Frames: moulding, profile, depth and strength
A frame is chosen by the depth of its rabbet and the strength of its joins first, and by its appearance second. This chapter sizes the frame to the package, tells canvas frames from frames for work on paper, and judges how the back is secured.
Start here Essentials
At minimum this chapter asks you to check two dimensions on a frame before you look at it properly. Everything marked Go deeper is optional on a first pass.
- Add the stack up and read the rabbet depth you need. The depth box does it (Sizes and the stack).
- Check the depth and the width of the rabbet on the frame you are considering. Use a tape, in the shop (Depth and width first).
- For a canvas, check the frame is a few millimeters larger than the stretcher. Decide the backing board as well (Frames for canvas).
- Decide how the back will be secured. Write it in the framing plan (Securing the package).
Minimum time: about 15 minutes in a shop with a tape measure. Done when: the framing plan names a frame whose rabbet takes the whole package, and a canvas has a backing board decision written down.
1. Depth and width first Understand
The rabbet is the step cut into the back of the moulding, and it has two dimensions that matter. Its depth decides whether the package fits at all. Its width decides how much of the package's edge is covered and held.
Depth is the one that rules frames out, and pressing harder will not fix it. A conservation institute gives the figure: the rabbet must be deep enough for the mat, the backing board and the glazing together, about 2 cm in total, and metal clip-together frames are often not deep enough for all three. The UK trade specification says the same thing as a requirement rather than a measurement: the chosen moulding must be strong enough and its rebate deep enough to hold and support the whole package of glazing, window mount or fillets, artwork, undermount and backboard.
A rabbet that is too shallow has two honest answers and one bad one. The honest ones: add wood to the back of the frame, which a conservation institute describes for paintings as adding strips with glue or wood screws, thick enough to sit flush with the back of the stretcher; or attach the backing board to the back of the frame with screws and cup washers rather than trying to get it inside the rabbet, which the same institute's framing note gives as the remedy when the rabbet is too shallow. The bad one is to leave a layer out, which is usually the backing board, and which removes the thing protecting the package from behind.
Width matters less and still matters. Too narrow and the package's edge is barely covered, so the mat's edge shows when the piece is seen from an angle and the points holding it in have little to bite on. Too wide and the window is effectively smaller than you planned. On an order form the rabbet width is rarely stated, so measure it.
One structural note before appearance. A framing tool maker puts the whole chapter in two sentences: the glass, matboards, artwork and backing board must fit into the frame without bulging out of the back, and the frame must be strong enough to bear the weight of all the materials it will hold, because glass is heavy and a large glazed piece requires a strong frame.
2. Wood, metal and composite
| Material | How it is joined | Where it is good | What to watch |
|---|---|---|---|
| Wood | Mitred corners, glued, with wedges driven across the join from behind (the trade calls them V-nails) | Depth is easy to find; it can be built up at the back; it takes points and tacks | Wood emits acids. A national library says that if the frame is wood, the rabbet should be lined with a barrier film such as aluminum or polyester tape with acrylic adhesive, to stop acids in the wood moving into the mat package |
| Metal | Extruded aluminum sections joined with metal L-shaped brackets or plastic inserts | Light, thin, consistent; sections come in even inch increments so a custom size is two cuts | Depth. A framing tool maker gives the typical metal profile as about 3/4 inch deep, which is near the limit for a full package. A conservation institute names clip-together metal frames as often too shallow |
| Composite | Mitred and joined like wood | Stable, often cheaper, takes a painted finish well | Heavier than it looks for its section; check the depth as carefully as with wood |
| Clips and strips | No frame at all: glass, art and backing held together by corner clips | Cheap, fast, fine for something temporary | A framing tool maker calls this framing temporary, because dust, air pollution and moisture can easily invade the package. A conservation institute goes further and says not to sandwich an artifact between two sheets of glass or between glass and board held with clips, because it offers little protection from dust or atmospheric pollution and puts unnecessary pressure on the artifact |
On the wedges that hold a mitred wood frame together: one maker publishes sizes of 3, 5, 7, 10, 12 and 15 mm, in softwood, hardwood and MDF grades, and says they can be stacked for deeper mouldings. What it does not publish, and what this research found nowhere, is a chart mapping wedge size to rabbet or moulding depth. If you are having a frame chopped and joined, the shop chooses; if you are buying ready-made, it is already done. Ask the authority
3. Frames for canvas
A stretched canvas is a different object and the rules change. It is rigid in its own right, it moves with humidity, and it usually has no glazing, so the frame is holding a wooden structure rather than a stack.
Clearance. A conservation institute asks for 3 to 6 mm, an eighth to a quarter of an inch, on all sides between the frame and the painting, so the wood components can expand as relative humidity rises, and says plainly that the painting should not fit tightly. The UK trade specification gives the same figure from the other side: the frame should be around 3 to 6 mm larger than the stretcher bars, to allow for future retensioning. Where the opening is slightly too large, the institute says to fill any gap wider than 3 mm with strips of stable compressible foam or cork attached to the side of the rabbet.
Padding the rabbet. The same institute says to pad the rabbet so the paint surface never touches bare wood, naming colorfast nylon velvet ribbon, synthetic felt, or closed-cell crosslinked polyethylene foam with an integral skin, and gives useful sizes of 1 cm wide by 1.5 mm thick, or 1.5 cm by 3 mm. It adds a practical detail worth keeping: wait 24 hours after gluing before inserting the painting.
The wedges. A stretcher has wooden wedges, sometimes called keys, in its corners. The institute's instruction is that every key must be present and secured, because a key loose enough to fall out of its slot can drop between the lower stretcher bar and the canvas and cause damage. The trade specification says the same thing in its own words, that wedges should be held with thread or wire rather than left loose. What neither of them wants you to do is tap them to tighten a slack canvas: the institute treats keying out as a procedure requiring paintings-conservator training, not a basic one, and publishes a separate note on it.
The backing board. This is the single strongest preventive instruction in the research for a canvas. A conservation institute states that all paintings should have backing boards as a preventive conservation measure, and lists what one does: it protects against puncture and tear from behind, keeps dirt from falling between the lower bar and the canvas, buffers humidity fluctuations, reduces the painting's response to shock, reduces canvas vibration in transit, blocks airborne pollutants, and discourages writing on the canvas back.
Its ranked materials, best first for buffering humidity: corrugated twinwall plastic sheet with a four-ply matboard liner; twinwall plastic alone; acrylic sheet; tempered hardboard at least 3 mm thick. Chipboard is ruled out because its components may emit vapors that degrade some materials. Screws should be corrosion resistant, number 5 or 6 for most paintings, generally penetrating the auxiliary support no more than about half a centimetre, spaced 10 to 25 cm apart for corrugated plastic and about 25 cm for hardboard, no closer than 1 cm to the edges and not directly over corner joints. And one instruction in capitals in the original: do not drill holes into the auxiliary support.
The institute also publishes its own climate caveat, which is the kind of thing this manual likes: the note is written for the Canadian climate, and in places with prolonged dampness a backing board could increase the risk of mold, so where there are dry days and damp nights above 80 percent relative humidity for more than 30 days, use a plastic board and put ventilation holes in it. Physical principle
Glazing a canvas. Usually not. Where it is used, the same institute says the glazing must never touch the paint surface and the framing method must include a spacer, and notes that humidity protection is greatly enhanced if a painting is framed behind glass or acrylic, and is necessary if relative humidity could fall below 30 percent. A glazed and sealed frame can give protection over many weeks to years, depending on the glazing, the backing board and the sealing materials, and on whether there are cracks or leaks.
Float frames. A canvas in a float frame sits inside a tray with a visible gap around it, attached from behind rather than held by a rabbet lip. The clearance rules still apply, and so does the backing board.
4. Securing the package into the frame
Four ways, with a clear order of preference in the published sources.
Points or tacks. The trade specification's top level requires the frame back to be secured with framers' points, tacks or better, and says flexible tabs are not acceptable at that level. A conservation institute describes the same thing: secure the backing board with finishing nails driven with a brad pusher or a tack hammer, or with glazier's points, taking care not to split the moulding. A professional wiki adds the sequence worth knowing: insert four framing points first, one at the midpoint of each side, to stabilize the package, then finish; three points per side is often sufficient for small to medium works.
Screws and cup washers, through the back of the frame, when the rabbet is too shallow to take the package inside it.
Brass mending plates, which a conservation institute's textile framing note gives as the alternative where the rabbet is too shallow.
Two things the wiki warns against on the way. Tack hammers are undesirable, it says, because they jar the art object; and nails hammered into the side of a frame with the artwork already in it risk jarring the frame, damaging the frame, or damaging the artwork. A brad pusher applies steady pressure instead of impact, which is the whole reason it exists.
The finish at the back. The trade specification asks for pads or buffers at least 3 mm thick on the two lower back corners, which hold the frame off the wall and let air move behind it, and for a label with the date and the framer's name, with existing labels preserved because they can provide provenance. A dust cover across the back is standard; a preservation center describes finishing the back with archival paper or a spunbonded sheet.
Whether to tape the join between the backing board and the frame is a real disagreement, and it belongs with the sealing question in Holding the art, where both positions are set out.
5. Strength, and when a frame is the second problem
Glass is heavy, and weight scales with area. A 24 by 36 inch sheet of 2.5 mm glass is about 7 pounds on its own; at 30 by 40 it is about 10. Add boards, a backing and the moulding itself and a large glazed frame is carrying real load at four mitred corners, each of which is two pieces of wood, some glue and a few wedges.
Two published cautions. A framing tool maker says the frame must be strong enough to bear the weight of all the materials it will hold, and that a large artwork that will be glazed requires a strong frame. A conservation institute notes that some large contemporary paintings are framed with strip frames that are too thin or inadequately attached to the painting to support its weight, and that the remedy may be adjusting the placement of the hanging hardware.
In practice, three checks on a large piece. Hold the empty frame by one corner and see whether the opposite corner moves. Look at the back of each mitre for the wedges. And think about where the hanging hardware is going: hardware fitted to a weak frame transfers the whole load through a join that was not designed for it, which is why Hardware by published rating treats frame-side hardware as its own problem.
This manual does not teach cutting or joining moulding, cutting glass, or restoring a frame. It gives the measurements and the questions so an order is right, and it says which frames are too shallow or too weak for a given package. If a frame is itself historic, that is a conservator's subject: a conservation institute asks that the aesthetic and historical importance of a frame always be considered, because many frames carry labels, inscriptions and stamps, and in some cases the original frame was chosen or prepared by the artist.
Go deeper 6. Finish, old frames and what we do not teach
Reusing an old frame. A conservation institute gives the checklist: look for damaged or missing moulding, lost gilding, loose joints and warping, and replace any glass that is chipped, broken or brittle. A state archives adds the sentence worth hearing before you replace a frame at all: a frame may be important in its own right, and an expert is worth consulting before replacing or repairing an existing one.
Gilded and antique frames. Not a subject this manual teaches. A conservation institute's cleaning note says the action of dusting an ornate frame can scratch the often fragile layers of its surface finish, and that wet-wiping a frame will remove surface finishes, especially traditional gilding. The household version is: a soft brush, lightly, and nothing wet. Cleaning has the rest.
Mixing frame finishes across a group. No source in this research gives guidance on it. One gallery suggests identical frames for a series by one artist and reclaimed or vintage frames for an eclectic collection, which is one practitioner's method; an art museum's caption describes variety through diverse sizes, frames and painting styles while uniting on genre, which is a caption rather than a rule. Arranging pictures treats it as preference and says so. Preference
What the trade qualification examines that this manual cuts. Cutting and joining mitres, cutting glass, cutting acrylic, stretching canvas onto stretcher bars, ruled lines with a traditional ruling pen, and V-grooves. The first three are hazardous or need a bench; the last three take practice on scrap and protect nothing. All of them are reasonable things to ask a framer for by name, which is what A framer or yourself is for.
What a large sheet of glass does to a join. There is no published formula here, and the manual is not going to invent one. The useful habit is to notice that the weight is distributed around the perimeter and carried by four corners, and that the hanging hardware concentrates it at two points. A rail system, which does publish per-component ratings, spreads it differently; Hardware covers that.
7. See it See it
Yusuf Ferrante's canvas. Yusuf has a 30 by 40 inch painting on stretched canvas by a friend, in a rented apartment where the long wall is an outside wall and summers are humid. Known: the stretcher measures 30 by 40 inches and 7/8 inch deep; two of the four corner wedges are loose in their slots. Assumed: that the apartment sees prolonged damp rather than brief damp, which he has not measured. Open: whether to use a solid or a ventilated backing board, which the institute's own climate caveat makes an open question for him: solid buffers humidity best, and in prolonged damp a plastic board with ventilation holes is what it recommends instead. He settles it by buying a hygrometer for the cost of a sandwich and reading it for a month. Meanwhile he secures the two loose wedges with thread rather than tapping them, because keying out is a trained procedure and tapping a loose canvas tight is how people put a crack in one. The frame he is looking at has a 30 1/4 by 40 1/4 opening, which gives him an eighth of an inch per side, inside the institute's 3 to 6 mm.
- Framing a painting, a conservation institute: clearance, rabbet modification, padding, the keys, and the frame as part of the work's history.
- Backing boards for paintings on canvas, the same institute: the ranked materials, the screw sizes and spacings, and the climate caveat.
- Framing works of art on paper, the same institute: the 2 cm rabbet figure and the warning about clip-together metal frames.
- Frame types, a framing tool maker: how wood and metal frames are actually joined, and why clip framing is temporary.
8. Practice Practice
Minimum: one frame measured in a shop and compared with your package depth. Time: about 30 minutes. Materials: a tape measure, your stack figure from the depth box, and a shop or a box of frames you already own.
- Write your package depth down before you leave the house. Without it you are guessing at the only dimension that matters.
- Measure the rabbet depth of three candidate frames. Use the end of a tape, hooked into the rabbet and read against the back face. Write all three down; the variation between frames that look alike is the lesson.
- Measure the rabbet width as well. Work out how much of your mat's edge will be covered.
- Flex test. Hold an empty frame by one corner and look at the opposite corner. On a large frame, a visible movement is a reason to go up a section.
- For a canvas (Go deeper; optional). Measure the stretcher's width, height and depth, check every wedge is present and secured, and write the backing board decision in the framing plan.
9. Your pictures Your pictures
10. Check your work Check your work
Criteria
- The rabbet depth of the chosen frame is written down and is larger than the package.
- If the frame is wood, the rabbet is lined or the package is sealed, so wood acids do not reach the mat.
- A canvas has 3 to 6 mm of clearance on all sides and a backing board decision.
- The back-securing method is named, and it is not flexible tabs on anything you care about.
- Pads at the two lower back corners, and a label with the date.
Worked example
Priya Raman (fictional) takes her package depth, about 9 mm, to a shop and tries three 26 by 32 inch ready-made frames. The first is a thin metal section with a 3/8 inch rabbet: it will take the glass and the mats but not the backing board, so it is out. The second is a wood moulding with a 5/8 inch rabbet and a 1 inch face: fine, with room for a dust cover. The third is deeper but flexes visibly when held by a corner at that size. She takes the second and notes in the plan that the rabbet is wood, so the shop will line it or she will seal the package. One frame ruled out on depth, one on strength, in ten minutes with a tape.
Common mistakes
- Choosing the moulding by eye and finding out about depth at assembly. It is the one dimension whose only remedy is adding wood to the back.
- Leaving the backing board out to make the package fit. That is removing the part that protects the package from behind.
- Tapping a canvas's wedges to tighten it. Keying out is a trained procedure, and the institute that says so publishes a separate note on it.
- Letting a canvas fit tightly. Wood expands; a conservation institute asks for 3 to 6 mm on all sides and says the painting should not fit tightly.
- Nailing the back with the artwork in the frame. A professional wiki names the risk: jarring the frame, the artwork, or both. A brad pusher applies pressure instead of impact.
What is still unresolved
- Whether the back should be sealed, and what sealed means: Holding the art.
- What the finished piece weighs and what will hold it: Weight and hardware.
- Whether to order a frame chopped and joined or buy ready-made: A framer or yourself.
- Which level of framing the frame and its securing put you at: The levels.