minerals Review Cobalt and Associated Impurities in Blue (and Green) Glass, Glaze and Enamel: Relationships between Raw Materials, Processing, Composition, Phases and International Trade Philippe Colomban 1, * , Burcu Kırmızı 2 and Gulsu Simsek Franci 1,3   Citation: Colomban, P.; Kırmızı, B.; Simsek Franci, G. Cobalt and Associated Impurities in Blue (and Green) Glass, Glaze and Enamel: Relationships between Raw Materials, Processing, Composition, Phases and International Trade. Minerals 2021, 11, 633. https://doi.org/10.3390/ min11060633 Received: 8 March 2021 Accepted: 11 June 2021 Published: 15 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 MONARIS UMR8233, Sorbonne Université, CNRS, 4 Place Jussieu, 75005 Paris, France; gusimsek@ku.edu.tr 2 Department of Conservation and Restoration of Cultural Property, Faculty of Architecture, Yıldız Technical University, Yıldız Yerle¸ skesi B Blok, Be¸ sikta¸ s, Istanbul 34349, Turkey; kirmizi@yildiz.edu.tr 3 Surface Science and Technology Center (KUYTAM), Rumelifeneri Campus, College of Sciences, Koç University, Rumelifeneri Yolu, Sariyer, Istanbul 34450, Turkey * Correspondence: philippe.colomban@sorbonne-universite.fr or philippe.colomban@upmc.fr Abstract: Minerals able to colour in blue (and green in combination with yellow pigments) are limited in number and geologically. After presenting a short history of the use of cobalt as a colouring agent of glass, glaze and enamel in the Western/Mediterranean, Islamic and Asian worlds since Antiquity, we will present the different forms (dissolved ions, natural and synthetic crystalline phases/pigments) of cobalt and associated elements regarding primary (transition metals) and secondary geological deposits (transition metals and/or arsenic, bismuth, silver). Attempts to identify the origin of cobalt have been made by many authors considering the associated elements but without considering the important modifications due to different processing of the raw materials (extraction/purification/formulation). We review the information available in the ancient reports and present literature on the use of cobalt, its extraction and production from the ores, the different geological sources and their relationship with associated elements (transition metals, bismuth, arsenic, and silver) and with technological/aesthetic requirements. (Partial) substitution of cobalt with lapis lazuli is also addressed. The relative application of non-invasive mobile Raman and pXRF analytical instruments, to detect mineral phases and elements associated with/replacing cobalt is addressed, with emphasis on Mamluk, Ottoman, Chinese, Vietnamese and Japanese productions. The efficiency of Ni-Zn-As diagram proposed by Gratuze et al. as a classification tool is confirmed but additionally, CoO-Fe 2 O 3− MnO and CoO-NiO-Cr 2 O 3 diagrams are also found as very efficient tools in this research. The relationship between the compositional data obtained from the artefacts and historical questions on the origin and date of their production are discussed in order to obtain a global historical view. The need of a better knowledge of (ancient) deposits of cobalt ores and the evolution of cobalt ore processing with time and place is obvious. Keywords: cobalt; composition; glaze glass; enamels; pottery; manganese; arsenic; lapis lazuli; global history; ore processing 1. Introduction Ancient glassy materials, either as glass objects or glassy coatings in the form of glaze and enamel, are basically made up of (alumino-)silicates. Colouring agents able to colour glassy silicates in blue are limited. They are also used to obtain green colours by mixing with a suitable yellow colouring agent. The limited methods available to produce blue colour are based on the following mechanisms: (i) Co 2+ ions dissolved in the glassy silicate network, (ii) Co 2+ ions located in specific sites of a crystalline phase, a pigment, prepared before and dispersed (and unreacted) in the glassy phase or formed by precipitation on cooling [1–3], (iii) S n - ions located in specific sites of a feldspathoid (lazurite, the main phase of lapis lazuli) or of a zeolite (synthetic ultramarine) [4,5], and (iv) V 4+ ions distributed in certain Minerals 2021, 11, 633. https://doi.org/10.3390/min11060633 https://www.mdpi.com/journal/minerals