Backing Boards for Canvas Paintings

Hangers can be attached on top of backing boards, but cutting out holes for the hanging systems is possible too (Image 4).

Some stretchers have stepped profiles with a groove cut into the inner edge. This makes it easy to accommodate a backing board without increasing the depth of the painting and making it stand out further from the wall, which some artists might dislike. Particularly, stretchers with T-profiles and some Heavy-Duty stretchers have stepped profiles (Image 5). Examples would be the Creative Mark Probar Stretchers, the Heavy Duty stretchers by John Annesley Company, and stretchers from Jack Richeson & Co., as well as Upper Canada Stretchers. When using regular stretchers, one can mill a 3 mm deep and about 5 mm wide recess into the reverse of a stretcher to fit in a backing board.

Humidity and Ventilation

The exact shape and ideal material for backing boards depends mostly on the environmental conditions of the space where the painting will live.

In cold climates the dry air of heated rooms is problematic. Low humidity causes increased tension and stiffening in ground and paint layers, while canvases relax. Therefore, it is important to seal the backing board well to keep the dry air out from behind the painting. The Canadian Institute for Conservation recommends using foam strips in between stretcher bars and the backing board to prevent air gaps (well sealed backing boards also create a better air cushion and thus dampening effect during travel).

In humid climates mold growth is the biggest concern. Relative humidity over 65% becomes risky.

Usually, air is more humid during the night than it is during the day. Even if the humidity rises way above 65% at night, that can be okay, as long as the space behind a painting can dry during the day, so that mold cannot grow. To allow the moisture to escape, backing boards must have sufficiently big ventilation holes. Cutting off the corners of a backing board is an easy way to provide ventilation (Image 6). Screens can be placed over the vent holes to keep insects and dirt out, or, in very humid climates, rigid backing boards can be replaced altogether with screens.

In temperate climates a wide range of practices are common and possible (cut corners/fully covering boards/type of material), although, humidity and mold still have to be considered. House walls, especially masonry walls, or shaded walls, tend to be cooler than the respective room temperatures. This leads to a more humid environment directly behind a painting. Backing boards help to insulate the cooler wall temperatures and thus can prevent mold growth.

Paintings in Transit

Transporting paintings on bumpy roads exposes them to damaging shock and vibration. Now, each material vibrates at its own frequency when exposed to certain movement. When backing boards create a resonance vibration with a painting, more, rather than less flapping of the canvas may be the result. It was found that thinner backing board materials (tested were 3 mm corrugated cardboard, polyurethane hard foam core board, and polycarbonate multiwall board) tend to increase vibrations at typical truck-transportation frequencies, while more sturdy corrugated cardboard (4.5 mm and better yet 8 mm), as well as honeycomb cardboard significantly dampen the vibrations of paintings (see http://www.gemaeldetransport.ch/kti/wp-content/uploads/2011/11/Baeschlin_ICOM-CC_2011.pdf).

This is relevant to know for those who care for paintings which travel a lot. The vast majority of paintings out there, however, remain where they are or travel only a few times during their lifetimes. For those stay-home paintings, backing boards do a lot to conserve their condition, regardless of the inherent so called eigenfrequency (natural frequency of a system) of the backing materials during transit.

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