Except for pictures of only temporary interest, a glazing should incorporate a filter to block almost all ultraviolet radiation (a UV filter) from penetrating the glazing. This filter slows the photocatalytic degradation of organic materials in the picture. Both glass and acrylic glazings are available with built-in anti-static properties. This option is necessary for objects with friable or degraded media, which would be pulled off the object and onto the glazing by static electric forces. Except for the most disposable or temporary displays, the glazing must be held off the surface of the picture in order to prevent the object from becoming adhered to the underside of the glazing, acquiring irreversible color changes due to compression of the media, and/or developing mold growths that otherwise would not occur. This distancing is accomplished with a mat, "spacers" tucked behind the glazing and hidden from view by the lip of the moulding, shadowboxing, sandwiching the glazing between two mouldings, and similar methods. Relieving the glazing is also necessary in order to prevent loose media, such as charcoal or pastel, from becoming smudged. The treatment of the back of the framed artwork also varies widely. All frame packages should incorporate some sort of stiff, dense board to protect against physical blows and the ingress of dirt, insects, moisture, and pollutants. Except for temporary displays of expendable objects, the backboard(s) should be made from good archival-quality material, such as matboard. Archival-quality corrugated boards, both paper and plastic, are sometimes used, and foam-core boards that are described as being archival quality are also used. As with many details of good framing, the advice of an expert is valuable. Behind the backing board(s), retaining clips or brads hold the package in place, mirroring the restraint provided by the lip of the moulding at the front of the frame. A dust seal (usually sturdy archival-quality paper) is adhered, over the back of the moulding. While these are almost invariably simply functional, there are examples of works in which they have been decorated and considered part of the artwork. Finally, hanging loops or similar attachments are securely screwed to the left and right sides of the molding. Plique-à-jour picture frames, made of enamel by Bulushoff, are among the most expensive frames in the world. Picture frames are generally square or rectangular, though circular and oval frames are not uncommon.