Carole Dignard, Bob Barclay and Carl Schlichting
Conservation treatments often involve setting parts of an object in position and applying pressure to them in a controlled manner, for example while an adhesive sets or while a humidified object dries. The range of clamping solutions that have been used in conservation is as wide and diverse as the practitioners who have described them. Yet the art of applying pressure remains a relatively unexplored topic in the conservation literature. This talk, based on the forthcoming CCI publication The Gentle Art of Applied Pressure, attempts to address the subject in its own right.
The effectiveness of applying pressure to a join will depend not only upon the tool and technique used but also upon the condition of the material being joined, how closely the pieces fit, the surface area in contact and the shear forces that occur. Assessment of the force to be applied, and its effect upon the materials, is essential in avoiding over-compression and potential damage. In cases where the materials to be joined offer only a very small surface area in comparison to the load they are expected to bear, it may be necessary to add strengthening devices such as dowels. Distribution of the clamping force is essential and can be ensured by increasing the surface area upon which the clamp operate, for example using wide pads and resilient materials such as softwood, dense rubbers and plastics. Another important aspect is to prevent shear stresses that can lead to slippage. If pressure cannot be exerted exactly at right angles to a break (as is often the case due to the object’s configuration), some amount of the clamping force will produce shear. Thus, instead of coming together evenly, there is slippage (often compounded by the presence of the adhesive that acts as a lubricant) and the pieces move past each other like wedges. Strategies to counter shear involve immobilizing the object in the two possible shear directions (for example, using clamps, weights, or walls), or redirecting the clamping force. The latter can be achieved using cauls, that is, shaped pieces that are follow the contours of the object. Cauls also have the advantage of distributing pressure on a wider surface.
The most common means of applying pressure are through mechanical devices such as clamps, of which cams and squeeze-and-lock clamps are common examples. Mechanical devices magnify force using three basic mechanisms – the lever, the wedge and the pulley – following the law of physics whereby to gain more pressure, one must apply more travel as a trade-off. When the application of pressure requires control and fine-tuning, as is often the case in conservation work, it is usually easier to achieve this using a tool that conveys lower force with longer travel (which often translates to a longer handle). Other mechanical devices besides clamps that can be useful as well in applying pressure in a localized and controlled manner are screw threads, bands and rings, turnbuckles, wedges, springy materials and elastic materials. Besides mechanical devices, other means of applying pressure are by making use of natural forces such as gravity, vacuum, air pressure, hydraulic pressure and magnetism. Interesting applications of some of these tools and techniques applied to solve conservation challenges are presented, in the hope of stimulating new ideas and adaptations.