Inlays in joinery: wood, metal, shell and stone

At a glance

Sizes and care notes are what our own inlay and shell pages say. Stone has no figure: none was read.

What an inlay is, and what it asks of the ground

An inlay is a piece of one material let into a recess cut in another and levelled flush, so the two read as one plane. Marquetry works the other way round: the pattern is cut and assembled as veneer first, then laid onto the surface, so nothing is cut into the ground (our journal article on marquetry). Parquetry is built from regular geometric pieces. Stringing is a thin line of inlay, and banding a wider strip.

The point for a specifier is that with an inlay the ground is cut. It has to be thick enough, flat enough and stable enough to take the cut and to hold what is set into it. In a veneered board the groove passes through the veneer into the core, so the veneer is only a few tenths of a millimetre and the core is what the inlay is bonded to (a general point of craft: see the review list).

Adhesive. The Wood Handbook says that where a structural bond is needed between wood and metal, only epoxy, polyurethane and other isocyanate-based adhesives may be sufficiently compatible, and that good adhesion often needs the metal cleaned and abraded, and sometimes primed, because clean metal quickly forms a weak oxide boundary layer (Chapter 10, p. 10-5). An inlay is decorative, not structural, but the same point holds: ask which adhesive the workshop uses and how it prepares the metal.

Movement: why the direction of the line matters

Humidity moves timber, and temperature moves metal. The two are not the same size. The Wood Handbook gives a coefficient for the width change of timber across the grain per 1 per cent change in moisture content (Table 13-5, for moisture contents between 6 and 14 per cent): US white oak is 0.00376 plain-sawn (tangential) and 0.00194 quarter-sawn (radial). A room at 21 C moves from 35 to 65 per cent relative humidity as wood moves from 6.9 to 12.0 per cent moisture content (Table 4-2), a change of 5.1 points. The width change is the width times the coefficient times that change (Equation 13-3).

That gives 5.75 mm across a 300 mm plain-sawn oak panel, 2.97 mm if it is quarter-sawn, and 11.51 mm across 600 mm. A brass strip 300 mm long warmed by 20 K moves 0.12 mm, so the oak moves about 47 times as far across the grain as the brass moves along its length. Along the grain timber moves far less: the Handbook gives 0.1 to 0.2 per cent from green to oven-dry for most species (p. 4-9), which on a linear reading is 0.17 to 0.34 mm in a metre over the same humidity change, close to brass's 0.41 mm in a metre.

Along the grain and across it

So the direction of the line decides the risk. A line along the grain moves with its ground by a similar small amount and can run in one length. A line across the grain of a wide piece of solid timber is held by a metal strip that will not follow the timber's movement, so something has to give: the line, the glue line or the timber. The remedies are a stable ground (a balanced veneered board moves far less: the Wood Handbook defines balanced construction as one that does not warp with moisture changes), a line broken into short lengths with a joint between them, or a different design.

No standard we could read gives a maximum continuous length for an inlay line across the grain, or a spacing for movement joints, so the figure in the wording below is left for the partner workshop. These numbers are for US white oak and for a 35 to 65 per cent humidity swing at 21 C; European oak and other species differ, and rooms with underfloor heating or air conditioning swing differently.

Making it flush, and what flush means

The usual order, as our metal inlay pages describe it, is: cut the channel fractionally tight, press the strip home and bond it, level the surface, then finish the metal and the timber together and check against the approved sample. Where the line meets another, turns a corner or stops, the junction is resolved on the drawing, because a seam is read at close range.

What flush means. No published tolerance for the finished flushness of an inlay was found, so the specification has to state a test the workshop can meet. Two work together: a step limit measured with a straight edge and feeler gauge, and a visual test, such as no gap, void or glue line visible under raking light from a stated distance. The figures are for the workshop to propose and for you to approve on a sample.

Finishes. A brass line will tarnish unless it is sealed or deliberately patinated, and the choice affects the whole surface. See lacquered or unlacquered brass and copper and patination, and finishes on timber joinery for the timber. For shell, our shell pages describe sealing and building up clear lacquer, then cutting back and polishing, so that the nacre reads through a smooth plane.

What to put on the drawing and the schedule

An inlay is quoted and made from what is on the drawing. A short list covers it:

1. The ground: material, species and cut, thickness, balanced construction and the moisture content at fixing.
2. The inlay: material and alloy, width and thickness, and the finish.
3. The groove: depth and the thickness left behind it, and the direction of the line to the grain.
4. Movement: lengths and joints for any line across the grain of solid timber.
5. Bonding: the adhesive, and how the metal is prepared.
6. Junctions: corners, crossings and stops, drawn full size.
7. Finish sequence and the flushness test, with the sample that governs.
8. Care: in the operation and maintenance manual at handover (see handover and O&M pack).

Care

Shell. Our shell pages advise dusting with a soft dry cloth, or a barely damp one dried at once, avoiding solvents, abrasive pads and standing water, and keeping dyed shell out of prolonged direct sunlight. They treat a lacquered shell surface as decorative rather than a heavy-wear one and say a worn lacquer can usually be cut back and repolished without disturbing the shell.

Metal lines. Keep abrasive metal polish off the surrounding timber, and follow the finish: a lacquered line needs no polish, and a patinated line is meant to change (trade practice: see the review list). Put the care instructions in the handover manual.

[Brass] line inlay [3] mm wide x [1.5] mm thick, let flush into a groove [3] mm wide x [1.5] mm deep cut in the [core / solid timber] of [element], running [along the grain]. Metal surface prepared and bonded with [adhesive and method stated by the workshop]. Finished flush with the surrounding surface: no step greater than [0.1] mm and no gap, void or glue line visible under raking light at [500] mm. Metal and timber finished together with [lacquer / patina] to approved sample [ref]. A line across the grain of solid timber wider than [150] mm is not permitted unless broken into lengths of not more than [x] mm with movement joints as drawing [ref].

[Natural / reconstituted] [mother of pearl / abalone] inlay [1.0] mm thick let into a recess [1.6] mm deep in a [stable MDF / plywood] ground; recess floor flat to [ ] mm over [ ] mm; bonded with [adhesive]; surface filled and built up with [clear lacquer] to flush; no lifting or cracking at the edges. Approved sample [ref] governs.

Ground: [balanced veneered board / solid timber, species, cut], at [8] per cent moisture content (+/- [ ]) at fixing, conditioned to the expected room humidity of [ ] per cent before inlaying. Inlay layout to be shown on shop drawings with grain direction, line lengths and joints. [One] mock-up of [one] junction and [one] corner to be approved, and checked under raking light, before production.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material, FPL-GTR-282 (2021): Tables 4-2 and 13-5, p. 4-9, p. 4-17, Eq. 13-3, p. 10-5, glossary. US species.
• Library sheet: thermal expansion of metals (TL-MET-23): the metal coefficients.
• Our metal inlay, shell and marquetry pages: strip sizes, bonding order and care. No standard was found for groove depth, movement joints or flushness.

Questions we are asked

What is the difference between inlay and marquetry?

In inlay, a recess is cut into a solid ground and another material is set into it. In marquetry, the pattern is cut and assembled from veneer first, as one thin sheet, and then laid onto the surface. Inlay is worked into the solid; marquetry is the veneer technique.

How deep should the groove for a brass inlay be?

The groove has to match the strip, so that the metal sits flush, and the ground has to be thick enough to take it. No published rule for depth was found. Our metal inlay pages describe strips typically 1 to 3 mm wide; the thickness and depth are for the partner workshop to propose and for you to approve on a sample.

Can I run a brass line across a solid oak top?

Not safely on a wide top without a plan. On a 300 mm width plain-sawn white oak moves about 5.75 mm between 35 and 65 per cent humidity, while a brass strip that length moves about 0.12 mm for a 20 K change. Use a stable ground, break the line into lengths with joints, or run the line along the grain.

Will brass inlay tarnish?

Unsealed brass changes colour with handling and air. A lacquer keeps it bright for a time and a patina is deliberate change; choose one with the whole surface in mind. See the sheets on lacquered and unlacquered brass and on patination.

Can mother of pearl be used on a table top?

Our shell pages say yes on a stable substrate under a built-up lacquer finish, treated as a decorative top rather than a worktop, with no sustained heat, standing liquid or abrasion.

How do I check that an inlay is flush?

Run a finger across it, lay a straight edge across the seam and look along the surface in raking light. Write the test into the specification (a step limit and a visual check) and approve it on a sample.