Emma and I recently took samples of metal thread and spangles from the Spangled Bed for the purpose of analysis. Using resources provided by the Institute of Archaeology at University College London. They were to be analysed with optical microscopy and scanning electron microscopy.
SEM: Scanning Electron Microscopy is a process by which the elemental composition of materials can be determined by exposing samples of material to an electron beam which reflects off the sample surface at an angle. The angle of reflection is measured by the machine, which uses standard angle measurements of different elements to determine the elements that compose the sample being tested.
By using this method it has been possible to determine, once and for all, the composition of the spangles which has long been an unanswered question. We took the opportunity while we had access to the machinery to analysis some of the metal threads from several other important pieces, such as the King’s Bed and one of X-frame chairs of state.
Additionally, photographs using a digital microscope were taken of the threads and spangle samples under high magnification. These show the construction of the threads and the corrosion products which have formed over time. Images of the spangles can be seen in the photographs below, before and after an experimental cleaning trial.
Some metal threads were also taken from the head cloth of the bed for analysis.
The sampled thread proved to include gold, silver, copper, and traces of several other elements which make up the corrosion products.
Manufacture and Deterioration of the metal threads:
Metal thread is made by hammering gilt silver very thin and cutting it into fine strips which are wrapped around a textile core. Silver and other metals present in such threads are susceptible to the agents of deterioration.
The chief component of the black tarnish that forms on household silver is silver sulphide, which starts in damp conditions (the rate of the tarnishing reaction accelerates about 70% relative humidity). It is caused by the reaction of silver with sulphur in the air. In the past, there were many sources of sulphur in historic houses, such as tobacco smoke and degrading wool fabrics. Many other gaseous pollutants also affect metal threads.
As metal thread is composed of two very different materials, with different strengths and properties, the cleaning of metal threads is one of the most difficult challenges in textile conservation. Methods which clean the textile elements (water, detergents, and solvents) are too gentle to clean the metal threads and methods for removing tarnish and corrosion products from the metal elements (stronger solvents, abrasive pastes, and acids) are too strong for use on the textiles. Silver sulphide is insoluble in solvents and must be removed with acids.
The results of the analysis have answered some long standing questions and will inform the National Trust textile conservators on exactly what materials they are dealing with for cleaning trials on parts of the bed fabric later this year.
MSc Conservation for Archaeology and Museums, UCL