The backing board

It appears to be a poor part of the whole framing package and has never really been given too much importance but, over long periods of time, it can play a rather fundamental role towards the final framing result.

What should be used as backing board? There are various materials that could be used but they must all have one essential characteristic: rigidity. In fact the main function of the backing board is to confer rigidity to the picture.
The choice between the various types of backing board will mainly depend on the specific circumstances. Lets see each material type one by one:

Grey board
Our survey concluded that it is the most popular backing board. About half of the framers interviewed make great use of it.
It is available in various thicknesses. The most advisable and the most used, is the 3 mm (1/8") thick size but for smaller pictures (for example 30x40 cm (8" x 10")), thinner ones can also be used. Obviously the thickness required varies in direct proportion to the size of the picture to be framed.
The quality of these boards also varies and in particular their rigidity and weight can vary substantially. A lighter board is drier and hence more rigid, whilst a heavier board tends to become flabby and wave at every temperature change.
One characteristic of grey board is its hygroscopicity, i.e. its ability to absorb humidity. This means that the board can deform after framing.
Grey board could be cut with a normal Stanley knife by repetitively passing over the same incision. It would be preferable though to use a professional guillotine. Strangely enough few framers have one.
Framers that use lots of grey board could cut them into standard sizes (60 x 80 cm, 50 x 70 cm, 40 x 50 cm, 30 x 40 cm etc.) (15" x 20", 12" x 14", 10" x 12", 8" x 10", etc.). Time would be saved and material would not be wasted due to the programming of the cut. Some suppliers supply grey board in the above mentioned standard measurements. From a conservation framing point of view grey board is far from being acid-free and is thus not advisable for framing highly valued pieces of works of art or works of art on paper. If grey board must be used then the picture should be separated by a thin piece of acid-free board or paperboard.

Medium density fibre
It is a material that has just recently made its appearance in the framing industry. It is currently almost exclusively used for clip frames and for volume market ready-mades. A saw is required to cut it. Unfortunately this limits its use for just occasional purposes.

Corrugated board
In the last few years its popularity has constantly grown. Its main characteristic is its lightness and hence its ease of cutting which can be performed with just a normal Stanley knife. In spite of its lightness it is rather rigid, particularly in the thicker sizes. Its main handicap is that the waviness can sometimes be detected from the front of the picture.
Its price is much inferior to that of grey board. It is generally not acid-free although only just recently various types of acid-free corrugated board have appeared on the market.

Foam board
Its lightness and rigidity have quickly made it a favourable product amongst framers. Its major advantage is the ease of cutting it. Without too much force it can be quite easily cut with a simple Stanley knife. A guillotine will tend to squash it and the edges will not be cleanly cut.

For large sized frames it must be used as it is the only material that possesses sufficient rigidity in big sizes. It is however difficult to saw. It can be purchased in pre-cut measurements.

For small sized pictures (up to 30 x 40 cm (8" x 10")), normal cardboard used for mountboards can also serve as backing board. Cut outs from larger mountboard apertures could be used. The limited thickness of these boards (generally 1.5 cm (2/5")) and their limited rigidity make them unsuitable for large frames.