Medieval Gilding Technology of Historical Metal Threads Revealed by Electron Optical and Micro-Raman Spectroscopic Study of Focused Ion Beam-Milled Cross Sections

Abstract

Although gilt silver threads were widely used for decorating historical textiles, their manufacturing techniques have been elusive for centuries. Contemporary written sources give only limited, sometimes ambiguous information, and detailed cross-sectional study of the microscale soft noble metal objects has been hindered by sample preparation. In this work, to give a thorough characterization of historical gilt silver threads, nano- and microscale textural, chemical, and structural data on cross sections, prepared by focused ion beam milling, were collected, using various electron-optical methods (high-resolution scanning electron microscopy (SEM), wavelength-dispersive electron probe microanalysis (EPMA), electron backscattered diffraction (EBSD) combined with energy-dispersive electron probe microanalysis (EDX), transmission electron microscopy (TEM) combined with EDX, and micro-Raman spectroscopy. The thickness of the gold coating varied between 70-400 nm. Data reveal nano- and microscale metallurgy-related, gilding-related and corrosion-related inhomogeneities in the silver base. These inhomogeneities account for the limitations of surface analysis when tracking gilding methods of historical metal threads, and explain why chemical information has to be connected to 3D texture on submicrometre scale. The geometry and chemical composition (lack of mercury, copper) of the gold/silver interface prove that the ancient gilding technology was diffusion bonding. The observed differences in the copper content of the silver base of the different thread types suggest intentional technological choice. Among the examined textiles of different ages (13th-17th centuries) and provenances narrow technological variation has been found.

Ion milling: the perfect cross-section of a painted textile

“…The use of ion-milling to produce high quality paint cross-sections was first proposed [8] who reported greatly improved SEM images of samples with 17th century lead white and modern acrylic paints. Weiszburg et al [9] reported the successful use of focused ion beammilled cross-sections to investigate gilt silver threads. However, such work has not been reported for use with painted textiles.…”

Unveiling the Complexity of Japanese Metallic Threads

“…However, the majority of the studies concentrates on European production from the 13th to 17th century. Flemish Renaissance tapestries [6], Italian gentry' mantles [7], Hungarian [7] and Portuguese [8] liturgical vestments, and Greek/Byzantine ecclesiastical textiles [9] date back to this period. Successively, the use of metallic threads did not decrease, and we can find it in Croatian liturgical vestments and festive folk costumes from the 17th to 20th century [10], Spanish coaches and furniture textiles from the 18th to 19th century [11], Turkish caftans and brocades dating between the 16th and 19th century [12], ornamentation for ecclesiastic clothing, religious statues, and church decoration from the Colonial Andes, which dates back to 17th and 19th century [13].…”

“…Some authors concentrate on material identification for conservation purposes, some on technological issues about gilding manufacture, others on degradation products. Even if some innovative approaches have to be mentioned [3][4][5]7,[24][25][26], the majority of works relies on routinely and welltested techniques. First of all, optical microscopy (OM) and scanning electron microscopy (SEM) are fundamental for a preliminary analysis of the sample dimension, morphology, and surface [1,[5][6][7][8][9]13,14,27,28].…”

“…Even if some innovative approaches have to be mentioned [3][4][5]7,[24][25][26], the majority of works relies on routinely and welltested techniques. First of all, optical microscopy (OM) and scanning electron microscopy (SEM) are fundamental for a preliminary analysis of the sample dimension, morphology, and surface [1,[5][6][7][8][9]13,14,27,28]. For example, degradation and corrosion of layers can be appreciated [12,13,29], as well as information about the manufacturing techniques [1,7,13,27,29,30].…”

A silvered shield grip from the Roman Period: a technological study of its silver coating

“…This method perhaps originated in ancient China. It may have been known in early medieval Europe, but in this continent it became more widespread in the High Middle Ages (La Niece 1993, 207-209; see also Blair and Ramsey 2001;Theophilus 1979: p 113;Ingo et al 2013;Weiszburg et al 2017).…”