# Pre‐industrial Use of Bauxite by Late Gothic Goldsmith Masters: Analytical Evidence and Experimental Study

## TL;DR

Late Gothic goldsmiths in Central Europe started using bauxite instead of imported red clay for gilding due to disrupted trade routes.

## Contribution

First evidence of pre-industrial bauxite use in gilding technology, linked to Croatian Minjera through mineral analysis.

## Key findings

- Bauxite replaced imported Armenian bole in gilding preparations by 1470.
- Diaspore found in samples confirms bauxite source as Croatian Minjera.
- Bauxite mining in Europe began shortly after 1453, earlier than previously known.

## Abstract

The turn of the 15th and the 16th century was marked by turmoil including invasion of the Turks in the Balkans. Consequently, trade routes to the Mediterranean were disrupted, resulting in sudden demand for alternative sources of imported materials. For example, direct import of potassium alum had been gradually replaced by its local production from alunite, alum schists or grey pyritic bauxite. It has now been proven that concurrently extracted red bauxite had been used as a substitute for imported high‐quality red clay, the so‐called Armenian bole, employed by Central European painting workshops in the preparations for gilding (“poliments”). Importantly, connection with the documented mining of grey bauxite in Croatian Minjera was evidenced by unique finding of diaspore together with dominant boehmite. Mineralogical analyses were performed by X‐ray powder micro‐diffraction and reference bauxites from Istria and Balkan Peninsula were used to evaluate their technological suitability for gilding. As it was found that the earliest appearance of boehmite in poliment dates back to 1470, the beginning of bauxite mining in Europe is shifted to the period shortly after the fall of Constantinople in 1453, and it also represents the oldest known evidence of the use of bauxite raw material in technology.

First analytical evidence and replication of yet undescribed substantial change in gilding technology in the early modern period ‐ the replacement of imported red clay (bole) with locally available bauxite. Proof of its source ‐ Croatian Minjera, according to a unique find of mineral diaspore. Earliest documented use of bauxite in technology ever, important for dating anonymous paintings.