RESTRAINT WITHOUT STRESS, HISTORY AND PROSPECTS: A LITERATURE REVIEW OF PAINTINGS AS STRUCTURES
Barbara Whitney Keyser
5 PROSPECTS FOR THE FUTURE
INTEREST IN PREVENTING THE STRUCTURAL deterioration of paintings is increasing for several reasons. Enlightened custodians of paintings are recognizing the conservator's role in forestalling as well as repairing damage. Awareness of the effects of environmental factors on works of art has also increased. Cleaning, varnishing, and lining controversies have led to reassessment of acceptable degrees of intervention in treatment of works of art. An accompanying realization is that if many previously accepted routine treatments are judged to be excessive intervention, then works of art must be protected so that they require less frequent and less drastic treatment. Experience with the fragile and irrepairable structures of contemporary paintings has also increased awareness that prevention of deterioration is more desirable than remedial treatment which may not only alter the work in unacceptable ways, but be totally ineffective.
Furthermore, when called upon to advise artists, conservators find that their perspective converges with that of artists concerned with permanence (62). Questions arise such as: How could crack-proof paintings be produced in various styles and media? What are the most stable painting systems? Can replacement of glue sizing and conventional priming with acrylic priming prevent cracking and cupping? If artists are not to paint on canvas, what should they paint on? How do the mechanical properties of acrylic or alkyd paintings compare with oil paintings? These are problems which should by systematically studied, since the sound construction of a painting is the most basic form of preventative conservation.
Statistically there must be a critical age range in which every type of fabric-supported painting becomes susceptible to structural deterioration. Therefore a key concept for future study is rational description of the rheological state of a painting, which all conservators in fact sense intuitively whenever they assess the structural security of any painting. The rheological state is the relationship among the mechanical properties of each layer of the painting composite relative to every other layer—both the interaction of changes in each layer due to ageing and the behaviour of each component material in response to ambient temperature and relative humidity.
For example, in a typical oil-on-canvas painting over fifty years old, the paint layer has become brittle, while the fabric support has decreased greatly in tensile strength and may have undergone both primary and secondary creep. Thus the fabric is probably in a permanent set, supported and restrained by the paint/ground. This is very different from a new painting whose fabric support is tough, strong, and elastic while the paint film is flexible and capable of enduring elastic and plastic strains without fracturing.
Parameters of vulnerability thus include age (brittleness of support and paint/ground being a function of age, though the rate of embrittlement differs with the painting materials used), internal stress (largely affected by relative humidity, but also by thickness of layers), and exposure to external stresses such as stretching/tensioning, shock and vibration in shipping, impact or trauma, and flexing.
It could prove possible to assemble and analyze statistically a data base describing paintings' technique/construction, scale/size and geometry, age, environmental history, and previous treatment. Then a conservator, like an actuary, could predict the probability that a given painting of a given age in a given environment will or will not structurally deteriorate in a given time. An appropriate stage in the life of the work could be chosen for a preventative minimal treatment, or an informed decision be made whether a given work should be exposed to the rigours of travel.
Another line of future research would be to select and apply appropriate paradigms from other disciplines and from the testing programmes of industry. This approach proved fruitful for Buck, Keck, and Mecklenburg and is needed today more than ever.
Appropriate experimental methods could also resolve questions already mentioned above, such as optimal stretcher design and long-term effects of constant-tension stretcher systems; physical properties and compatibility of aged and new fabric/adhesive systems; benefits and limitations of protection from hygroscopic backing boards, loose lining, and panel stretchers; and actual measurement of the changing physical properties of painting materials as they age.
Other questions to be addressed in the future include: Can lining prevent structural deterioration, or should lining always be delayed until active deterioration has begun? If a lining treatment is chosen, how is the lining really affecting the painting as a mechanical system, how should it affect it, and what is the best way to achieve the desired mechanical properties without undesirable side effects? What are the effects of vibration, as experienced in travel, and flexing on crack formation? What forces arise within the paint film, especially thick or variable films, and what role do they play in the structural deterioration of paintings? If a painting's support is restrained from moving, will the paint layers develop stresses which would have been relieved by movement of a flexible support? What, indeed, is the optimal balance of strength and flexibility among the paint layer, ground or sizing, lining adhesive, and lining fabric or solid support?
Now that the need is so apparent, surely research topics first suggested thirty, forty or even fifty years ago and since neglected can now be taken up; and with the sophisticated tools now available—institutional, methodological, and intellectual—yield the essential information the conservation profession so urgently needs.