Completed doctorate Hannah Flock

Technique of single thread bonding in conservation (Bild: Heike Fischer/TH Köln)

Dr.-Ing. Hannah Flock M.A. successfully defended her doctorate in Saarbrücken on 7 May 2021; the defense took place in a hybrid format at Saarland University due to the pandemic. She completed her doctorate cooperatively under the supervision of Prof. Dr.-Ing. Stefan Diebels, Chair of Applied Mechanics, from Saarland University and Prof. Dr. Elisabeth Jägers, Cologne Institute of Conservation Sciences (Faculty of Cultural Sciences at TH Köln). The dissertation title is "Single Thread Bonding in Painting Conservation: Adhesives, Testing Systematics and Results".

The technique of single thread bonding is used in conservation to close tears and cuts and apply fabric inserts in losses of textile supports of paintings. Different adhesive systems based on natural and synthetic binders are used in various bonding techniques. However, there has been a lack of systematic research in this field.

The work aimed to develop a suitable testing system. In addition, recommendations for conservation practice were to be made based on tests to be carried out.

An initial test system was implemented to evaluate the mechanical properties of bondings with different adhesives and techniques. The tests were designed as a consecutive set-up, in stepwise extension of the parameters to approximate the application in the painting. The dissertation presents uni- and biaxial short-time tensile tests on uncoated and coated fabrics, with and without bondings.

In addition, a new test set-up for long-term biaxial tests is presented.

The evaluation discussed the advantages and disadvantages of the experimental tests and the selected parameters. On this basis, a future testing system was proposed, and an outlook was given on further considerations. As a result of the test series, suitable adhesive systems for different applications were named, and additional adhesive options were identified.

The basis of the cooperative doctoral project was Hannah Flock's master's thesis from 2013 on the topic of "New Investigations on Adhesives for the Tear Repair in Canvas Paintings: Uni- and biaxial Tensile Tests on Specimens of mended Threads and Canvas as well as free Adhesive Films". The thesis aimed to evaluate the adhesives based on a comprehensive test series showing the correlations and dependencies between different adhesives, bonding techniques, and test methods. In cooperation with the Chair of Applied Mechanics (LTM) at Saarland University (UdS), a biaxial test setup was used for the first time.

Within the framework of the cooperative doctorate, the topic was continued, deepened, and expanded to gain deeper insights into the research field of bonding tears and cuts. The conservation technique of single thread bonding to close disruptions in textile supports of paintings was carried out using fine tools under the microscope in different possible bonding techniques as well as with different adhesive options on test specimens. The requirements profile for adhesives is highly complex. The research focused on evaluating various adhesive applications through material tests, primarily uniaxial and biaxial tensile tests.

Of particular interest in this context are, among other things, the achievement of homogeneous, reproducible bonding qualities, the restoration of the original mechanical-physical properties, the durability of the bonding under permanent stress, the aging and long-term behavior, the compatibility with the original painting components as well as the long-term reworkability of the system.

The overriding goals of the dissertation were to provide direct guidelines for adhesive testing and adhesive application for painting conservators. The interdisciplinary cooperation between the LTM of Saarland University and the CICS of the TH Köln made it possible to benefit optimally from the mutual experience and opportunities in interdisciplinary exchange to establish essential principles of materials testing in restoration and conservation science.