A mount serves four broad functions:
1. To support and protect the subject:
The windowmount (or combination of mounts) ensures the subject does not come into contact with the glass. An additional piece is used as an undermount which when placed behind the subject protects it from impurities in the frames backing. Both these points are very important; first, if the artwork is in direct content with the glass there is the possibility that the image will be transferred to the glass and should there be any condensation form on the inside of the glazing giving rise to the possibility of 'tide lines', image distortion, mould and eventually the destruction of the artwork. The undermount should be hinged to the windowmount along its longest edge this provides a stable support onto which the artwork is hinged. Correct hinging using 'T-Hinges' of the appropriate tape helps prevent both cockling and buckling; often artwork bought from galleries and art suppliers is incorrectly hinged which does eventually cause problems. Please see Conservation Mounting and Hinging
2. To determine the image size:
A mount can be cut to conceal sections of the subject being framed and/or to enlarge the subject to fit a specified frame size. With the use of a windowmount an aperture can be cut to show only the main subject, obscuring a lot of the unwanted picture. The widths of the border can be be sized to the customers requirement.
As far as borders around artwork is concerned there are no particular rules, it is really up to personal choice. Aesthetically, there should be a small difference in the bottom border as opposed to those on the sides and top. I am of the belief that the artwork should be given space and consequently set my borders as 70mm on the sides and top with 80mm on the bottom, on occasions I will increase these to 100/110mm but this depends on the artwork itself.
Of the two mounts above the one on the right has borders cut to the above specifications whilst the one on the left is cut to historical dimensions used by the British Museum based on imperial sizes and their own border proportions.
3. To enhance the subject:
By creating a given space between the subject and the frame in a complementary colour, the eye is naturally drawn to the subject giving a more focused appreciation of the subject.
4. To provide a decorative medium:
The windowmount can be decorated to enhance the subject. An extra window aperture can also be cut allowing for a title or inscription to be added. Two or three mounts (double or triple mounts) of different colours can be used with different aperture sizes and shapes when mounted on top of each other. Triple mount especially with a deep bottom mount given the impression that one is falling into the mounted picture.
The use of washlines and panels originated with early watercolourists who used this form of mount decoration to enhance and provide a boundary to their artwork. Even the simplest washline can have the effect of breaking up by adding that extra dimension and value added to to a wide border between the artwork and frame. The use of a marbled effect, gilded or decorative papers can provide an interesting effect or more effectively the use of differing colours, tones and hues can provide for a subtle watercolour wash. Whichever is chosen this should always be appropriate to and never overwhelm the artwork. Whilst lines can be produced by CMCs these are limited in the media used; without a doubt the use of watercolours or acrylics are the most effective and demonstrates the expertise of the framer using, by hand, split leafed pens - a traditional art.