Mounts: standard and museum grade, widths and a weighted bottom

What a mount is for

A mount (also called a passe-partout, or a mat in American usage) is a board with a window cut in it, placed between the photograph and the glazing. It has two jobs:

• protective: it keeps the surface of the photograph away from the glass. A photograph pressed against glass can stick to it as humidity changes, and moisture can condense under the glass,
• visual: it separates the image from the frame and the wall and gives it space. A small photograph in a generous mount looks precious, not lost.

A full mount consists of a window mount (with the cut-out) and a backing board (without one), to which the photograph is attached. The window is cut at an angle, usually 45°, so a light edge of the board’s core shows around the image.

Standard and museum-grade mounts

	Standard mount	Museum / conservation mount
Core	wood pulp, bleached and buffered	α-cellulose or 100% cotton (rag)
Over time	the core and bevelled edge can yellow	stable in colour and chemistry
Use	decoration, short-term display, reproductions	originals, Fine Art prints, family photographs

The Library of Congress recommends mounting photographs in acid-free rag or museum board. Materials that touch a photograph are covered by ISO 18902; part of it is the PAT under ISO 18916, which expressly applies to mount boards too. A board maker’s statement “PAT passed” is therefore worth more than a general “acid-free”.

Buffered or unbuffered? Most museum boards are buffered (they have an alkaline reserve). The Library of Congress points out that buffered materials are unsuitable for some processes, such as dye transfer prints and cyanotypes. For rare historical photographs, consult a conservator.

How wide should a mount be?

There is no fixed rule; the values below are established guide figures from framing practice:

• the mount should be noticeably wider than the frame moulding; equal widths look indecisive,
• for small sizes (up to A4) around 5–8 cm, for medium and large sizes 8–12 cm or more,
• the smaller the image and the larger the wall, the more the mount can “breathe”.

A weighted bottom

If the mount has the same border all round, the image looks as if it is sinking in the frame. So the bottom border is made slightly wider — commonly by 1 to 2 cm, or by 10 to 20% compared with the other borders. Optically, that brings the image back to the centre. With very narrow borders the weighting is often left out.

Example dimensions

Inner frame size	Photograph	Side borders	Top / bottom border (weighted)
30 × 40 cm	20 × 30 cm	5.0 cm	4.5 / 5.5 cm
40 × 50 cm	30 × 40 cm	5.0 cm	4.5 / 5.5 cm
50 × 70 cm	40 × 60 cm	5.0 cm	4.5 / 5.5 cm
60 × 80 cm	40 × 60 cm	10.0 cm	9.0 / 11.0 cm

The window is usually cut a few millimetres smaller than the photograph on every side, so the photograph cannot “drop” through the window. If you want to show the edge of the paper or a signature, the window is cut larger and the photograph “floats” within it.

How to attach the photograph

• Yes: photo corners made of PAT-tested material, or Japanese paper hinges fitted by a framer or conservator; the attachment is reversible.
• No: self-adhesive tapes, glue applied directly to the photograph, double-sided tape. The Library of Congress recommends not using adhesives directly on photographs and entrusting the mounting to an experienced framer or conservator.
• Mounting to board (bonding the whole photograph to a board, for example by dry mounting) is irreversible. It is fine for decoration, not for originals.

Mount colour

White and off-white are the safe choice because they do not compete with the photograph. A black mount suits high-contrast black-and-white images. A coloured mount takes over the attention; if you want one, choose a tone from the photograph, not from the wall. You can try combinations in Frame Lab.