The benefit of a tough skin: bullet holes, weathering and the preservation of heritage

Abstract

Projectile damage to building stone is a widespread phenomenon. Sites damaged 100 years ago during the First World War still see daily use, while in a more contemporary setting numerous reports show the damage to buildings in Babylon, Mosul and Palmyra. While research has been carried out on the long-term effects of conflict such as fire damage, little is known about the protracted damage sustained through the impact of bullets, shrapnel and other metal projectiles outside of the field of engineering focused on ceramics and metals. To investigate alterations to mineral structure caused by projectile damage, impacts were created in medium-grained, well-compacted, mesoporous sandstone samples using 0.22 calibre lead bullets shot at a distance of 20 m. Half these samples were treated with a surface consolidant (Wacker OH 100), to mimic natural cementation of the rock surface. These samples were then tested for changes to surface hardness and moisture movement during temperature cycles of 15–65°C. Petrographic thin section analysis was carried out to investigate the micro-scale deformation associated with high-speed impact. The results surprisingly show that stress build-up behind pre-existing cementation of the surface, as found in heritage sites that have been exposed to moisture and temperature fluctuations for longer periods of time, can be alleviated with a bullet impact. However, fracture networks and alteration of the mineral matrices still form a weak point within the structure, even at a relatively low impact calibre. This initial study illustrates the need for geomorphologists, geologists, engineers and heritage specialists to work collectively to gain further insights into the long-term impact of higher calibre armed warfare on heritage deterioration.

Bullet impacts and built heritage damage 1640–1939

“…While there are differences between the ballistics used during the Civil War and the ballistics used in this test, a.22 calibre lead bullet is soft and deforms easily [32] reaching a maximum of 275-365 m/s, thus also falls into the 'low impact' group of ballistics, penetrating relatively little into the impacted surface [33]. We hypothesise that the relative strengthening of the surface within the impact areas observed on Powick Church is due to realignment of the clay matrix within the sandstone surface, in a process comparable to that observed in the simulation tests of .22 calibre lead bullets on a similar sandstone surface in previous research [10]. Surprisingly, this could indicate that a singular impact point from a lowimpact projectile (such as a musket ball) could actually be stronger than the material surrounding it.…”

“…Therefore, ballistic damage was not superimposed to any previous weathering process and any differences in deterioration behaviour can therefore be attributed to ballistic damage. This is in contrast with the previous case studies where the sites had been created centuries before ballistic impacts hit their surfaces, and where weathering inheritance might alter the long-term response of the stone to ballistic impacts [10,52].…”

“…Within this newly exposed surface the clay content within the sandstone matrix will have compacted and realigned itself with the impact, thus reducing permeability and susceptibility of the material to deterioration processes. While it was not possible to sample the stone at this listed Grade I building, this principle has been tested using similar sandstone [10]. Thin section analysis showed that at the point of impact of a low-calibre bullet, the clay minerals in the cement matrix realigned with the direction of impact (see Fig.…”

“…These illustrate the wide ranging occurrence of this type of damage and highlights the complexities that are created by the high-velocity/high-energy impact of a bullet upon a stone surface. Despite the prevalence of this type of damage in our built environment, relatively little is known about the short-and long-term implications of ballistic impacts [10]. What is even more concerning is the rapid pace of development of ammunition and the ever-increasing risk it poses to heritage.…”

Ineducable us: the applications and contexts of microscopy used for the characterisation of historic building materials

“…Looking at the effects of consolidants, for example microscopy is used for determining penetration depth and textural relationships of treatments within the microstructure of materials [27]. A related activity is the experimental simulation of damage caused by acute events, for example stressors such as fire and armaments/projectile impacts [28,29], in which microscopy can play a role in evaluating effects (Fig 3).…”