Tuesday 17 February 2009
When Woodcarving reader Frans van Waardenburg asked us for some advice on gluing wood, we asked Dick Onians to step in and put the record straight
Mr van Waardenburg's questions about the gluing of wood are very pertinent. As it is rare to find large pieces of seasoned wood that are free from splits, we have to laminate or settle for small sculptures, or tolerate shakes. Carved wood, like furniture, historically has often been jointed with glue, sometimes dowelled. Grinling Gibbons often used nails. It is more economical to build up wood only where it is needed than to carve away large amounts of waste (see photo 1). Joints have often been masked by the use of paint or gilding.
Timber yards generally reduce logs to boards of 100mm (4in) thickness or less, as these are the dimensions that their main customers - the building and furniture trades - want. Besides, it is quicker and cheaper to season thinner timbers. Thinner wood moves more easily without splitting.
When laminating wood, the most important considerations are wood movement, colour, figure, and angle of the grain.
Wood movementWood will always respond to changes in the relative humidity of its environment; this is why doors sometimes stick in damp weather. Mr van Waardenberg is right in believing that quarter sawn wood is the most stable; this is because the wood shrinks in the radial direction about half as much as around the annual rings. Technically wood is described as quarter sawn if the annual rings, as seen from the end of a plank, meet the broad surfaces at between 45 and 90 degrees (see photo 2). This means that a radially sawn plank shrinks comparatively little across its width. It does, however, shrink in thickness more on the edge towards the outside of the tree than it does on the side towards the heart.
If you look at the ends of a well-made table top, which has boards glued side by side - butt jointed - you should find that it is made of quarter sawn boards, which have the heart edges together and the outside edges together (see photo 3). This is to prevent the step that occurs if an outside edge shrinks next to a heart side edge.
One result of this different shrinkage behaviour is that boards that are not quarter sawn - which is what most boards are nowadays - shrink more on the surface towards the outside of the tree. This is called 'cupping' (see photo 4). If you butt joint this plain sawn timber to make a wider panel for relief carving, you must reverse alternate boards to prevent the finished piece curling up from side to side (see photos 5-8). The slight waviness that results is usually negligible. When you laminate to make a thick block, you must counteract this cupping tendency by putting heart surfaces together, or outside to outside (see photos 9-10). Most modern adhesives can cope with the stress if the wood moves, unless it is unseasoned when assembled.
Colour and figureIf you join the pieces in the ways described, it is likely that the figure and colour will match. However, as can be seen, matching of figure is impossible if there is a knot (see photo 5) or some sudden change of grain. To avoid problems with matching colour and figure, use woods such as lime (Tilia spp.) (see photo 11) and jelutong (Dyera costulata ) (see photo 12), which have very little figure and colour change. Alternatively, a wood with a wild figure such as you find in English walnut (Juglans regia), can hide a join.
Grain directionIf the pieces you laminate have grain with a definite slope, you will need to judge whether any discrepancy in colour or figure is of more or less importance than having grain lying at contrary angles. If a plank sawn in two is laminated by, as it were, bending it back on itself (see photos 9-10), the colour match and the angle of grain should both find the best solution.
PreperationThe strongest and least visually disruptive joints are achieved by careful planing of the surfaces to be joined. If you have access to a planer thicknesser, you can save yourself effort, but you will still need to use a smoothing plane and a cabinet scraper for the best results. Another successful method is to rub the surfaces to be joined on a sheet of medium grade abrasive paper, glued to a slab of polished marble or granite. You must be careful not to rock the wood when doing this. Wash any grit deposited out of the grain before gluing.
AdhesivesMost woodworkers today use the white wood glue, PVA. This provides a strong, and as far as we know, durable joint. The only glues that have really been tested by time are the animal glues and natural resins and gums. These are likely to be affected by heat and damp, and the animal glues, being rich in protein, are an invitation to insects and moulds if there is moisture present. Although some PVA glues are advertised as suitable for outdoor use, it is best to use a formaldehyde resorcin such as Bison Glue or a polyurethane glue such as Gorilla Glue.
Titebond 2, an aliphatic resin glue which I have tried, leaves a yellow glue line. The resorcin glues leave a reddish brown line such as we see in exterior grade plywood. The polyurethane glues leave a line of foam unless the joint is very tight. They do, however, fill gaps. PVA can also fill a gap, leaving a grey line that tends to blend with the wood.
One possible drawback with PVA is that if you are gluing oak (Quercus robur), it may react with the tannin in the wood and go black, even staining surrounding wood if the surplus is not wiped off immediately. Epoxy resins are very strong but quite expensive and, if a thick glue line is left, may turn yellow with age. Cascamite is used less now than formerly. It gives a very strong but brittle joint which, if not perfect, leaves a hard glue line that sets like glass, and takes the edge off chisels.