Glues for veneer and geometric intarsia

Why the choice of glue matters more in geometric intarsia than in solid wood

When solid wood is glued, the glue line is wider, the material thicker and the species uniform. Errors are absorbed by the tolerances of material and working. In geometric intarsia things are different: veneer is between 0.5 and 1.2 millimetres thick, the glue line is narrow, and every joint lies between two species whose properties differ.

Contrast is not a decorative element here but an amplifier for everything that shows at joints. A glue stain that would be invisible at an oak-to-oak joint can be seen at once at a wenge-to-ash joint. Glue squeezed out of an open joint under pressure onto a light area leaves contamination that no amount of sanding fully removes.

Veneer is also no material for correcting mistakes. Its thickness decides how many rounds of sanding are possible before it is used up. If you try to remove glue residue by sanding hard, you risk sanding the veneer too thin or damaging neighbouring areas. So the strategy has to be: apply cleanly, do not correct.

Kinds of glue for veneering

In practice, five kinds of glue come into question for veneering. Each handles differently, cures by a different chemistry and needs different pressure and pressing time.

PVA wood glue (polyvinyl acetate) is the most widely used glue for veneering. It is water-based, easy to apply, non-toxic and available with a wide range of open times. It cures thermoplastically, which means the bond stays reversible under heat and moisture. For precision work, a PVA with medium viscosity and a controlled open time is the standard choice.

Urea-formaldehyde glue (UF) cures by chemical cross-linking and is hardly reversible once cured. It is more water-resistant than PVA, more resistant to creep and harder. It calls for more discipline about pot life, because once mixed it can only be worked for a limited time. The bond is brittle enough not to soften in sanding.

Hide glue (animal glue) is the classic reversible glue of cabinetmaking. It is applied warm, sets as it cools and can be released again with hot water or steam. In intarsia it matters when repairability or later dismantling is planned. Its short open time calls for experienced working.

Contact adhesive is applied to both surfaces, left to dry and then bonds instantly on contact. It forgives no errors of positioning: the bond is active at first contact. It is unsuitable for precision intarsia with tight tolerances, because the veneer can no longer be corrected after the first contact.

Film glue and glue sheets are pre-impregnated sheets or layers of adhesive that are laid between veneer and substrate and activated by heat. They give an even thickness without variation in application, but need a heated press. On small workpieces of irregular geometry, the transfer of heat is hard to control.

Veneer tape: the tape used in assembly to hold pieces together before pressing is not a glue in the sense of the bond; it holds things temporarily on the show face. But its water-activated adhesive takes up moisture from the glue and sits ever more firmly over time; if the tape stays on the veneer for long, it tears out fibres more easily when peeled off. Veneer tape should be removed as soon as possible after pressing.

Open time and pressing time

Open time is how long an applied glue stays workable before it grabs and no longer makes the best bond. With PVA, the open time lies between one and eight minutes, depending on the product and temperature. Cooler temperatures lengthen it; larger glued areas need more time to lay up.

For geometric intarsia with many small single pieces, too short an open time is a practical problem. If the veneer assembly consists of thirty single pieces, the whole laying-up has to be finished within the open time. A PVA with two minutes of open time is unsuitable here; a product with four to five minutes allows calm, precise laying down. With urea-formaldehyde glue the pot life, the time after mixing, can be the limiting factor, not the amount applied.

Pressing time is the minimum time the glue has to cure under pressure before the pressure can be taken off. For PVA at room temperature, typical pressing times are thirty minutes to two hours for a bond that can be handled. Full cure takes up to twenty-four hours. If you sand straight after pressing, you risk sanding into glue that has not fully cured, which can leave scratches.

Applying glue, thickness of the layer and pressure

The amount of glue decides how much glue is squeezed out under pressure. A thin, covering layer, with no shine and no runs, is enough to make a complete glued area. More glue gives more squeeze-out at the edges and at gaps, not a stronger bond.

Pressure has to be spread evenly over the whole veneer surface. Uneven pressure creates zones with too little glue and zones with too much. In a vacuum press the pressure is spread evenly; with clamps, it has to be ensured by enough clamps and a rigid caul. Details of the pressing stage are described in gluing and pressing veneer.

Too much pressure on veneer that is too thin can crush it or push glue stains through the material. Open-pored species in particular, such as oak, have large pores through which glue can come straight to the show face under high pressure. Oak veneers therefore call for reduced pressure and a check on the amount of glue.

Glue creep, moisture movement and warping

PVA glue is thermoplastic: it stays slightly elastic after curing and can creep a little under lasting mechanical load or changes in temperature. In small precision objects in stable room conditions, creep is hardly noticeable. In large intarsia panels, or in rooms with a strongly changing climate, it can lead to visible offsets at joints over months.

Moisture movement is a separate risk. If the glue is water-based, it brings moisture into the veneer as it is applied. The veneer swells on the glue side and can ripple. This effect is short-lived and goes back once the glue sets and the moisture evaporates, but during this phase a freshly laid veneer assembly can build up tension in the press that leads to slight distortion once it comes out.

Warping after pressing happens above all when veneer and substrate move by different amounts with moisture. A substrate that is more stable than the veneer, such as MDF or multi-ply, holds the veneer in place. A substrate that moves on its own can distort the veneer. For geometric intarsia objects, MDF and multi-ply are the more reliable substrates. Solid wood substrates call for the grain directions of substrate and veneer to be matched.

Glue bleeding through and contaminating the show face

Glue can come through the veneer in two ways: from below, when too much glue lies on the substrate and is pressed through the veneer, and from the side, when glue gets out of a joint under the veneer tape or into an open gap. Both end on the show face with a layer of glue that spoils the finish.

Bleeding through from below happens especially with open-pored veneer. Oak has large earlywood pores that you can see with the naked eye. With too much glue and too much pressure, these pores fill completely. After curing and sanding they appear as hard, non-porous dots in the show face. Oil does not penetrate there, and wax sits on top and reflects differently. The result is a restless finish with a visible pattern of dots.

Contamination from the side, out of gaps, happens when two neighbouring pieces of veneer do not close completely in the press. Glue forced into the gap under pressure flows out onto the show face and cures there. In sanding the film of glue is removed, but the pores underneath stay sealed. This contamination cannot be fully removed even by careful sanding: the surface looks clean afterwards, but as soon as oil or wax goes on, the sealed pores show.

Dark-to-light contamination at contrast joints

The most critical form of glue contamination in geometric intarsia is the transfer of dark material onto light neighbouring fields. Wenge veneer gives off dust particles and extractives that are deeply pigmented, in sanding and in contact with moisture. If moisture from the glue moves from the wenge field into a neighbouring light field of ash or oak, this pigment goes with it.

The result is a greyish brown discolouration in the light wood that shows as a shadow after the finish. This fault can no longer be fully undone after pressing, because the pigment has got into the pores of the light wood. Sanding removes the surface, but not the particles that have got deeper.

Protection at high-contrast joints includes: masking light fields with painter's tape before applying glue, working with minimal amounts of glue, avoiding too much pressure that pushes glue into joints, and removing glue residue with a clean, dry cloth straight after pressing and before it fully cures. Working with a narrow brush that applies glue precisely to the substrate and leaves the veneer edges free reduces glue getting into joints with small geometric pieces.

How the glue behaves in sanding

Cured PVA stays tough and slightly elastic. It sands along with the wood, but not at the same rate as the surrounding wood, and it clogs the abrasive quickly. That means slight glue residue on the show face is not removed evenly in sanding. Instead it creates sanding shadows: spots where the abrasive removes the surrounding wood faster than the glue, which leaves a slight hollow around the glue.

UF glue is brittle after curing and sands much like wood. It leaves fewer visible differences between glue and wood surface in sanding, but can dull abrasives faster, because the cured urea film is more abrasive.

Veneer sanded over glue residue does not lose thickness evenly. If you sand a veneer assembly with spots of glue, you have to go deep enough into the layer to remove the film of glue completely. That lowers the whole veneer thickness by that amount. At 0.6 millimetres of veneer, every round of correcting a fault uses up reserves planned for the later fine sanding.

How glue interacts with the finish

Cured glue is not porous. Oil, wax and shellac get into the pores of the wood and create their optical effect there: depth, saturation, sheen. Where glue covers the surface, that effect is missing. The finish sits on the glue, not in the wood. The result looks different from the surrounding area: flatter, more matt or with a different sheen.

This interaction is especially visible on oiled surfaces. A dry field of wood takes up oil evenly and darkens. A glue stain next to it takes no oil and stays lighter. After the finish, the difference shows as a light spot on a surface that has got darker. More on how different species behave under finish is in finishing geometric intarsia.

Shellac and wax show the same principle: shellac cures smoother on a film of glue than on wood pores, which creates local differences in sheen. Wax is harder to rub into glue stains and gives a different texture there. If you want a fine oil or wax finish, you have to make sure no glue residue is left on the show face after sanding.

Reversibility, repair and precision of assembly

PVA bonds can be softened with heat and moisture. Hot steam releases the bond, but at the same time it swells the veneer. In a geometric intarsia assembly of twenty single pieces, releasing a single piece with steam is hardly possible without affecting its neighbours. The practical reversibility of PVA in veneering is therefore limited.

UF glue is practically irreversible once cured. Hide glue, on the other hand, was used in cabinetmaking for centuries precisely because it can be released with warm water. If an intarsia is designed so that single fields can be replaced or corrected later, hide glue is the only glue that reliably meets that demand. Its short open time and the working temperature it needs make it more demanding than PVA.

Small geometric objects forgive no attempts at repair at the bond. Assembly is the last step at which errors of alignment can be corrected without using up material. Whatever is visibly open or offset after pressing needs an intervention that costs veneer thickness. The consequence is that the strategy for glue has to aim at cleanness and control from the start, not at the possibility of a late correction.

Geometric patterns are especially unforgiving here: an offset strip in a repeating pattern shows the error in every line that continues from it. If you understand the effects of errors in planning the pattern, you know why geometric errors get no second chance after pressing.

Which glue Reihax uses

Reihax glues every piece with PVA wood glue, the pen holders as well as the wall pieces, and presses them in a veneer press. For objects used indoors, PVA glue with a medium open time is the usual and reliable choice.

More important than the kind of glue is that the glue does not show. It should bond without colour, without stains and without affecting the finish. Three rules prove themselves for that: as little glue as possible, as a thin, closed layer on the substrate, not on the back of the veneer; enough pressure for full contact, but not so much that glue is forced through the pores of light woods such as oak or ash; and remove glue that squeezes out at once, before it grabs.

Every joint where dark meets light wood is a place where glue can become visible with dark dust or colour. A pattern that shows a shadow at the wenge border after the finish loses its sharpness, however exact the joint is mechanically. Geometric sharpness comes from cutting and assembly, and from glue only where it belongs.