Conservation and restoration of panel paintings

The conservation-restoration of panel paintings involves preventive and treatment measures taken by paintings conservators to slow deterioration, preserve, and repair damage. Panel paintings consist of a wood support, a ground (linen or parchment sized with glues, resin, and gesso), and an image layer (encaustic, tempera, oil). They are typically constructed of two or more panels joined together by crossbeam braces which can separate due to age and material instability caused by fluctuations in relative humidity and temperature. These factors compromise structural integrity and can lead to warping and paint flaking. Because wood is particularly susceptible to pest damage, an IPM plan and regulation of the conditions in storage and display are essential. Past treatments that have fallen out of favor because they can cause permanent damage include transfer of the painting onto a new support, planing, and heavy cradling. Today's conservators often have to remediate damage from previous restoration efforts. Modern conservation-restoration techniques favor minimal intervention that accommodates wood's natural tendency to react to environmental changes. Treatments may include applying flexible battens to minimize deformation or simply leaving distortions alone, instead focusing on preventive care to preserve the artwork in its original state.
Preventive Care.
Preventive conservation involves the mitigation of potential threats to the stability of an artifact or specimen through a number of means. Threats such as fire, flood, natural disaster, physical force, and theft cannot be foreseen and inherent vice can only be addressed through prudent risk management, care, and handling of museum objects.
Conservation and curatorial staff can mitigate risks by correcting improper storage conditions which may lead to damage from pests, water, humidity, and mold. These issues can be further catalyzed by regional and seasonal environmental fluctuations and pollution. Proper handling protocols can help prevent chemical damage caused by contact with skin oils and structural damage from physical stress. Deterioration also occurs as a result of light exposure when objects are examined, exhibited, photographed, and transported.
Environmental Conditions.
Fluctuating humidity and temperatures can lead to structural damage of a painted surface due to long-term cumulative damage. In panels that are made up of multiple pieces of wood composing the larger surface area, the damage is most often visible where the pieces of wood are attached. Temperatures should range between +/- 5% of 70°F and the humidity between +/- 5% of 50% Relative Humidity. A Relative Humidity over 65% can lead to mold growth and dry rot. 
Panel paintings are subjected to various environmental conditions during exhibition. Regulating temperature, relative humidity, light intensity and duration of exposure, and atmospheric pollutants are essential factors in painting conservation. Air filtration via HVAC systems can reduce gases and airborne pollutants within exhibition spaces and storage areas. Maintaining light levels as low as possible, between 50-150 Lux, will minimize cumulative and irreversible light damage.
Integrated Pest Management.
Integrated Pest Management policies are created to reduce or limit any problems relating to pests damaging objects within collections. Basic concepts discussed in Integrated Pest Management Policies include restricting food and sugary drink consumption in exhibition, storage areas and surrounding areas along with maintaining cleanliness in exhibition and storage areas. As noted, storage also comes into play when it comes to pest management, climate control, assessment of HVAC systems, and contamination due to exposure.
Handling.
Prior to any handling, an examination of the work should be completed, including looking for signs of weakened structure and material stability. Both the painting and the space to which it is being transferred should be measured. These notes should be shared with preparators, registrars, and curatorial staff. The recommended handling method for panel paintings is similar to that of canvas and other two-dimensional works. Conservators are the only persons that should touch the front or back of a painting. The panel should be held by the edges using nitrile gloves and carried with two hands or in tandem by two people, depending on size. Paintings should never be handled more than one at a time. The procedure includes planning the move, handling, and communication with staff. Painting carts and other tools may be used, including a pallet jack or furniture dolly for moving a panel within a crate. The Metropolitan Museum of Art, the Canadian Conservation Institute, and the Australian Institute for the Conservation of Cultural Material provide general guidelines on proper care and handling. 
Storage.
Typical storage units in museums include sliding screens or slotted shelves made out of MDF and heat-treated wood coated in polyurethane. "S" hooks are used to hang objects with the proper hanging hardware (e.g. D-Rings or eye brackets with picture wire) onto a screen. Temporary storage solutions include the use of ethafoam, moving/packing blankets, and archival corrugated cardboard. These storage methods are employed in order to minimize exposure to heat, light, and humidity, and pests. All exposure is cumulative so it is important to plan carefully the challenges of exhibition conditions, as well as transportation and storage of objects when they are not being displayed. As objects become more fragile over time, the ability to display them can be challenged by deterioration concerns.
Agents of Deterioration.
Damage may occur for a variety of reasons, generally known as agents of deterioration. Typical agents of deterioration include:
Physical Force.
Mishandling, overhandling, being dropped or stored improperly can all cause physical damage to panel paintings. 
Fire.
Fire may destroy a painting entirely, or damage may also be caused due to the heat and smoke of a fire. Paint may become brittle or wood may warp.
Temperature and Relative Humidity.
Paintings may be affected by fluctuating temperature and Relative Humidity. Low Humidity reduces the chemical changes to the materials in a panel painting but raises the risks of mechanical damage to the paint by making it more brittle. High temperatures and humidity in enclosed storage or exhibition space may lead to excess moisture that warps the painting. High Humidity reduces mechanical damage such as brittle paint but raises the risks of biological organisms, e.g. white efflorescence and green-to-black stains on a panel painting. High Humidity also raises the risks of curving or warping of the wood over time, forcing the paint to flake off.
Water.
Water damage may cause swelling, as seen in a case study by the Getty Conservation Institute, and water-soluble paints and other materials may dissolve. A wooden panel may also be distorted, split, shrunken, or stained when subjected to water. Mold may also occur as the materials are organic.
Pests.
In this case, pests are defined as living creatures able to damage or destroy material culture. Microorganisms such as mold or bacteria are considered pests, as are insects and rodents. Any of the three may cause damage to a panel painting, in particular wood pests such as termites or carpenter ants.
Light and Ultraviolet Radiation.
Overexposure to light and ultraviolet radiation may cause fading over time. A material's colorant sensitivity, or the estimate of how much light exposure it should be able to tolerate before fading, determines how an object may be stored and exhibited. When handling panel paintings they should never be exposed to heat including photographic lamps It is recommended that HMI lamps are used for photographing panel paintings.
Examination.
The first step in developing a conservation-restoration plan is close examination to estimate the condition of both the panel and paint. This allows conservators to ascertain the full extent of damage and make a risk assessment. Techniques include visual observation and technically-assisted examination through diagnostic studies and analysis.
Structural.
Technologies commonly used to examine the structure of paintings and identify previous treatments include radiography and infrared reflectography (IR). Radiography reveals the type, condition and density of the wood, as well as any pest damage or activity. Furniture and Larder Beetles 
 can burrow into wood compromising its internal structure and should be treated before proceeding with the restoration. IR, raking light photography, and low-magnification observation are useful in determining damage and distortion to the wood and paint surface. 
Surface and Painting.
Photographic documentation, magnification, and visual observation with the naked eye under diffuse orraking light are common methods of surface examination. Radiography can also be used to revel the opacity of specific paint colors. Degradation to the surface of panel paintings is often a sign of underlying structural issues, temperature and humidity fluctuations, and wood-boring pests. 
Examples of Technology.
These technologies have been employed in the conservation of Millais's painting, "Ophelia," at the Tate museum. Although this example is not directly related to panel paintings, it is a valuable demonstration of the technology.
Treatments.
Treatments applied to paintings in order to mitigate damage caused by various agents of deterioration are varied. Treatment consists of any efforts made to stabilize, restore, or repair an object; panel paintings, being heavily wood-based, may need treatment for pests, weakened structural supports, failed adhesives, temperature and relative humidity damage, general dirt and grime, and more. Documentation of all treatments applied to a panel painting is crucial, as it determines the course of future conservation-restoration efforts. The following treatments are often seen in conservation of panel paintings: 
Pest Management.
Exposure to pests may cause structural damage to wood panels and supports and should be eradicated prior to conservation to ensure the longevity of the object and the effectiveness of conservation efforts. Pest management methods include:
Overexposure to fluctuations in Relative Humidity and temperature should be monitored when managing pests to prevent damage to the painting.
Creating Supports.
Each panel undergoing conservation has unique considerations when creating support structures. Conservators must draw upon their experience in formulating treatment plans in order to develop the best treatment plan for each individual panel painting. Two supportive structures commonly used in conservation include: 
Applying Adhesives.
Cracks, loosened joints, and other such damage to a panel painting, which may be caused by various agents of deterioration, can be treated by applying the proper adhesive. Adhesive treatments include:
Types of adhesives:
Transferring Panel Paintings.
Transfer of Panel Paintings from an unstable panel to a new support is considered to be a delicate process. This process occurs when there has been a form of degradation in the original panel (possible causes include pest damage, burrowing, warping, or panel thinning). The transfer should be made to a similarly aged panel, made of the same wood type. These transfers have become almost obsolete given advances in the conservation field, but were common in Italy into the 1950s. Early efforts to transfer panel paintings were unrefined and lead to distortions in the paintings, but in later years the technique became more refined.
Drying.
The recommended procedure for panels submerged due to flooding is to exposure to a long drying process prior to any further conservation work. This process should not be rushed in order to prevent further damage to the panels.
Conserving Painted Surfaces.
The conservation of the surface of a painting varies depending on the materials used in a painting as well as its condition. Generally, conservation of a painted surface includes cleaning, removal, and replacement of degraded varnish and the restoration of paint losses. According to the Tate Museum, "cleaning is a particularly delicate and demanding part of conserving and restoring paintings. Layers of dirt, discolored varnish, and old restorations that may be disfiguring or obscuring parts of the composition are painstakingly removed."
The evolution of modern materials that can be used in panel paintings and their conservation has positively impacted conservation techniques. "The introduction of synthetic binders—most notably acrylic, alkyd, polyvinyl acetate, and nitrocellulose—has resulted in paints that exhibit fast drying times, reduced yellowing tendencies, a vast range of appearances and handling properties and, in the case of emulsion formulations, great flexibility and the elimination of organic solvents as thinners and diluents," according to the Getty Conservation Institute.
Conservation ethics resources.
The care and management of cultural heritage materials should be addressed with consideration for the physical and symbolic integrity of the artifact. While the virtues of object integrity may be contested in professional circles, the ultimate goal is preservation. Institutions are governed by their mission statement, and international, federal, and local laws surrounding cultural heritage objects. It is critical that thorough research is conducted not only on the materials but also on the ethical and moral implications of handling figuratively and literally valuable artifacts. The following links address conservation ethics, including the restoration of panel paintings.