Fayum boy 2nd century encaustic on wood panel
Wood has been used for centuries as a support for paintings with some good and not so good results. Wood naturally expands and contracts with moisture changes in the air, usually across the grain. This can lead to major cracks developing even to the point where the painting is no longer in one piece, as you can see in the picture above.
If wood is used it should be a close grained wood, and preferable a hardwood like poplar, birch, walnut, etc. Oak tends to have open grain and requires many layers of ground to fill them in, even though it was the most popular wood used in the past. Also, it is preferred that the panel be cut across the grain from heartwood to minimize expansion or contraction.
Even with precautions, including climate controlled environments, solid wood panels will have problems down the road. Many paintings on wood from the past are being transferred to more stable supports where possible, but since we have better options now solid wood panels should be avoided if there is not some compelling reason for using them.
Plywood is a much more stable support than a solid wood panel and is a good choice for a support. Plywood is a series of thin wood veneers glued together with the grain of each successive layer going in the opposite direction. Since wood tends to expand in only one direction, across the grain, the different layers pull against each other preventing the plywood from expanding or contracting much.
But all plywoods are not equal and lower quality or construction grade plywood should be avoided since the glue holding the layers together may not last as long and the support may fall apart. A cabinet grade plywood should be used for the best longevity as they are expected to last for many years and the wood used in them is a higher quality. Marine plywood should be avoided as chemicals are often sprayed onto the wood veneers to make them water resistant, but the chemicals could react with the size, ground, and paint of the painting preventing proper adhesion or other unpredictable results.
As with solid wood panels a hardwood plywood with close grain should be used. The surface veneers should be significantly thicker than several sheets of paper, especially if the laminate glue does fail and your painting has to be transferred to another support.
Hardboard is made up of millions of tiny wood particles that are compressed and are held together by either the natural lignan in the wood or by a resin. Hardboard has two main varieties, untempered which is held together with lignan and tempered which is held together by resin. Because of its density and multiple directions of the fibers, hardboard is less likely to expand or contract with fluctuations in moisture.
Artist’s were cautioned against using tempered hardboard since it used to be made with large amounts of oil, but with modern manufacturing techniques if oil is used at all a light sanding before sizing and applying the ground will take care of it. Also tempered hardboard is stronger and lasts longer than untempered since the natural lignan that holds untempered hardboard together will break down over time, and is more hydrophilic.
The question of which to use is becoming a moot question as untempered hardboard is becoming rare, leaving tempered hardboard as all that is available.
MDF, or medium density fiberboard is a compressed mat of saw dust that is held together with glue or resin. Due to the manufacturing process MDF usually has a smooth and hard outer surface with a comparatively softer less dense inner core.This is a concern for some artists as the inner core can absorb moisture, but this can be mitigated to some extent by painting the ground on both faces and edges.
Since MDF is softer than hardboard it is not recommended as highly, but it is still an acceptable surface for a painting.
General Concerns for Wooden Supports
All wood based supports will benefit from perimeter cradling and cross bracing on the back to prevent the panel from warping over time. It is recommended that anything 1/8 inch thick and over 12 inches square be braced by the addition of good wood strips glued to the back, except for a solid wood panel which would need a special bracing system. Gluing thinner panels to thicker plywood would also serve as a good bracing system.
If a panel is small, applying the ground to the back of the panel is a good practice as the pull of the dry ground will be countered by the same pull on the back. If you don’t want to use an expensive ground on the back, left over commercial household paint can also be used. But if a panel is cradled and braced, applying ground or paint to the back of the panel is not necessary.
Tomorrow we’ll get to paper, and hopefully also cover some of the more unusual supports.