Mats: board, borders and the window

A mat holds the art off the glazing and carries the mount. This chapter chooses the board grade from what the piece is worth keeping, sets the window from the sheet, settles the borders, and names the cases where a mat is the wrong answer.

1. What a mat actually does Understand

A mat does two jobs, and only one of them is visual. It carries the mount, so the piece is attached to a board rather than to the frame. And it holds the art off the glazing.

That second job is the one the decorating literature treats as optional and the conservation literature does not. Physical principle Every publisher in this research says the same thing: a national library, a preservation center, a conservation institute, a federal leaflet, a conservation center, a state archives and the conservators' own professional wiki all state that nothing inside the frame may touch the glazing, and that the gap is made either by the thickness of a window mat or by a spacer. The professional wiki puts it most directly, calling separation of the object from the glazing the most important function of matboard, and naming what contact causes: adhesion, ferrotyping, staining, and further damage from condensation such as mold growth.

The mechanism is moisture. A conservation institute notes that the inside of glass is subject to moisture condensation, which can be transferred to paper, and that a breathing space is essential for friable or flaking media, which tend to adhere to the glazing. The reader who has ever peeled a poster off its glass knows the result.

How much space does a plain mat give? One numeric anchor exists. A practitioner supplement hosted by a national library states that the minimum window mat package is a four-ply window plus a four-ply back mat, and that this combination gives a space of a sixteenth of an inch in front of the art, while being rigid enough to be a steady platform for hinging and handling. The same source rules out two-ply at both positions: too shallow for a window, not enough support for a back mat. A conservation institute gives a different figure for a different object, a minimum of half a centimeter between the highest point of a painting and the glazing.

So a plain four-ply mat gives you a sixteenth of an inch, which is enough for a flat, stable piece and not enough for several other cases. When a mat is wrong covers those.

2. Board grades, and what the words mean

Three grades cover almost everything you will be offered, and the trade names for them are confusing on purpose.

A practitioner supplement hosted by a national library untangles it in one paragraph. Wood pulp refined to remove lignin and hemicellulose is alpha cellulose, and boards made from it are often sold as "conservation" board. Boards made of cotton are sold as "museum" or "rag" board. And "rag" does not mean reclaimed textiles: the cotton comes from linters, the fiber left on the boll after textile cotton is removed. The third grade is the one that causes the damage: a sandwich of colored surface papers over a core of poorer-quality groundwood board. A federal leaflet names it as unacceptable and names its result, the yellow or brown line on the artwork that it calls acid mat burn.

What the trade specification actually requires at its top level is worth reading once, because it is the only place in this research where a framing grade is written as a specification rather than as an adjective: 100 percent cotton fiber, no post-consumer waste, free of metal particles, waxes, plasticizers, residual bleach and peroxide, under 0.0008 percent reducible sulphur, iron not over 150 parts per million and copper not over 6, no optical brightening agents, and a stated pH range. And then the sentence that makes the whole chapter possible: the use of the term "acid free" must be avoided in product specifications. Ask the authority

Why does "acid free" fail as a specification? A preservation center explains it: lignin, groundwood and alum-rosin sizing are what generate acid, so a storage material has to be free of all three, and a pH above 7 does not by itself mean a material is of preservation quality, because some newly manufactured wood pulp board is alkaline when made and becomes acidic rather quickly. Ask what the board is made of, not what its label avoids saying. What "archival" means takes this further.

One default, with the reasoning. Choose cotton rag for anything irreplaceable or possibly valuable, alpha cellulose for things you care about but could live without, mid grade for a reprintable poster. The board is the cheapest decision in the package to get right and one of the hardest to undo, because what it does wrong it does to the art rather than to itself.

Buffered or unbuffered is a separate question, and it is one where publishers differ. The default this manual gives: unbuffered for traditionally processed photographs, buffered for paper documents. That is a federal leaflet's rule, stated as neutral pH 7 board for photographs and buffered or alkaline-reserve material at pH 8 to 8.5 for paper documents. The three publishers who answer it differently are set out under Buffered or unbuffered.

3. Borders and the window

Setting the window

The window comes straight out of the overlap: sheet size, minus the overlap on each side. A conservation institute cuts the bevel at 45 degrees to improve the appearance and reduce shadows, and removes the burr on the back of the cut with fine sandpaper. The window and back mat are joined along the longest edge with water-activated gummed linen tape, the top for a horizontal format and the left for a vertical one, and the tape is used only on the matboard, never on the object.

Choosing the border widths

Here the manual's protection advice is strongest and its taste advice is weakest, and it is worth saying so plainly. There is no published rule for how wide a border should be. There is one published range and one published worked example, and everything else is preference.

The range: a federal leaflet gives mat margins of 2 to 7 inches, with 4 to 5 inches suiting a 24 by 30 inch object and 2 to 3 inches a 5 by 7 inch one, and says either all four margins equal, or equal top and sides with the bottom about 30 percent wider. Convention A conservation institute says the same shape in fewer words: top and side margins equal, bottom slightly larger.

The worked example: the UK trade body's practical examination requires a candidate to submit a piece with a double mount aperture of 200 by 287 mm and borders of 90, 90, 90 and 105 mm. That is a real set of numbers from a real specification, so it is worth seeing: equal top and sides, a bottom about 17 percent wider, on a window a little taller than it is wide.

Why weight the bottom at all? The usual explanation is that a frame hung above eye level is seen from below, so an equal bottom border appears narrower than the top. That is an explanation, not a finding, and no source in this research tests it. Both answers are in use; a federal leaflet publishes both. Choose one, write the four numbers down, and apply the same choice across a group so a wall reads as one thing.

4. When a mat is wrong

Four cases, each with its own answer.

The edges are the point. A deckle edge, a plate mark, a signature in the margin, a hand-torn sheet: covering the edge hides part of the object. The answer is a float mount, where the piece sits on the backing board with the window larger than the sheet, or no window at all. The professional wiki gives the clearance: the inside dimension of the window is slightly larger than the object and never closer than an eighth of an inch to the object's edge. Floated objects need not have a window mat at all, with spacer walls built onto the edges of the glazing instead.

The piece is not flat. A cockled sheet, a collage with raised elements, a seal, a wax medal. The answer is a sink mat: a built-up frame of filler strips, the thickness of the object or of its highest point, with a window mat over it. The professional wiki is specific about the construction: filler strips at left, right and bottom butt the backboard edge and come within a fraction of the object's edge, and the upper filler strip is not adhered, so it neither supports the object's weight nor prevents its movement. It also notes that objects cannot be floated in a sink mat, because a sink mat is always overmatted. A conservation center reaches the same place from the other direction, saying that works with planar distortion, thickly applied media or very large sheets need additional depth, achieved with acrylic spacers wrapped in alkaline paper or extra layers of ragboard.

The design calls for close framing. A piece framed to its own edge with no visible border. This is legitimate and it needs a spacer, because something still has to hold the piece off the glazing. Spacers covers the detail, including the arrangement that keeps a spacer from falling onto the object: a preservation center asks for spacers arranged post-and-lintel, the top piece resting above the sides and the bottom piece below, so no piece can fall onto the object if its attachment fails.

The piece is friable. Pastel, charcoal, chalk. A single four-ply mat gives a sixteenth of an inch, and that is less than the published minimums: the trade specification asks for at least 5 to 6 mm of space, and the professional wiki asks for at least an eight-ply mat's worth. A conservation institute's older note suggests the same thing differently: eight-ply board rather than the usual four, or a double mat with two superimposed windows. Friable media finishes the question, because the glazing choice goes with it.

Two things the manual leaves to the trade: ruled lines drawn with a traditional ruling pen, and V-grooves cut into the mat surface. Both appear in the trade qualification's practical examination, both take practice on scrap, and neither protects anything. If you want them, they are a thing to ask a framer for by name.

6. See it See it

• Preservation guidelines for matting and framing, a national library: a component-by-component checklist you can hand to a framer, including the four-ply minimum and the gummed linen tape rule.
• Preservation matting for works of art on paper, a practitioner supplement hosted by the same library: where the trade names come from, and the four-ply-over-four-ply figure.
• Matting and framing specifications for objects on paper, a conservation center, updated July 2022: short, specification-shaped, with the 3.5 percent buffering figure and the five-inch hinge spacing.
• Quality standards for mountboard, a UK trade body: what each board grade must actually contain, and the sentence about "acid free".

7. Practice Practice

Minimum: one piece, with a board grade chosen and four border numbers written down. Time: about 40 minutes. Materials: your measurements, a few free board samples from a frame shop, a pencil, and some plain paper or card to cut mock borders from.

1. Collect board samples. Most frame shops will give you corner samples. Look at the cut edge of each: a cotton or alpha cellulose board is the same color through its thickness, while a colored surface over a groundwood core shows a darker middle.
2. Choose the grade from what the piece is worth keeping. Write it in the framing plan by what it is made of, not by its trade name.
3. Mock up the borders. Cut four strips of plain card to your candidate widths and lay them around the piece on a table. Look from a few feet away, then from the height the picture will hang at. Try the equal version and the weighted version.
4. Put the sheet size, the overlap and your chosen borders into the size calculator. Write the window and mat outside sizes into the plan.
5. Check the gap (Go deeper; optional). Use the depth box with the mats you have chosen and see how much space sits in front of the art. If it is under 5 mm and the piece is friable, add a spacer or go to eight-ply.

8. Your pictures Your pictures

9. Check your work Check your work

Criteria

• The board is named by what it is made of (cotton rag, alpha cellulose, mid grade), not by a trade adjective.
• The window is derived from the sheet and a chosen overlap, and written down.
• Four border numbers are written down, and the same choice is applied across a group.
• Something holds the art off the glazing, and you know how much space that is.
• If the piece is a photograph, you know whether the board is buffered and why.

Worked example

Nia and Marcus Okonkwo (fictional) are matting fourteen photographs to one outside size, 16 by 20 inches, so the frames can be bought in quantity and rearranged later. The prints vary from 5 by 7 to 8 by 10. That means fourteen different window sizes in fourteen identical mats, with the borders absorbing the difference: a 5 by 7 print gets a window of 4 3/4 by 6 3/4 and borders of 5 5/8 and 6 5/8, while an 8 by 10 gets 7 3/4 by 9 3/4 with borders of 4 1/8 and 5 1/8. Board: alpha cellulose, unbuffered, for the three traditionally processed prints; the rest are modern lab prints and the shop's standard conservation board. Their bottom borders are all an inch wider than their tops, which is their rule, kept throughout.

Common mistakes

• Buying board by its adjective. "Archival" and "acid free" are not specifications, and the trade body that publishes the board standard says so about the second one in print.
• Letting the mat touch the art with tape. The linen tape joins the two boards. It does not touch the object, and a federal leaflet puts that in capitals in its own leaflet.
• Assuming a mat is enough space. A four-ply window gives about a sixteenth of an inch. For a flat print that is fine; for a pastel it is a long way under the published minimums.
• Mixing border rules across a group. Equal on some and weighted on others reads as a mistake from across the room even when each one looks fine alone.
• Cutting the window to the image rather than the sheet without deciding. Both are legitimate; doing it by accident is not.

What is still unresolved

• Whether the gap the mat gives is enough, which depends on the glazing: Glazing.
• How the piece is attached to the back mat: Holding the art.
• Whether the board grade you chose matches the level you want: The levels.
• Whether to cut the mat yourself or have it cut: A framer or yourself.

10. Knowledge check and scenario

Checklist

• Board grade chosen and named by its fiber, in the framing plan.
• Window size derived from the sheet and a chosen overlap.
• Four border numbers written down, with the same rule across the group.
• The art is held clear of the glazing, and I know by how much.
• Buffering decided for any photograph, with the reason. (Go deeper; optional)