High‐temperature performance of two‐layered ceramics and the implications for Roman crucibles

Abstract

Roman metalworking crucibles are frequently characterized by an extra outer layer of clay, applied to a pre‐formed vessel. Three‐point bending and standardized dead‐weight loading tests were conducted to determine the advantages offered by the extra outer layer. Deformation and fracture behaviour, at temperatures up to 1100°C, of two‐layer, monolithic and tempered‐monolithic briquettes were compared. Measurements indicated that the two‐layer briquettes were more resistant to fracture at high temperatures: > 850°C they showed more extensive plastically deformed regions than monolithic briquettes. The influence of temper was consistent with previous results: tempering caused quasi‐stable fracture and reduced fracture strength. The results suggest extra outer layers assisted in preventing catastrophic failure.

On diverse arts: crucible metallurgy and the polymetallic cycle at Scandinavia’s earliest Viking town, Ribe (8th–9th c. CE), Denmark

“…Higher alumina contents in group B crucibles, similar to high alumina crucibles types 2 and 3 from Hedeby (Merkel 2016, p. 213), would enhance their heat resistance properties. Nevertheless, given that the Ribe crucibles are not highly refractory, additional layering to reduce the effects of brittleness (Rehren 2003;Bayley and Rehren 2007;Gardner et al 2020), might not have been needed, as exterior layering was found in just one crucible (base fragment ASR9 ×147.17, S7, no. 3).…”

“…3). The increase in group B crucibles from the early to the later period suggests that more refractory fabrics were used after 790 CE, possibly based on the craftspeople's practical experience (Braun 1983;Freestone and Tite 1986;Gardner et al 2020). Alumina-rich crucibles were imported to Hedeby and Kaupang in the 9th c. (Pedersen 2010(Pedersen , p. 191, 2016Merkel 2016, p. 211) and later (eleventh c. CE) in Viborg, Denmark (Jouttijärvi and Andersen 2005, p. 356).…”

“…9) and bulk compositions (Fig. 11) could represent contamination by fuel ash from charcoal as a result of interaction with the ceramic during heating of the crucibles (Tylecote 1980(Tylecote , 1982(Tylecote , 1987Misra et al 1993;Crew 2000;Rovira et al 2007;Wood et al 2009;Müller 2017;Gardner et al 2020). However, there is a lack of increasing K 2 O concentrations (~6 ± 3 wt%) in the ceramic body and crucible slag as would be typically expected for fuel ash contamination.…”

“…As metallurgical ceramics performed the most challenging role of all ceramics (Gardner et al 2020), their manufacturing choices directly impacted the efficiency of metalworking. Crucibles were made of higher alumina clays and had increased refractory properties from 790 CE onwards suggesting that craftspeople at Ribe reflected upon the ceramics' production and achieved better performance after of almost a century of on-site metalworking.…”