Non‐metallurgical Slags in the Masonry of Obřany Castle in the Czech Republic: Evidence for the Local Production of Hydraulic Lime in the 14th Century?

Two types of compositionally heterogeneous slags have been found in the wall masonry of the Gothic Castle ruin Obřany. The bright phase is composed of quartz, tridymite, cristobalite, and porous glass, while the dark one also contains feldspars, hercynite, magnetite, clinopyroxenes, and a suite of accessory phases. The bright slags have a similar chemical composition as the mortar from the castle masonry or local sandstones, based on rare earth elements (REE) and other trace element abundances. In contrast, the dark slags are characterized by elevated contents of Al 2 O 3 (15.4–18.6 wt.%) and Fe 2 O 3 tot (4.1–10.0 wt.%), and consequently resemble the local clay. The investigated slags are not related to iron metallurgy in terms of their composition and texture. They probably originated in a medieval lime kiln through heat sintering of the inner lining during the burning process under relatively high temperatures (1080–1500°C). Simultaneously, the finding of relics of clinker‐like material together with products of its hydration in the hydraulic mortar from the wall masonry infers the production of hydraulic lime or Roman cement during the construction of the castle in the 14 th century. The Obřany Castle appears to represent one of the first applications of hydraulic binders in Moravia.