Monday 9 July 2018
Placing veneer in a press allows it to be held firmly in position while the glue sets, enabling you to use a modern slow-setting adhesive. But while this overcomes some of the problems of hand veneering with hot hide glue, there are other issues associated with pressing veneered work. For one thing of course you need access to a press, which might sound tricky to arrange, but veneer presses come in many shapes and sizes.
While in theory there is no limit to the area you can hand veneer, in practice dealing with larger dimensions becomes very awkward as one part sets while you are still pressing down another. Also, hide-glued furniture is not good news in warm moist climates or locations prone to warmth and dampness.
Other than for restoration work or very small pieces, makers nowadays nearly always use synthetic glues for veneering.
Ideally veneers should be laid out flat and stored that way in the workshop, but for practicality long pieces often need to be rolled along the grain direction.
Rolls of veneer should be kept to a large diameter, with the outside wrapped in cardboard to prevent strain from the ties. This will also guard against marking and fading. Tight rolling would set a curl in the veneer, making subsequent use difficult.
Burr veneers are harder to flatten. In the process of drying after the veneer is cut, differences in grain direction cause different movement and hence the veneer buckles. These undulations must be flattened out before you apply the veneer. Also, it will be prone to buckling later if there is any weakness in the glue bond.
Brittle and badly buckled veneers are best avoided but they can be softened if necessary. Spray with clean water and leave to soak for a day or two by pressing between damp layers of paper towelling. Donï¿½t leave the veneer damp too long as mould is likely to grow and discolour or destroy the veneer.
The groundwork is also referred to as the substrate or the core. The most widely used groundwork nowadays is MDF (medium density fibreboard). It has no grain direction and suffers practically no movement with changing humidity.
Although MDF does not move with moisture, the veneer layer does. Balancing veneer on the reverse side will ensure the composite boardï¿½s reaction to changes in air moisture level is balanced.
Unlike chipboard, which is much coarser and loosely packed in the middle, MDF has a uniform density and strength. The disadvantages are its featureless surface, a tendency to creep under load and a relatively poor strength-to-weight ratio compared with hardwoods.
A good alternative to MDF is plywood, with the veneer placed at right angles to the grain direction of the plywood face.
Traditionally, furniture was often veneered on oak or softwood carcasses, but veneering over solid wood substrate can result in severe stresses on the glue and on the veneer.
This is caused by movement of the substrate, so the grain of the substrate and the veneer must be aligned. Designs that allow for solid wood movement are more difficult to implement with veneers applied. Small cracks that develop in the substrate will telegraph through the veneer and may cause it to delaminate and split.
This is not to say that solid wood substrates should not be used ï¿½ some very fine furniture has been made this way, but it does demand extreme care.
Laying it out
A large panel may take several pieces of veneer to cover. It is necessary to cover the surface in one go to make the joints tidy and avoid making the glue lines visible.
The first stage before any cutting is to lay out the veneers on the substrate to see how the grain will run between adjacent pieces and what patterns the figuring will make.
Before you choose which surface will be the show face, slide adjacent veneers alongside each other to find where they make the best match at the edge joint. Conventional pattern matching such as book match and quarter match works well on traditional pieces but can look old fashioned on contemporary furniture.
Mark each piece with chalk once you have decided where and which way round to put it.
It is wise when estimating the requirements for veneer to buy an extra sheet as backup in case something goes wrong. As well as holding the job up if you need to get more, veneers from different flitches will not match.
To mark out roughly the size of veneer needed, lay the substrate on top of the veneer and use it as a template to trim oversize. For more accurate cuts clamp the ends of a straightedge to the bench and use a bevel-edged knife so the flat side runs along it. Score lightly first time so the veneer is not pushed away from the knife if it catches.
When two or more sheets need to be edge jointed they must be cut absolutely straight if the glue line is not to show. Veneer edges can be jointed with a plane on a shooting board, but it needs to be a long board that provides a means of clamping the veneer on top. Watch out for buckling in the veneer that causes the edge to distort when it is released from the shooting board.
Get it taped
Having cut and jointed the veneer sheets it is important they stay in tight alignment. Thin or buckled veneers are particularly likely to move when the press is applied.
Sheets of veneer, show face up, are taped together across the seams every 100mm or so, pulling the edges together and along the seams to secure the joint. Veneer tape is gummed and with water it becomes extremely sticky. The tape is thin so as not to leave depressions in the veneer.
It requires careful handling to get the tape in the right place while holding the veneers in alignment. Press the edges together and feel with your fingertips for any gap or overlap before applying the tape.
After the veneer has been glued you can scrape or sand away the tape or else damp it to peel it off.
Roll on the glue
Modern resin powdered glues and PVA types do not have any significant grab and they offer a long open time, time enough to spread the glue on the substrate, lay the veneer on it and then apply pressure with the veneer press.
Every time you open powdered resin it absorbs moisture from the air which limits its shelf life so it pays not to buy too much at a time. Paint rollers or rubber ink rollers are ideal for applying the glue.
Apply synthetic glue to the substrate, not to the veneer, otherwise the veneer will curl.
Unlike hide glue, resin is not good grain filler and it is difficult to remove from the show face. If you apply too much glue it will squeeze through open grain and cracks in the veneer. The excess must be scraped off otherwise it will glaze the show face, making it repellent to finishes.
In the past small mechanical veneer presses were made from cast iron with a hand screw to apply pressure to flat plates. Commercially built mechanical veneer presses, large enough to take a tabletop, use hydraulic rams to apply even pressure to the platens.
For small-scale veneering a press can be improvised using a series of pairs of wooden rails pulled together at each end by G-cramps or sash cramps. By slightly crowning the rails along their length they can be shaped to form cauls that apply pressure from the centre outwards. A pair of laminated kitchen worktop boards makes ideal pressure plates to clamp between the cauls.
The substrate and the veneer are then sandwiched in the middle.
Work the clamps progressively to apply pressure evenly from the centre outwards, giving the glue time to squeeze out. This system works all right for small areas but is impractical for large panels. To apply sufficient pressure needs a great number of clamps, while the cauls are likely to slip as they tighten.
Before applying a mechanical press, any step caused by small differences in the thickness of veneers must be padded out to ensure even pressure is applied to both sides of the joint.
A vacuum press consists of either a rubber sheet stretched across the veneer work on a table or else a vinyl bag containing the work. A vacuum pump removes practically all the air so atmospheric pressure presses on the veneer.
Wood and MDF are semi-permeable to air so within a few minutes the vacuum evens out. The atmosphere applies one kilogramme per square centimetre which amounts to 10 tonnes to each square metre. To put this in perspective it is equivalent to parking one end of a fully loaded HGV on an average-size dining table, quite a force.
Once the air is removed from the bag the pump only has to cope with leakage. This means a small pump consuming little power will do the job adequately.
Vacuum presses are relatively cheap, compact and powerful and have the advantage that the pressure is uniform regardless of the area, shape or the thickness of veneer.
When it has set
With the veneer removed from the press, clean off glue squeeze-out with a chisel before it becomes brittle. The thinness of veneer could easily lead to it being penetrated by over-enthusiastic scraping. If you sand the surface, avoid any tendency to round the edges as this will thin the veneer or remove it.
Allow veneered boards to dry evenly from both sides with equal ventilation otherwise they may suffer cupping and distortion.
Bubbles are less likely to form with resin in a cold press than with hot hide glue and hand veneering, but they are harder to deal with. Glue needs to be injected into bubbles with a syringe before the veneer hardens, then pressed again.