Abstract
This paper presents applied research on stretchers for double-sided canvas paintings based on experimental tests and conservation practice. The mechanical behaviour of canvas and its inner tensile distribution are discussed, and tensioning systems aiming to provide even distribution of force are proposed and put into practice.
Turbulence caused by currents of air, which might lead to deformations and consequent mechanical damages, can be mitigated by a mechanism able to absorb and dissipate vibrations using a viscoelastic material. Studies and research carried out to date have led to the practical construction of tensioning structures made from wood, aluminium and other materials from other fields. Applied structures and devices, as well as some prototypes, are described.
Keywords
Two-sided painting, stretcher, frame, continuous tension, vibration control
Self-expansion stretcher for two-sided paintings: floating auto-adapting suspension system
Franco Del Zotto RCA
Via Nanino 28
I – 33010 Reana del Rojale (UD), Italy
Tel/fax: +39 432 857 857
E-mail: francodelzotto@libero.it
Introduction
The aim of this work is to offer some technical and practical guidance on the proper conservation of canvases painted on both sides, for example, standards and other decorative objects. It illustrates the phases of a research process dating from 1980 and details observations made along the way.1
Background theory and some other established aspects of the preservation of canvas paintings are omitted here due to space constraints. Essential reference material can be found in the bibliography.
There are, however, a number of fundamental guiding principles that should find universal application in restoration (even in non-invasive interventions such as environmental control). We constantly abide by four of them.
•Minimal intervention. The physical media necessary for the intervention should interfere as little as possible with the delicate ‘equilibrium’ of the artefact. Every addition, modification and deletion has the potential to shift the aesthetic, historical, material and functional balance of the work of art.
•Preservation of the artefact in its entirety and articulation, i.e. aesthetic, material and functional succession and layering. A work of art is such not only for its ‘skin’, its polychromy, but also for its ‘flesh and bones’ – in a painted canvas, the textile support and the supporting stretcher. We must become accustomed to upholding a work of art for what it really is: if it is born a sheep (frail) we shall not transform it into a lion (strong) because its structural characteristics have intrinsic value per se.
•Preservation of the equilibrium by providing proper microclimatic conditions. A large number of studies focus on the influence of the climate on the artefact: relative humidity (RH), temperature, chemical and micro-biological attacks serve as risk factors and may cause degradation. There is widespread consensus that RH and temperature cycles are very significant for the accumulated damage to the complex layer systems (hereafter, ‘canvas-painting layers’) that paintings on fabric supports represent.2 The obvious approach towards limiting these climatic oscillations is a proper microclimate; but this is difficult to establish systematically even in the most advanced museums.
•Avoid alienating the artefact from the context in which it was created or through which its presence has become ‘historical’. Turning every instance of art into a museumlike venue in order to solve the conservation issue is hardly an appropriate proposition. Neither is encasing the artefacts in ‘controlled environment coffins’ which contribute only to the mummification of the art. Isolating the ‘specimen’ from is own roots will deprive it of its historical, social and cultural role that is unique to its peculiar reference environment.
It follows that works of art will be subject to the environment in which they are located, and this will be the situation in the future.
We have focused on the problem of double-sided canvas paintings, emphasizing solutions integrating their proper presence in the relevant context and their optimum conservation. Perimeter cushioning and constant tension stretchers have been the outcomes of our experimental application of studies into the tension distribution in canvas paintings. This paper addresses more specifically the reduction of mechanical stress in the ‘canvas-painting layers’ system and the development of an auto-adaptive floating suspension system that can be integrated in proper display frames as appropriate.
Every material expands and contracts, thus dissipating some of the stress being generated within it by temperature and RH variations and the physical changes occurring with ageing. The painted canvas can be approximated to an orthotropic multilayered membrane. Tensioned in the X–Y plane it will strain according to an elastic modulus that is the result of the combined moduli of all its constituents.
There is obviously no guarantee that all the strata will be kept within their yield point.
Furthermore, different expansions in the layers generate shear forces at the interfaces between layers. If the shear overcomes the tenacity of the bond, separation and ‘peel-off’ will occur. If the said bond is strong enough to resist the shear force, this will be transferred to the bulk of the layer and turned in tangential tensions, causing cracks perpendicular to the direction of force. In canvas paintings – special multi-layered membranes – the phenomenon is particularly evident.
The old rule of ‘fat over lean’, indicating the precept of layering more elastic strata (i.e. fat) over strata that are less toleratant of expansions (i.e. lean), breaks down at the canvas interface.3 As illustrated by the basic theory of composite materials, it appears that the largest discontinuity in the canvas-painting layers system is really the canvas itself. Preliminary treatment of the canvas by stretching renders it ‘lean’ by the application of high levels of tension (stress), thus ensuring the complete distension of the fabric (strain), i.e. the final positioning of the woven network of yarns in a stable configuration. On this ‘lean’ layer, the subsequent painting layers are comparatively ‘fatter’, thus assuring the congruency of the layers.
Time will inevitably alter the status of the artefact in a number of aspects:
•fabric stability, evidence of localized or generalized repositioning of yarns due to internal movements
•dimensional stability of the individual yarns with variations in RH and temperature change; volume changes (thickness and length) in individual yarns
•changes in the elastic modulus and yield point (general degradation) due to ageing of the fabric and other layers
•localized alterations in the paint film and foundation layers.
The instinctive tendency to ‘stretch the canvas’ in such a way as to plasticize all the areas capable of flowing and to reach a stable configuration involves, by definition, exceeding the yield point of the canvas: it is per se the cause of some premature ageing modes. This happens when the Maximum Sustainable Tension (MST) – the tension that the fabric can sustain indefinitely – is exceeded. In a traditional wedged stretcher this happens regularly.
Cyclic re-tensioning of wedged stretchers and rigid strainers bear silent witness to the continuous creeping of the canvas. Dimensional stability within a canvas painting can be defined in rigorous geometrical language as the absolute constancy of the dimensions of any area in the canvas-painting layers system. But it can be also defined in a more technological way as equality of the deformation of the canvas and painting layers which results in the minimum residual stress averaged across the layers.
We believe that the solution best suited for the long-term conservation of the object is to maintain the fabric within its MST, with the added constraint that the tension itself shall be maintained constant in such a way as to preserve the fabric’s configuration. This implies the preservation of the internal geometry of the canvas that is bonded to the painting layers.
This forces us to abandon the idea of a falsely immobile canvas bonded to an ideally rigid stretcher and directs us instead to search for a ‘stabilized’ canvas in continuous equilibrium of internal and external forces. We advocate what seems
to be a contradiction in terms: we want to maintain in static equilibrium something that is constantly on the brink of movement.
Developing the concept of our ‘self-adjusting continuous tension stretcher’ for single-sided canvas paintings,4 we propose a further enhancement which is applied to the conservation of double-sided paintings. The system is called floating, as it decouples entirely the canvas-painting layers system from any rigid constraint, both orthogonal and tangential to its perimeter, and auto-adaptive as it intervenes in a way strictly correlated to the real instantaneous condition of the canvas.
The stretcher frame for both-side canvas paintings
‘Both-side paintings’ is how we classify those artefacts on textile supports exhibiting paintings on both recto and verso, either the same or different subjects; examples are standards, flags, decorated partitions, organ doors. It is in these instances that the suspension technology we are proposing best expresses its functionality.
In the design phase of this project, the fundamental problems relevant to a double-sided painting were identified. Some of the most significant are as follows.
•Image availability of 360° and proper positioning. If the image has been conceived in this way, it is incorrect to limit the perception range of the viewer.
•Sensitivity to micro-climatic variations. The complete recto–verso coating by size, ground and paint film is not enough to shield the fabric and does not provide the system with complete stability. Exposure with conventional paintings of one face to the environment. A standard has two faces exposed.
•Dynamic loads generated by air movement. A two-sided canvas painting may act rather as a sail, subject to the physical action of the surrounding moving air. It is almost common practice in contemporary restoration to protect the verso with membranes or panels to cushion this effect by air damping, but this approach cannot be applied for a standard.
•Simple and non-invasive system. Planarity of the object shall be achieved through a technological solution that has limited dimensions and provides a ‘soft’ aesthetic impact.
•The stretcher shall provide uniform and tuneable tension. It shall enable the designer to calculate the parameters in the analysis phase and be adjustable on site in order to deliver the necessary stress–strain performance.
•Aesthetic finish. If necessary, the system shall be suited to constitute a display frame with the necessary shapes, forms and decorations.
•Long-term maintainability. The floating auto-adapting suspension systems shall enable easy and reliable tuning, verification and serviceability.
Design
On this basis, we directed our research to the design of a floating auto-adapting suspension system integral to the frame.
The stretcher is designed to intervene in the correct way, to properly react to the expansion or contraction of the canvas-painting layers system, which contracts and expands with its own characteristics. A number of issues were identified.
First, during contraction the stretcher shall ensure that the canvas does not exceed its MST. Second, during expansion the stretcher shall not allow the initial pre-set force, and hence tension, to drop.
If the canvas is maintained within its MST it will not exhibit creep. This means that a painting supplied with a suspension system respectful of its MST shows a minimum value of relaxation. For this reason we have chosen the implementation of the first point (auto-reducing stretcher), considering fabric distortion the most serious problem. Hence, our aim is not to deform the fabric, and thus not have to ‘recover’ its deformation later.
Specifically, for the second point, in addition to a mechanism that responds to canvas expansion, a tuneable reduction of the tensioning springs has been provided. It enables us to establish a ‘neutral position’ or equilibrium point in the tension in early canvas mounting. If necessary, expansion can be prevented totally.
Figure 1. Stretcher for double-sided paintings in wood. a: Fixed wooden stretcher, b: lid as a decorative frame in wood, properly carved and decorated, c: C-section aluminium profile, d: segmented hard-wood reinforcing sandwich, e: perimetral canvas extension, f: plunger, g: suspension spring, h: aluminium sleeve, i: aluminium/Teflon slide, l: tensioning register, m: dead stop
The first design of a load-bearing structure for a double-sided painting was built around a rigid wooden stretcher, aesthetically appropriate, split into two sections: one (A) incorporating the suspension system, the second (B) as a lid (see Figure1). The joint between the sections allows for structural co-operation of the two members, enabling the use of slender cross sections. Inside the stretcher/frame there is a second free-edge aluminium stretcher with a lowered C-section (C), in which the perimeter of the canvas (E), reinforced as sandwich between two profiles of hard wood (D), is free to slide.5 This outer profile is interrupted every 4 to 6 cm to facilitate longitudinal sliding and a proper force distribution. The perimetral fabric buffer extension contributes to a better distribution of the longitudinal strain. A number of spring-loaded plungers (F, G), an interface between the C profile and the wooden stretcher all enable final tensioning of the canvas. The proper design of the load transmitted to the canvas is carried out during the dimensioning of the spring and can be corrected on site via the tensioning registers (L). This allows positive control for establishing the desired MST condition. The system features a ‘dead stop’ (M) between the free-edge C-stretcher and the wooden stretcher to limit the travel in the event of catastrophic failure of the canvas. If necessary and proper, the whole system can be hidden by a carved, gilded or polychrome set. The connection between the two components is via pressure latches, which enable easy access (see Figures 2 and 3).
To optimize the device, an aluminium system with reduced section and weight was developed (see Figure 4). A further dimensional reduction was achieved via the use of a PEEK® (polyether ether ketone) profile (D) within a pocket obtained by folding back the perimetral fabrics (E) (see Figure 5).
According to the various configurations, three different mechanical solutions have been developed: long bracket, short bracket and bayonet. The short bracket is for non-straight-line contours and the long bracket allows a significant reduction in the number of plungers. Both are equipped with rounded, dry lubricating sliders for the canvas.
The most compact is the bayonet configuration. Being more rigid it provides a more even load distribution. The spring tuning system is an internally threaded backrest which slides inside the plunger recess. All friction points are lined with Teflon or PEEK.
Underlining our use of a systematic and holistic approach towards the painted artefact and its support, we have been striving towards further approximations of the ideal solution, well aware that we may not establish any ‘truth’ but moving in small steps towards the ultimate comprehension of the complex system. Reconsidering the general problems, we have two main classes of loads:
•quasi-static loads: changing slowly over time and far from resonant •dynamic loads: rapidly changing or close to resonant.
A chief example of the first type is the contraction of the painting. Its challenges are addressed by the floating auto-adaptive system, notwithstanding a limited
Figure 2.
paintings in wood with a second softer spring for vibration control and progressive restraint. Pressure latches connect the two sections. These enable easy access to the internal mechanism
Figure 4. Stretcher for doublesided paintings with reduced section and weight. a: Fixed aluminium stretcher, b: lid as a decorative frame in wood, properly carved and decorated, c: C-section aluminium profile, d: segmented hard-wood reinforcing sandwich, e: perimetral canvas extension, f: plunger, g: suspension spring, h: aluminium sleeve, i: aluminium/
increase in tension with the contraction due to the spring linearity. If properly catered for by design, the initial pre-set force (Fi) can be maintained within the MST for the whole range of contraction.
Aerodynamic loading
An example of the second group is the aerodynamic load. A painting located for both recto and verso viewing is easily subject to the full effect of the surrounding moving air. It is worth recollecting that wind induces a primary effect of about 100 Pa for each km/h (purposely disregarding the Cx[Q1] ). Since 1 Pa corresponds to 1 kg/m2, a painting 1 m2 exposed to an orthogonal 10 km/h wind is subject to about 100 kg force. It is easy to imagine the effect of gusts and the induced overpressures. Aerodynamic loads are generally rapid and large. It is likely that a system unable to compensate will systematically exceed the MST. In addition, the higher frequency ripple effect, induced by the wake turbulence, loads the painting if the wind has components parallel to the stretcher. If the force causes the canvas to deflect, the force is maintained within the design parameters by the springloaded plunger, which reinstates the original conditions upon cessation of the load. Some questions, such as load frequency and spring linear elasticity, require further analysis.
Usually the force is intended as static: we could not find reference to rapid loading–unloading cycles and tension-induced backlash. A more accurate and, we hope, simple description is necessary here. If we assume that the painting is inextensible, what happens in the centre and at the border? (For didactic reasons, the description here is at the level of ‘finite differences’.) As an example, a painting measuring 2 m x 2 m (semi-aperture: 1 m) is deflected by 40 mm in its centre. The centre has moved by 40 mm while the border has moved only by 0.8 mm (theoretical angle: 2.28° deflection). For this kind of movement, the centre of the canvas moves about 50 times faster than the periphery as it covers 50 times the distance in the same time. Apparently, movement at the stretcher is negligible. Careful analysis reveals how a small movement at the border is amplified towards the centre of the canvas. If the stretcher causes a rapid return to the initial configuration, the canvas in the centre will be accelerated past its rest position and overshoot in a bulge in the opposite direction. The cycle the repeats itself, dampened only by internal friction and the surrounding air.
The ‘deflected configuration’ graph illustrates such an occurrence (see Figure 6). The elongations with 20-, 40- and 60-mm deflections are 0.27, 1.07 and 2.4 mm respectively. Without an auto-adaptive floating suspension these necessarily have to be furnished by the canvas through internal deformation. Furthermore, and
Figure 5. Stretcher for double-sided paintings with PEEK profile. a: Fixed aluminium stretcher, b: lid as a decorative frame in wood, properly carved and decorated, c: brackets or bayonet clasp, c1 and c2: dry lubricating slides, d: PEEK profile, e: perimetral canvas extension, f: plunger, g: suspension spring, h: aluminium sleeve, i: dry lubricating lid, l: tensioning register, m: damper
Figure 6. The elongations with 20-, 40- and 60-mm deflections are 0.27, 1.07 and 2.4 mm respectively. Without an auto-adaptative floating suspension these necessarily have to be accommodated by the canvas through internal deformation [Q4] [Q4]
Figure 7. a: High damping in a polymer reduces the impulse peak of a shock wave over a long time frame. Sorbothane reduces the impact force up to 80% and brings the mass to rest slowly, facilitating better. b: Low amplification at resonances shows Sorbothane’s damping superiority over other elastomers. Isolation at large frequency ratios also shows Sorbothane’s capability to isolate vibration. c: By comparing the area under the curves, Sorbothane removes most of the impact energy from the system
perhaps more importantly, the back and forth flexing of the painted film is an obvious cause of degradation and detrimental to the life span of the standard.
We aimed to explore new avenues to address this challenge. As a preliminary measure, a second softer suspension spring was added, counteracting the primary for vibration control and progressive restraint (see Figure 2). It was installed with very low pre-loads and enabled a neutral point tuning.
The dissipation of the energy can best be achieved through a viscous medium. Even a conventional rubber can be improved upon towards enhancing its purely viscous characters. An appropriate commercially available material is Sorbothane® (see Figure 7). Results are evaluated as shown in Figures 3, 4 and 5.
To avoid the progressive increase in tension with displacement, from the linearity of the springs, a non-linear kinematic link was introduced[Q2] . Implementation was through a progressive curvature arm (R) and a pullrod (S) interposed between the spring and the canvas fixtures (see Figure 8). The force is transferred to the arm via a sliding bracket (Q). The whole mechanism is made of
a dry self-lubricating, low-creep material such as PEEK. The arm slides inside the bracket (sliding restraint). One end pivots around a fixed point whose distance to the plunger is constant. The other end is linked to the bayonet profile via the pullrod. The arm is not straight, as it will enact a variation of the lever’s purchase following a prescribed angular relationship.
This provides a modulation of the force, which can be designed not to be a function of the displacement but instead as a constant value practically independent of the spring elongation. We therefore reach the ideal situation: the painting is intrinsically stable within the MST, a stress that can be sustained indefinitely. It is, in fact, essentially the creation of a ‘mechanical potential valley’, a stability centreposition towards which the canvas returns once the perturbing event is exhausted. This is possible only if the suspension system provides a re-centring force resultant for every displacement of the canvas from the geometric centre of gravity, as set during tuning. A few percentage points of variation over the whole travel are sufficient to relocate the canvas in the centre of the auto-adaptive floating suspension system.
Conclusion
Our research on both-sided paintings was developed on the basis of previous studies on single-sided paintings. For the research and experimental programme developed in our studio, we chose to pursue the benefits of minimum intervention in order to respect historical and cultural properties of the whole elements in a work of art. Bearing in mind that the a priori intervention for an even microclimate favours ‘mummificaton’ of the work of art, we know that this condition is difficult to achieve satisfactorily. The only remaining possibility for mitigating thermal and hygrometric variations and the influence of turbulence is to intervene on the suspending mechanism of the painting.
Our first solution was a stretcher able to sustain the canvas in plane using an autoadapting floating system. The second dealt with two other variables: the linearity of the springs (which can alter the force transmitted by the canvas movement), and air turbulence, which can cause vibrations inside the composite structure. Research is still underway, but results so far reveal two practical solutions: a shaped arm, connected to the main spring, that is capable of establishing linear response of the spring and a damper of viscoelastic material able to absorb turbulence vibrations.
Mathematical and laboratory data and results will be the subject of a further report. Note however that they support the results of the experimental tests and applications made to date and reported here. The positive outcome suggests this is a fruitful line of inquiry, worthy of further investigation.
Acknowledgments
The author thanks eng. Luca Piciaccia, arch. Roberto Saccavini, eng. Agostino Bruschi and Mrs Francesca Tonini for their indispensable co-operation.
Notes
1In his diploma thesis (Del Zotto 1983), the author shows, among other things, technical solutions for the preservation of the original stretchers and the ‘elastic’ mounting of paintings.
2Painting layers are size, ground, paint film and varnish.
3An outer ‘fat’ (i.e. soft) film bonded to a ‘lean’ (i.e. rigid) foundation resists climate changes and mechanical damage exceedingly well. An outer layer originally more rigid than its substrate, or one that has become so with ageing, invariably cracks and lifts.
4The first ‘self-adjusting continuous tension stretcher’ was completed by the author in 1985 and the details published in 1989 (Del Zotto 1989).
5If necessary the border areas can be integrated with a consistent insert of fabric butt-joined. This ‘expansion’ has to be realized in the appropriate way and material, taking into account a number of variables such as size, preservation condition, typology and conservation environment.
References
Berger, G A and Russell, W H, 1988, ‘An evaluation of the preparation of canvas paintings using stress measurements’, Studies in Conservation, 33(4), 187–204.
Berger, G A and Russell, W H, 1990, ‘Changes in resistance of canvas to deformation and cracking (Modulus of Elasticity „E“)[Q3] as caused by sizing and lining’, in Grimstad, K (ed.), Preprints of the 9th Triennial Meeting of the ICOM Committee for Conservation, Dresden, Paris, International Council of Museums, 107–112.
Bilson, T, 1996, ‘Canvas shrinkage: a preliminary investigation into the response of a woven structure’, in Bridgland, J. (ed.), Preprints of the 11th Triennial Meeting of the ICOM Committee for Conservation, Edinburgh, London, International Council of Museums/James & James, 245–252.
Colville, J, Kilpatrick, W and Mecklenburg, M M, 1982, ‘A finite element analysis of multilayered orthotropic membranes with application to oil paintings on fabric’, in Bromelle, N S and Thomson, G (eds), Science and Technology in the Service of Conservation, Preprints of the IIC Congress, Washington, DC, London, IIC, 146–150.
Del Zotto, F, 1983, Un Dipinto di Francesco Floreani: La Trasfigurazione (1584), diploma thesis in conservation.
Del Zotto, F, 1989, ‘Preservation of canvas paintings: structural solutions in relation to environmental changes’, in Proceedings of the European Symposium: Science, Technology and European Cultural Heritage, Bologna, Oxford, Butterworth-Heinemann, 717–722.
Del Zotto, F, 1990, ‘Preservation of canvas paintings: structural solutions in relation to environmental changes’, in Grimstad, K (ed.), Preprints of the 9th Triennial Meeting of the ICOM Committee for Conservation, Dresden, Paris, International Council of Museums, 113–118.
Hedley, G A, 1988, ‘Relative humidity and the stress/strain response of canvas paintings: uniaxial measurements of naturally aged samples’, Studies in Conservation, 33(3), 133–153.
Mecklenburg, M F, 1982, Some Aspects of the Mechanical Behavior of Fabric Supported Paintings, Smithsonian Institution Report, unpublished.
Young, C R T, Hibberd, R D, 1999, ‘Biaxial tensile testing of paintings on canvas’, Studies in Conservation , 44(2), 129–141.
Materials
PEEK® (Polyether ether ketone), www.goodfellow.com. Sorbothane®, www.sorbothane.com
Delzotto queries
There’s a lot of text but very few headings. I have added one – Design – but is it possible to include some more to split the entry into more manageable sections?
[Q1] Cx. Correct or C x ?
[Q2] Sense? Increase in tension was due to linearity of the springs?
[Q3] „E“ correct?
[Q4] This is a direct repeat of the text. Please supply an actual caption, e.g. ‘deflected configuration graph showing…’