Conservation of Wall Painting Department COURTAULD I NSTITUTE OF ART Somerset House, Strand, London WC2R 0RN Telephone +44 (0)20 7848-2848 [email protected] This is a dissertation accepted for a Postgraduate Diploma in the Conservation of Wall Painting in the Conservation of Wall Painting Department, Courtauld Institute of Art, University of London. It is unpublished and copyright is held by the author. No quotations or information derived from it may be published without the prior written consent of the author, who may be contacted through the address above. AN INVESTIGATION OF THE USE OF SOLVENT GELS FOR THE REMOVAL OF WAX-RASED COATINGS FROM WALL PAINTINGS TOBIT CURTEIS Courtauld Institute ofArt/Getty Conservation Institute, Conservation of Wall Paintings Department. July 1991 CONTENTS Acknowledgements. 1. Summary. 2. Introduction. 3. Solvent Gel Principles. 4. Gel Components. 4.1 Carbopol 4.1.1 Thickening mechanisms of Carbopol. 4.1.2 Amines for Neutralisation. 4.1.3 Surfactants. 4.1.4 Clearance. 4.1.5 Possible drawbacks with Carbopol. 4.2 Pemulen. 4.2.1 Thickening mechanisms of Pemulen. 4.2.2 Drawbacks of Pemulen. 4.3 Cellulose materials 5. Desiderata for Test Sites. 6. Test Methodology and Procedure. 6.1 Examination and Analysis. 6.2 Preparation of Carbopol gels. 6.3 Test Procedure. 6.4 Clearance Tests. 7. Holcot Church. 7.1 Introduction. 7.2 Painting Condition. 7.3 Cross-section analysis and SEM. 7.4 FTIR and Thermomicroscopy. 7.5 Solvency parameter tests. 7.6 Solvent gel tests. 8. The Holy Sepulchre Chapel, Winchester Cathedral. 8.1 Introduction. 8.2 Technique. 8.3 Conservation History. 8.4 Painting Condition. 8.5 Cross-section analysis and SEM. 8.6 FITR and Thermomicroscopy. 8.7 Solvency parameter tests. 8.8 Solvent gel tests. 9. Westminster Abbey Chapter House. 9.1 Introduction. 9.2 Technique. 9.3 Conservation History. 9.3.1 G. G. Scott. 9.3.2 Prof A. H. Church 9.3.3 H. M. Office of Works. 9.3.4 English Heritage. 9.4 Painting Condition. 9.5 Cross-section analysis and SEM. 9.5.1 Binding Media. 9.6 FTIR and Thermomicroscopy. 9.7 Solvency parameter tests. 9.8 Solvent gel tests. 10. Analysis for Residual Materials. 11. Conclusion and Further Research. 12. Bibliography. 13. Appendices. 1. Recipes for wax and wax resin preservatives advocated by Professors Church and Tristram. 2. Solvent Gel formulas. 3. Teas Chart showing the solvency parameters of the most effective solvent gels in relation to the solubility region of beeswax. 4. Solvent Gel Tests,Summary of results reported in the proformas. 5. Fourier Transform Infrared Spectra. 6. Nitromors paint stripper. 7. Solvent Gel Tests Carried out in the Chapel Of Our lady Undercroft, Canterbury Cathedral. 8. Site proformas. 14. Plates. Acknowledgements For permission to examine and sample the paintings at the four sites studied I am grateful to the Dean and Chapter of Canterbury Cathedral and to Mr. Wolfgang Gartner of the Wallpaintings Workshop; the Dean and Chapter of Winchester Cathedral and especially to Mr. John Hardacre for his help and advice, the Parish Council of Holcot Church and the Rev. Anthony Watkins; and to Mr. Jan Keevil, Head of the English Heritage Conservation Studio, for Westminster Abbey Chapter House. For technical information on materials and for product samples I would like to thank Mr. G. Stead and Mr. John Gallagher of B. F. Goodrich and Mr. C. Drake of AKZO Chemicals. Ms. Zahira Veliz, Mr. Alan Phenix (Courtauld Institute) and Ms. Lucia Scalisi (Victoria and Albert Museum) gave much helpful advice on formulation of the gels. I am particularly grateful to Ms. Marianne Odlyha of Birkbeck College for her constant help and valuable advice especially in regard to the analysis including Fl'lK, DSC and Thermomicroscopy. I am also very grateful for the help of Mr. Raymond White, Scientific Department, National Gallery, particularly in the area of interpretation of IR spectra. For their advice and support throughout and in all areas of this research I would especially like to thank my two supervisors Ms. Aviva Burnstock, Scientific Department, National Gallery, who also carried out the SEM and EDX analysis, and Ms. Katherine Powell of the Courtauld Institute. Finally I would like to thank Mr David Park and Ms. Sharon Cather of the Courtauld Institute for their continual help during the course of this work. -5- Summarv The effects of the preservative' coatings commonly applied to wall paintings in the 19th and first half of the 20th centuries, have been both damaging and disfiguring in the longer term. Typically these coatings consist of either pure beeswax or a mixture of beeswax and a natural resin applied to the painting surface. The penetration of the coatings due to the porosity of wall paintings and the fragile nature of many of the paint surfaces, when treated, has made the removal of such materials extremely difficult. The solvent gels formulated for this study were intended to have very specific solvency parameters allowing them to dissolve a particular material, without endangering the other vulnerable components of the paint surface. The system was not be intended as an all-encompassing solvent mixture for the removal of all coatings on the paint surface. If there were more than one coating of a different nature, for instance a varnish layer covered by a wax resin coating, it would be necessary to use two different gels, each tailored by their solvent mixtures to the specific problem of each layer. The complete removal from the painting of all components of the gel system, forms an integral part of gel cleaning process. The long-term effects of the non-volatile components and their interaction with the materials of wall paintings are not fully understood and to ensure that no damage is caused by these materials the implementation of an adequate system of clearance must be undertaken. The investigations into the feasibility of such a cleaning system drew heavily on the work of Richard Wolbers who was responsible for developing the theory and formulating a range of organic solvent gel systems for particular cleaning problems in the field of easel painting conservation. The formulas developed by him provided the starting point for the development of a series of solvent gels that were applicable to the problems faced in wall paintings. Materials were chosen that had a known history of usage in general conservation, although some of these had not been applied to wall painting conservation. Much of the initial work was theoretical, involved with the chemistry of producing a gel system with the necessary attributes. The main gelling agent, a high molecular weight polyacrylic acid, Carbopol, was chosen due to its ability to effect very viscous gels at relatively low percentages. Due to the high wax component of the coatings it was necessary to produce a series of highly aromatic solvent gels. A program of tests was undertaken on a number of wall paintings that had been treated with a wax or wax/resin coating in order to evaluate both the cleaning effect of the gels and their subsequent clearance. The possibility of conducting the tests on laboratory models was evaluated, but despite the complications caused by the lack of chemical control on actual paintings, it was felt that due to the problems of reproducing the necessary conditions on a model, the results obtained from actual paintings would of be more value in examining the effects of the gels. Cross-sections were taken to establish the stratigraphy of both the coatings and the painting. This enabled the characterisation of differing surface coatings and the their relative thickness to be established in order to make a comparison with the situation after cleaning. These samples were examined both on the surface and in cross-section using visible and ultra-violet microscopy and scanning electron microscopy. Samples of the surface coating were analysed using FTIR in order to establish the exact nature of the material. This was necessary so that the solvent -6- component of the gel could be prepared to dissolve that particular material. After the cleaning and clearance tests had been carried out, further samples were taken in order to evaluate the effects of the gels as well as to check for possible residues. Examination was carried out using visible, ultra violet and scanning electron microscopy. In order to examine the problem of clearance more fully, controlled tests were carried out on a laboratory model, allowing a more detailed series of experiments to take place. The results of the project were extremely encouraging indicating that it is possible to produce a solvent gel system with the necessary solvency parameters to remove the commonly encountered wax coatings. No residues of the material from the cleaning system were detected with FTIR or SEM after clearence had taken place. This latter result must however be qualified. A negative result such as this means simply that no residue has so far been found, it is not a guarantee that no trace of the material is present and further more sensitive analysis should be carried out to ensure this. -7- 2. Introduction During the nineteenth and the first half of this century, wax and resin based treatments were commonly believed to be the most effective method for the 'preservation' of wall paintings. The original premise behind the application of such coatings lay in the mistaken belief that classical Roman paintings were executed in encaustic technique, thus leading to the conclusion that wax was responsible for their longevity and survival. The writings of both Pliny and Vitruvius appeared to confirm that the use of wax was advantageous in protecting fresco paintings.1 The saturation of the painting with such coatings was seen as a general cure for the various types of damage encountered and with the active involvement of respected men such as Professors E.