4 INVESTIGATIONS OF PAINTINGS AS STRUCTURES 1970–1984

WIDESPREAD QUESTIONING OF traditional structural treatments for paintings has resulted in a proliferation of lining adhesives, supports, techniques, and philosophical approaches. For example, Caroline Keck (59) and Gustav Berger (9, 10, 11); have debated the merits of wax lining. Mehra (68, 69) and Fieux (35) have developed gentle lining techniques using relatively weak adhesives, while Berger (11) has favoured strong lining adhesives. Berger has argued that the appropriate use of tension can extend the life of paintings (6, 7, 12), while Mecklenburg (70) has argued against it. Some conservators believe many works would benefit from marouflage, while others prefer that no work receive any structural treatment unless absolutely necessary.

The positive result of this diversity of views is that a variety of treatments are available which can be applied to individual paintings as appropriate. On the other hand, there is still disagreement about what, in physical and mechanical terms, any structural treatment is supposed to achieve and what treatment will accomplish it safely, because there is still a paucity of experimental and empirical data on the causes of structural deterioration and the long-term effects of preventative and remedial treatments on paintings.

Two individuals, already mentioned in passing, have recently attempted to remedy this situation. Gerry Hedley has concentrated on externally applied stress to fabric-supported paintings. He has:

1. Determined that stretching a linen canvas on a stretcher results in large strains, especially in the corners (52);
2. Investigated the mechanical properties of wax-impregnated canvases (49);
3. With Caroline Villers, stressed the need for high-stiffness, non-creeping, non-hygroscopic lining (51, 53) and artists' canvases (95, 96);
4. With Stephen Hackney, studied the mechanical properties of naturally-aged linen samples (44); and
5. Measured the strength of wax-resin mixtures (93).

Hedley also called for:

1. Improvements in stretcher design,
2. Investigation of partial or preventative treatments to prolong the life of a fabric-supported painting before lining is required, and
3. more empirical observation of paintings which have survived well without treatment, or the state of preservation of paintings which have been given a known treatment at a known time in the past. (48)

On the other hand, Marion Mecklenburg (70, 71), with J.S. Colville and William Kirkpatrick (26), has quantified internal stresses in fabric-supported paintings by adapting the engineering technique of computer modelling to selected representative structural components of fabric-supported paintings, first separately and then together, at various relative humidities. The model accurately predicts some of the common crack patterns seen in fabric-supported paintings. Practical implications of these studies include:

1. The importance of environmental control, especially relative humidity, in preventing damage;
2. The extreme activity of the glue size layer;
3. The powerful effect of variations in relative humidity on the paint film and the utility of exposure to high relative humidity in plasticizing and relaxing cupped paint films; and
4. The magnitude of the structural burden carried by the size, paint, and ground layers in a fabric-supported painting, especially at low relative humidities, which is a, if not the, fundamental cause of cracking.

In fact, this approach to analysis of paintings as structures represents a major advance toward the mathematical model Urbani had called for, and is a valuable paradigm (63) for future research.

In spite of the important advances in the structural study and treatment of fabric-supported paintings since 1970, the number of significant ideas mentioned in the literature 50 years ago and still not systematically studied is striking:

1. Preventative treatments involving buffering of changes in relative humidity (74, 2, 21, 30, 37, 58, 84, 88, 98);, such as hygroscopic backing boards, panel stretchers (32), enclosure (67), and loose lining.
2. Ageing characteristics and reversibility of lining materials, as well as the efficacy of various lining methods in structurally stablizing paintings over time.
3. Assembly and study of naturally aged samples of artists' and conservators' materials (44), and exploration of appropriate artificial ageing and mechanical testing techniques. (3, 22, 41, 43, 99)
4. Intellectual integration of related disciplines, especially paint/polymer/adhesive science (78, 17, 29, 72, 80, 90); rheology (15, 33, 36, 82, 83); mechanical engineering (40, 89, 92); including brittle fracture phenomena (31, 39), effects of vibration and fatigue (24), composite materials science (61), and further application of computer modelling; textile technology (19, 25, 42, 45, 46, 47, 73, 87, 97); and textile conservation (14, 66, 76, 97, 100).

One might ask why such investigations have not taken place. Some of the reasons are the following:

1. Most practicing conservators lack time and training for conducting viable research projects.
2. Individuals with the necessary expertise, such as industrial technologists, have not been asked for their input.
3. Until the present, there have been few, if any, institutional structures encouraging or even permitting the kind of interdisciplinary collaboration required for appropriate research to take place.
4. Custodians of works of art have not demanded research on preventative conservation, since conservators traditionally have been seen as repairers of damage after the fact.
5. Last, the fabric-supported painting problem is extremely complex. It is difficult to isolate variables to study systematically the behaviour of a structure made up of diverse materials, combined in diverse ways, exposed to diverse environments, with all these factors changing over time.