Distinguishing replacement rivets on armour
The flexibility of plate armour relies on internal leather straps which hold together the individual elements. Over long periods of time this leather gradually decays and needs to be replaced. In the past the original rivets that attach the leather to the armour were sometimes discarded and replaced with new ones. Could the original brass-capped rivets on a 1610 Greenwich half armour (II.79) be distinguished by compositional analysis?
Results of analysis
Semi quantitative XRF analysis suggested 3 groups of rivets based on composition of zinc and tin.
The compositional differences are likely to reflect technological developments in the production of metallic zinc and its use in the production of high-zinc brass. On many rivet heads the brass sheet had partially worn away exposing the lead-tin solder below, which gave a high tin content and prevented accurate determination of the brass composition.
The remaining rivet heads were found to be composed predominantly of alloys of copper and zinc alone. These fell into two distinct groups: Rivets with a zinc content of about 30% are likely to post-date the “English” zinc extraction process, patented by William Champion in 1743, which enabled the production of high Zn brass. By contrast those with zinc contents below 22% are typical of the earlier cementation technology that struggled to achieve high zinc contents and are thought to be original or early replacements.
This research addressed, and offered a solution to an on-going problem for museums with collections of plate armour: the differentiation of replacement and original brass-capped rivets. It also effectively demonstrated the potential of chamberless XRF analysis for undertaking comparative quantitative analytical studies.
The work has been presented, for example at a British Museum conference in April 2005 entitled Metallurgy – a touchstone for cross-cultural interaction, where it contributed to the debate on the value of analytical studies for dating historical brass. It is also recently been published in an Arms and Armour Journal article, focussing on the identification of surface coatings.