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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Editor: Mary Gridley
Contributors: Dennis Calabi, Laszlo Cser, Mark van Gelder, Debra Daly Hartin, Perry Hurt, Cynthia Lawrence, Montserrat Le Mense, Gordon Lewis, Michael O'Malley, Robert Proctor, Allyn Rosser, Victoria Montana Ryan, George Schwartz, Laurent Sozzani, Carolyn Tomkiewicz, and Frank Zuccari.
This wiki entry was created from content contributed by the above to the Paintings Specialty Group Listserv in 2012.
Backing boards can be described as any stiff material fastened to the reverse of a painting's stretcher or strainer and whose function is protection from various physical and environmental hazards.
Backing boards in painting conservation have a relatively short history. Their use parallels the availability of cheap, large scale cardboards, pressed boards and the like, as well as the recognition by conservators that they are a relatively low cost, non-invasive method for mitigating a number of potential problems. They have been in regular use for the last 40 years.
Backboards were generally made of cardboard. Occasionally hardwood, wood laminates, fabric or wire mesh screening were employed.
Developments in the paper and plastics industries have widened the range of materials suitable for backing boards. Lightweight, large scale boards tend to be either paper based or plastic.
Backing boards serve to protect a painting from several common dangers:
--dust and dirt
--physical blows from the reverse
--cracks and deformations from handling
--rapid changes in humidity
--mild changes in temperature
--vibration during travel
Commonly used paper products:
Foam Core is comprised of two sheets of clay-coated white paper encasing a film of extruded polystyrene. It is non-archival, relatively cheap, widely available, lightweight and easily cut to the required shape. It is hygroscopic, and therefore allows a moderate degree of moisture transfer. It is a relatively good thermal insulator. It can build up static electricity. It is stiff enough to cover large, unsupported areas. It has poor puncture resistance; being essentially soft, it tends to deform before breaking. Fome Core is a available in several depths up to 1/2 inch, and in sizes up to 96 x 48 inches.
Archival Fome-Core® is structurally identical to the above except that the paper outer layers are of archival quality materials, and lack the smooth quality imparted by the clay coating. All the components are buffered. It generates little static electricty. It has moderate stiffness. It is expensive, not widely available, and is manufactured in 1/8 and 3/16 inch depths, with sheet sizes up to 96 x 48 inches.
Gatorfoam® is an extruded polystyrene foam board bonded between two layers of Luxcell® wood-fiber veneer. It is non-archival and more expensive than foam core. It is slightly less hygroscopic than foam core, probably due to the adhesive in the wood fiber veneer. It is the stiffest of the paper products: although a great deal of force is required to break it, when punctured or broken it can shatter into very sharp fragments.
Heritage board is comprised of two layers of paper--one blue-grey, the other white--laminated to a corrugated paper interior. It is archival, buffered, and moderately hygroscopic. Like foam core, it has moderate stiffness and deforms before breaking. It is available in various thicknesses, in sheet sizes up to 96 x 48 inches.
Matboard is a generic term for layers of higher quality cellulose fiber paper pressed together to form thick sheets. It can be archival or not, and is very hygroscopic, often deforming out of plane in moderately humid conditions. It is widely available in several thicknesses, (only 8-ply and above are stiff enough to be used as backboards) and at several price points. Sheet sizes vary.
Cardboard/Millboard is a generic term for pressed paper boards made of low quality cellulose fibers, often with many impurities. There is often a thin coating on either side to improve handling qualities. It is non archival, highly hygroscopic and widely available in many thicknesses and sheet sizes, with or without interior corrugation.
Plastic based materials used include:
Coroplast: Coroplast® Archival grade is a chemically inert polypropylene copolymer, extruded into a twinwall fluted plastic sheet. It is non-hygroscopic but can build up a high static charge. It is lightweight and easily cut to the required shape. Under pressure it deflects rather than deforms, and while piercable, does not shatter on impact.
Corrugated plastic sheets, twinwall plastic or corriboard are sold under a variety of tradenames (Coroplast, IntePro, Correx, Twinplast, Corriflute or Corflute, etc.) and are available in different types. Many products are identical to the above in handling qualities but may have various additives.
Acrylic Sheeting (sold the under tradenames of Plexiglas, Lucite and Perspex) is a Poly(methyl methacrylate) transparent thermoplastic sheeting. It has low hygroscopic properties, high static charge, and is stiff at room temperature but becomes embrittled at low temperatures. It can develop creep over time. It is inexpensive and widely available in a range of thickness and sheet sizes. Its main use is in a situation where there is extensive information on the reverse of the canvas which needs to remain easily accessible.
Mylar® is an archival pure polyester film (also known as Hostaphan 43SM or Melinex 516). It is hydrophobic, builds up static charge, and retains its dimensional flexibility better than acrylic sheeting. It provides adequate puncture protection at the thicker weights. It is available in a variety of thicknesses (although only .7 and above is stiff enough for backing), in sheets or on rolls. Like acrylic sheeting, it is useful when the back of the canvas needs to be visible, but where thickness, excessive weight or large size is an issue.
Fabric backings provide protection only from dust and some insects. Fabric is used in cases where size, weight, travel constraints or lack of resources preclude the use of other materials.
There is no consensus among paintings conservators about an ideal material for backing a painting. The choice of material to some extent depends on what environmental challenges a given painting is subject to. Extremes of humidity and temperature, dirt, and handling in situ or in transit, in addition to the size and weight of the painting, dictate the choice.
In general, a backing board is held on to the stretcher or frame with screws and then sealed with tape. Paper products allow for some moisture/air exchange while plastic products provide a far tighter seal, reducing the painting's exposure to short fluctuations in relative humidity (day-night cycles). The downside of a sealed plastic board has been observed in extremely humid conditions (a climate of prolonged dampness, or conditions in which moisture will condense within the enclosed space due to a drop in temperature or flooding), in which the moisture becomes trapped between the painting's back and the backboard, encouraging mold growth.
Testing done at the Canadian Conservation Institute1 ranked the effectiveness of various boards in preventing change in RH in the interior space when the exterior RH was changed. From most effective to least:
1. Coroplast, with 4 ply matboard on inside - Very Good without glazing (WG); Excellent with glazing (G)
2. Coroplast - Good (WG); Excellent (G)
3. Acrylic sheet (Plexiglas) – Good (WG); Excellent (G)
4. Millboard – Fair (WG); Fair (G)
5. Hardboard - Fair (WG); Fair (G)
6. Hardboard, 1 coat latex - Fair (WG); Fair (G)
7. 8 ply matboard - Fair (WG); Fair (G)
8. Gatorboard – Poor (WG); Fair (G)
9. foam core – Poor (WG); Poor (G)
10. Corrugated Cardboard - Poor (WG); Poor (G)
To utilize the advantages of both materials in a single backboard, a plastic backing can be fitted with an interior rag board layer to buffer humidity, or a paper board backing can be lined with mylar film.
A backing board will reduce vibration of the canvas in transit and will reduce the painting's response to shock. In addition, on large paintings, the backing board, often attached in sections to the cross-bars, stiffens and stabilizes the stretcher/strainer against movement during handling and transit. When movement of the canvas against stretcher bars and rails is an issue, the backboard can be fitted with individual pieces of foam that nestle within the spaces created by the stretcher bars.
Manufacture Information and Sources
Art Care Foam Core Board:
1Information provided by Debra Daly Hartin, Senior Conservator, Fine Arts, Canadian Conservation Institute, Ottawa, Canada. The conclusions printed here are from past testing done at CCI which were used for recomendations made in CCI Notes 10/10 Backing Boards for Paintings on Canvas available on the CCI Website (The revised Note 10/10 will be avaialble in Summer 2012).
PSG Stretchers and Strainers Catalogue, IV. Treatment Variations, section: N. Backing Boards.
Michalski, S. “Risk analysis of backing boards for paintings: damp climates vs. cold climates.” Minimo intervento conservative nel restauro dei dipinti, CESMAR7, Secondo congresso internazionale Colore e Conservazione. 2004, pp. 21–27.