze, lead does not dissolve and sits as globules in between the grains in the bronze matrix, causing inherent weakness, lowering the overall tensile strength and ductility of the bronze.
What this means is that when the bronze is heated, its endurance upon impact is significantly lowered and if it is hot-worked at a temperature too high, it will simply break. This of course is an advantage if one is hoping to fragment their object for recasting/recycling.
It is important to note, however, that at the moment I have yet to encounter the literature informing me that it is the lead causing hot-shortness in leaded bronzes, though given the description of this process for other metals, it seems a likely proposition.
I am, however, more than happy to be corrected by anyone with a greater material science knowledge than myself!
At this stage, I’m just pleased to have moved beyond pictures of hot shorts on Google.
Notes and Acknowledgements
This post represents an ongoing thought-process, which in no way reflects the end of the road in my understanding of this topic. If you know better than me, please please please get in contact! For now though, I have to thank the assistance of Giovanna Fregni and Maikel Kuijpers in getting me to this level of understanding and for their direction towards appropriate literature. Any imperfections represent my own misunderstandings!