Making a frame that closes
The mitre is everything. If two 45 degree cuts are not exactly 45, the corner shows a gap and nothing later hides it.
The mitre decides everything
A frame corner is two 45 degree cuts meeting. If each is out by a quarter of a degree, the corner shows a visible wedge of light, and no amount of clamping or filling hides it because the moulding profile no longer lines up either.
That is why picture framers use a mitre trimmer or a dedicated saw rather than a general mitre saw. A trimmer shears the timber with a guillotine blade against a fixed 45 degree fence, and it produces a face smooth enough to glue directly and an angle that does not drift with the blade.
- A quarter of a degree shows
- Multiplied over four corners it is a visible gap. This is the whole reason for specialist tools.
- Trimmer over saw
- A guillotine shear against a fixed fence, giving a glue ready face and an angle that cannot drift.
- Cut both halves of a pair together
- Opposite sides identical matters more than either being exactly right.
- Check with a set square
- Two offcuts held together should make a perfect right angle. It takes seconds and it is conclusive.
Joining the corners
A glued mitre alone is weak, because it is end grain to end grain. It needs mechanical reinforcement, and the standard is a wedge, or V nail, driven up into the joint from underneath by an underpinner. The wedge pulls the two faces together as it goes in.
For occasional work, corner clamps and glue with panel pins driven from the outside will hold, and the pin holes then need filling. An underpinner leaves nothing visible on the face, which is why every frame shop has one.
- Glue alone is not enough
- End grain to end grain is a weak joint. It needs a wedge, a pin or a spline.
- Underpinner wedges pull it closed
- Driven from below, so nothing shows on the face and the joint tightens as they go in.
- Two wedges per corner
- One near the inside edge and one near the outside, so the corner cannot rotate.
- Clamp and pin for occasional work
- Corner clamps, glue and panel pins from outside. The holes then need filling.
Cutting mounts and bevels
A window mount is cut with a bevelled edge so the aperture opens outward toward the picture, which is what gives the depth. That bevel is cut by a mount cutter running a blade at 45 degrees along a guide, not by a knife freehand.
The classic mistake is equal borders all round. A mount traditionally has a slightly deeper bottom border, because the eye reads a visually centred aperture as slightly high. Around 10 to 15 percent more at the bottom is the usual allowance.
- Bevel opens outward
- Toward the picture, which is what gives the aperture its depth and shadow line.
- Deeper bottom border
- About 10 to 15 percent more. A geometrically centred aperture reads as too high.
- Cut on the back
- So the blade enters from the reverse and any slight overrun is on the hidden face.
- Fresh blade every few mounts
- A dull blade drags and tears the core, which shows as a fuzzy bevel edge.
Glass, acrylic and what goes behind
Glass is cut by scoring once with a wheel and snapping along the score. Once is the rule: running the cutter back over a score never improves it and it ruins the wheel. Acrylic is cut by scoring repeatedly or with a fine saw, and it scratches far more easily than glass.
Behind the picture goes a backing board, then it is all held in with flexible points or a points driver, and the back is sealed with tape. The sealing matters more than people expect, since dust and insects get in through an unsealed rebate and cannot be removed later.
- Score once, then snap
- Rerunning a score never helps and it destroys the cutter wheel. One firm pass.
- Acrylic scratches
- Leave the film on until the last moment, and never clean it with a paper towel.
- Points, not nails
- Flexible points hold the stack in and can be lifted later to reopen the frame without damage.
- Seal the back
- Tape around the rebate. Dust and insects enter through an unsealed back and cannot be got out.
If the picture matters
Ordinary mount board and tape are acidic, and over years they burn a brown line into the edge of a print where they touch it. That damage is not reversible. For anything of value, conservation grade board and acid free tape are the difference between framing and slowly destroying.
The same applies to the mounting method. Never dry mount, glue or tape a print down permanently if it might ever need to come out. Hinge it at the top with acid free tape so it hangs free and can be removed.
- Acidic board burns the print
- A brown line at the mount edge within a few years, and it does not come back out.
- Conservation board and acid free tape
- For anything you would be sorry to damage. The cost difference is small.
- Hinge, never glue down
- Acid free tape at the top only, so the print hangs free and can be removed later.
- Keep the picture off the glass
- A mount or a spacer, because condensation makes a print stick to glass permanently.
Frequently asked questions
- What tools do I need to start picture framing?
- A mitre trimmer or fine mitre saw, an underpinner or corner clamps, a mount cutter, a glass cutter, a points driver, a steel rule and set square, glue, and a soft mallet. That set will produce a properly made frame with a closed corner and a bevelled mount.
- Why does my frame corner have a gap?
- Because the two mitres are not exactly 45 degrees. Even a quarter of a degree out on each shows as a visible wedge, and it multiplies across four corners. Check by holding two offcuts together against a set square, and use a trimmer rather than a general mitre saw.
- What is an underpinner?
- A machine that drives a wedge shaped V nail up into a mitre joint from underneath. The wedge pulls the two faces together as it enters and leaves nothing visible on the front of the frame, which is why it is the standard way to join a picture frame.
- What is a mitre trimmer and why not use a mitre saw?
- A trimmer shears the moulding with a guillotine blade against a fixed 45 degree fence. It gives a face smooth enough to glue straight away and an angle that cannot drift as a saw blade can. A mitre saw leaves saw marks and its accuracy depends on the blade and the setup.
- How do I cut a bevelled mount?
- With a mount cutter, which runs a blade at 45 degrees along a guide bar with adjustable stops. Cut from the back so the blade enters the reverse face, and change the blade every few mounts because a dull one drags and leaves a fuzzy bevel.
- Should mount borders be equal all round?
- Traditionally not. The bottom border is made about 10 to 15 percent deeper, because the eye reads a geometrically centred aperture as sitting slightly high. Equal borders look correct on a measurement and slightly wrong on the wall.
- How do I cut picture glass?
- Score once with a glass cutter along a straightedge, then snap along the score by lifting or tapping from underneath. Never run the cutter back over a score, since it does not improve the cut and it ruins the wheel. Wear gloves and eye protection.
- Does it matter what mount board I use?
- For anything of value, yes. Ordinary board and tape are acidic and burn a brown line into the edge of a print over a few years, which is not reversible. Conservation grade board and acid free tape cost a little more and prevent it entirely.
