Student and ECR Research Support Award (June 2021) - Nicolas C. Ciarlo

Biringuccio was right! Bronze gun-founding and scrap metals from a mid-17th century Dutch shipwreck

Nicolas C. Ciarlo, National Scientific and Technical Research Council (CONICET) and University of Buenos Aires

The use of waste goods (scrap) as raw material to obtain new products has been a long-standing activity in human history. Melting down old bronze pieces, exploded guns, and failure castings, to manufacture serviceable ordnance was a common practice in England, Spain, and other European countries during the 17th century. Nonetheless, no previous archaeological research has been developed on discarded failed products transported in ships as cargo or in stowage.

This study, carried out in collaboration with Josefa Martí (Andalusian Centre of Underwater Archaeology – CAS-IAPH, Spain) and Manuel Bethencourt (University of Cadiz – UCA, Spain), seeks to provide novel information on the technical challenges linked with the manufacturing of bronze guns, through the study of scrap bronze guns recovered from the Delta III site, a mid- to late 17th century Dutch merchantman lost in Cadiz, Spain (Fig. 1).

Figure 1. A section of a bronze gun recovered from the Delta III site: general view (a) and detailed side-view of the barrel, where the trunnion and severe casting defects can be appreciated. Images: N.C. Ciarlo. © CAS-IAPH.

The production of bronze guns at this time was a remarkable and highly-demanded metallurgical activity. Smooth-bore muzzle-loading pieces comprised culverins, cannons, perriers, and mortars, used both for field and naval service. Yet much more expensive than iron ordnance, bronze guns had widely acknowledged advantages. In particular, the latter were easier to produce: to cast, to rectify, and to decorate. Also, when needed, gun-founders could re-cast bronze scrap to obtain new pieces without losing the required properties.

A variety of bronze goods were melted down in bronze foundries, including guns and bells. For instance, a plate from Nova Reperta titled Pulvis Pyrius (Invention of Gunpowder), engraved by Philip Galle (ca. 1590), provides a late-16th century snapshot of the variety of activities carried out in the interior of a gun foundry. In it, a man is depicted melting down parts of broken guns which were hoarded in a pile near the furnace (https://library.si.edu/image-gallery/110721).

In the art of gun foundry, expertise and a rigorous control over several interrelated variables were required to obtain a suitable product. Accidents in production were not unusual, deriving in a wide diversity of defects, from microscopic imperfections to macroscopic features such as blowholes, cracks, and misruns (incomplete casting). While the former was unnoticeable to the naked eye and probably had no significant consequences for the gun’s performance, the latter would have resulted in a product unsuitable for military service or completely useless.

Drawbacks related to casting large objects were probably well-known when bronze artillery was introduced, given previous experience with bell-founding. This empirical knowledge was then further developed and disseminated in publications on metallurgy and artillery. In Pirotechnia, Vannoccio Biringuccio (1540) provided a detailed technical insight into the art of gun-founding as nobody had done before. For him, it was a laborious, expensive, dangerous, and difficult art, noting that "the outcome of this art is dependent upon and subject to many operations which, if they are not all carried out with great care and diligence and well observed throughout, convert the whole into nothing, and the result becomes like its name [cast away]" (Smith & Gnuci, 1990, p. 213). 

From his work and current knowledge on the foundry process, a series of usual errors associated with misinformation, negligence, shortcuts, and savings during mould making and actual casting (melting, pouring, and cooling) and their potential functional and aesthetic defects on the final product, can be extracted or inferred. To understand the whole process and assess failures, most of these factors should be viewed as interdependent phenomena.

How can we, as archaeologists, study these issues through a materiality which was never meant to be used, and in turn, was usually recycled to obtain new and usable goods? Numerous seventeenth century bronze guns are widespread across Europe and overseas. Several were located in shipwrecks (or underwater archaeological contexts) and have been subjected to detailed studies. Moreover, there are pieces showing manufacturing defects on their surface. Guns damages caused by fires were also recorded in wreck-sites. Yet, there are but a few examples of bronze guns with clear signs of being the result of a failed casting. Thus, the studied pieces recovered from the Delta III site are valuable to better understand the gun-founding practice of the time, as data obtained can contribute to discuss the factors involved in the failure castings.

Both macroscopic and microstructural evidence (Fig. 2) will allow to determine whether defects on these unserviceable products are associated with the use of a low-quality or non-uniform clay in mould making, incomplete drying or baking of the mould, deficient reinforcement of the mould, narrow or small vents, poor-heating of the furnace, unsuitable managing of fusion times, inadequate alloy composition, insufficient charging material, poorly controlled pouring of the molten metal, or uneven or uncontrolled cooling, among other factors.

Figure 2. Photo-micrographs of polished unetched sample no. 835.1 (an amorphous bronze mass attached to a gun’s barrel fragment): (a) as-cast bronze microstructure with some gas porosity and inclusions; (b) globular lead and sulphur impurities (light grey) located at grain boundaries, and silicon particles (dark grey) scattered through the microstructure, probably linked to mould’s material.

In sum, this work will shed light on the poorly known but common faults that lied on the way to master a practice that largely relied on the founders’ knowledge, skills, and practice.

References

Smith, C.S. & Gnuci, M.T. (1990). The Pirotechnia of Vannoccio Biringuccio: The Classic Sixteenth-Century Treatise on Metals and Metallurgy. New York: Dover Publications, Inc.