X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects

Summary

Modern high resolution X-ray powder diffraction (XRPD) in the laboratory is used as an efficient tool to determine crystal structures of long-known corrosion products on historic objects.

Abstract

The crystal structure determination and refinement process of corrosion products on historic art objects using laboratory high-resolution X-ray powder diffraction (XRPD) is presented in detail via two case studies.

The first material under investigation was sodium copper formate hydroxide oxide hydrate, Cu₄Na₄O(HCOO)₈(OH)₂∙4H₂O (sample 1) which forms on soda glass/copper alloy composite historic objects (e.g., enamels) in museum collections, exposed to formaldehyde and formic acid emitted from wooden storage cabinets, adhesives, etc. This degradation phenomenon has recently been characterized as "glass induced metal corrosion".

For the second case study, thecotrichite, Ca₃(CH₃COO)₃Cl(NO₃)₂∙6H₂O (sample 2), was chosen, which is an efflorescent salt forming needlelike crystallites on tiles and limestone objects which are stored in wooden cabinets and display cases. In this case, the wood acts as source for acetic acid which reacts with soluble chloride and nitrate salts from the artifact or its environment.

The knowledge of the geometrical structure helps conservation science to better understand production and decay reactions and to allow for full quantitative analysis in the frequent case of mixtures.

Introduction

Conservation science applies scientific (often chemical) methods in the conservation of artifacts. This includes investigations of the production of artifacts ('technical art history': How was it made at that time?) and their decay pathways as a prerequisite to develop proper conservation treatments. Oftentimes these studies deal with metal organic salts like carbonates, formates and acetates. Some of them have been deliberately manufactured using suitable compounds (e.g., vinegar), others derive from deterioration reactions with the atmosphere (carbon dioxide or carbonyl compounds from indoor air pollution)¹. As a matter of fact, the cryst....

Protocol

1. Sample Preparation

1. Collection of material
   1. Carefully pick a small amount (less than 1 mg) of sample 1 under a digital microscope using a scalpel and tweezers from settings of opaque blue-green cabochons on a historic clasp, belonging to the collection of the Rosgartenmuseum Konstanz (RMK-1964.79) (Figure 6).
   2. Carefully scratch a few mg of sample 2 with a scalpel from the surface of a glazed ceramic tile, dating at early modern times, manufactured in Southern Germany, with size of 41 x 29 x 3.5 cm, and part of the collection of Landesmuseum Württemberg (no. E 3004) (Figure 7).

Results

High resolution XRPD was used to determine the previously unknown crystal structures of two long-known corrosion products on historic objects. The samples were taken from two museum objects and carefully grinded before they were sealed in transmission and capillary sample holders (Figures 6, 7). Standard measurements using a state of the art laboratory high resolution powder diffractometer in transmission and Debye-Scherrer geometry using monochromatic X-rays were perform.......

Discussion

XRPD is a suitable technique for conservation research as it is non-destructive, fast and easy-to-use. XRPD data can be used in routine qualitative analysis, owing to the fact that the powder pattern is a fingerprint signature to the corresponding crystal structure. The biggest advantage of XRPD over other analytic techniques is the ability of performing simultaneous qualitative and quantitative analysis of crystalline constituents in mixtures by using the Rietveld refinement method⁵. Moreover, the presence of.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Stadi-P	Stoe & Cie GmbH		Powder Diffractometer
Mythen 1-K (450 μm)	Dectris Ltd.		Position Sensitive Detector
Mark tube borosilicate glass No. 50, 0.5 mm diameter	Hilgenberg GmbH	4007605	Low absorbing capillaries
Topas 5.0	Bruker AXS Advanced X-ray Solutions GmbH		Powder Diffraction Evaluation Software