Materials and Methods of Preservation Framing - Matting
The window mat and back mat should be made from cotton fiber (commonly called 100% rag or museum board).
Chemically purified wood fiber (sometimes called conservation board) is considered by some conservators to be an acceptable alternative. Cotton fibers are almost pure cellulose however, and to remove lignins and other impurities from wood pulp is a complicated process. At Bark Frameworks we prefer to use the material which requires the least purification, since industrial processes are not always reliable.
The matboard should be entirely free from lignins or groundwood and should be sized with a neutral or alkaline size. The pH of the board should be from 7 to about 9, and it should contain a calcium carbonate alkaline reserve, or buffer, of up to 3%.
Boards made specifically for framing certain photographs do not contain an alkaline reserve and have a neutral pH. Dye transfer prints and Cyanotypes should be framed in these unbuffered mats. Chromogenic color prints and albumen prints should also be kept away from alkaline materialss, according to many conservators. RC and fiber-base black-and-white prints should be safe in alkaline mats (though some conservators may disagree). Cibachromes can be safely framed with either buffered or unbuffered materials. In general mat boards that have passed the Photographic Activity Test are the safest to use in framing photographs.
Mat boards of conservation quality are made in a range of colors. These are made with non-acidic direct dyestuffs or pigments, unlike conventional dyes which require acidic mordants to set the dyes. Nevertheless, conservators often feel that these colored boards should be used with caution, since the pigments may scuff off and there is a risk of dyes bleeding under certain conditions, such as very high humidity. And colored mat boards should not be in direct contact with photographs.
Special mat boards are made that incorporate zeolite materials to adsorb volatile organic compounds and other indoor pollutants. Matting can also include silica impregnated sheets to help control relative humidity.
The window mat has an esthetic role, in providing a field around the work of art, which isolates it from both the frame and its surroundings.
The major protective function of a window mat is to separate the work of art from the glazing surface. The mat also helps support the perimeter of the paper sheet. Window mats are made commercially as thin as 2-ply, about 1/32" thick, and up to 8-ply which is 1/8” thick. At Bark Frameworks we custom mount thicker mats, up to about 3/8” thick. Thick window mats are sometimes preferable on esthetic grounds, and often are safer for the framed work.
There is no rule that above a certain size a window mat of a given depth should be used. In this as in so many other matters, it's a question of judgment, in which such issues as display conditions and media must be weighed.
Sometimes a window mat is not deep enough to protect a wavy or large work on paper. In such cases, and when for esthetic reasons a window mat is inappropriate, a spacer (known as a fillet) between the glazing and the back mat may be used.
We use fillets from 1/8" deep up to about 3" deep. Fillets may be made from mat board, acrylic or other stable plastic, or wood, which should be sealed.
Fabric Covered Mats
Silks and linens are often mounted on matboard for use as both window mats and back mats. When we tested a number of silks and linens for pH we found them all to be acidic. Whether or not a particular piece of cloth tests in the acid range, silk and linen should not be viewed as archival materials. If they must be used in a frame they should be isolated from the artwork by mat board of at least 2-ply thickness. For a window mat this means lining the underside. When we are required to float an artwork on a silk or linen covered back mat we cut a sheet of matboard slightly smaller than the work of art and fix it between the work and the back mat. None of these solutions is as safe as simply using rag mat board.
The back mat is the support to which works of art are hinged. It is usually attached along one of its long sides with a long tape hinge or "spine" to the window mat.
Back mats should be of 4-ply mat board, unless the frame is over-sized. For frames over 40" or so in both directions it may be necessary to make a more rigid backing. One option is to use 8-ply mat board. A solution we developed and sometimes choose at Bark Frameworks is to make a “laminated back mat”; we mount mat board to both sides of a core material, such as archival Coroplast or acrylic sheet. For very large works a panel can be constructed with aluminum clad material. Bark Frameworks stocks rag paper manufactured for us to our specifications for seamless surfaces of such panels. Our paper is 79” wide, and is sold in our store.
The backing board is installed behind the back mat. Its main role is to provide another layer of protection to the back of the work of art. It is the surface to which labels are usually applied. Backing boards can be made from either acid-free corrugated board or archival quality polypropylene (such as Coroplast) or archival polycarbonate.