There are many opinions on how wood should be glued up. I am not offering another opinion I am merely telling you how this was done in the past. After examining thousands of old pieces of furniture and other woodwork of the period there are common techniques that traditional craftsman invariably follow. There are exceptions some and many of these have caused problems for restorers today. After making many reproductions using the same original techniques I have determined that our ancestors had figured it out and what they did and what they documented works.
The first rule about wood is to orient it the way it grew in the tree, the outside to the outside, the top to the top, etc. While this is not always possible it is a good general guideline to follow. Modern scientific studies have shown this to be the case, the outside of wood weather better than the heart side.
While I am a traditionalist and mainly use hot hide glue, these techniques will work for other glues. I also use liquid hide glue that is readily available for many applications such as minor repairs or veneer work. For gluing up wood I use hot hide glue and I also always key or tooth the surfaces prior to gluing. This is done with a toothing plane, toothing scraper or even a rasp. Anything to intentionally roughen the surface to provide an additional key for the glue will provide for a stronger joint. A toothing plane or veneer plane can also smooth and flatten a joint and the fine ridges left in the wood will even slightly interlock as the joint is clamped together. Exercise caution when tightening the clamps. You can over tighten the clamps and actually starve the joint of glue. Apply just enough pressure to make the joint snug. With hide glue it is a good idea to go back after 10 or 15 minutes and slightly tighten the clamps. This is done because the water in the glue will be absorbed in the wood and this technique produces a tighter joint and helps prevent over tightening of the clamps.
When it comes to gluing up wood, you can’t have enough clamps. An old saying is ‘the worth of a cabinetmaker is determined by the number of clamps’, and that couldn’t be truer. If you are going to be making a lot of the same size glue up, you may consider making a dedicated clamping system such as notched boards and wedges. When gluing together boards that are wider than your longest clamp, instead of using two clamps interlocked an old clamping appliance can come in handy. This tool called a clamp extension and is illustrated below. Another advantage to this tool is that one edge can be used to keep the boards flat as you glue them together. Other types of panel clamps can be helpful when gluing up cabinet sides or tabletops that are simple to make, adjustable and will leave your other clamps free.
Different species of wood have diverse gluing characteristics but all gluing is better if the wood is keyed or toothed prior to gluing. It may seem like I mention this a lot, but most old pieces show evidence of this technique and I can always make a joint that ‘will pinch a hair’ using this ancient method. Some woods such as chestnut glue up well and appear as groundwork for veneer in many old pieces. Pine is also a fairly easy wood to glue but some with a lot of pitch can cause a problem. When gluing up pitchy conifers I will wipe the glue joint down with turpentine to remove surface pitch that can cause a problem with the glue adhering to the wood. Exotic woods such as rosewood and teak can have oils that make it difficult for the glue to stick to the wood. There is some discussion about using acetone (1839) or other solvents to clean these oily woods prior to gluing, some say it is a good idea, others say it draws more oil to the surface. When gluing up these woods I do two things, first key or tooth the wood then I etch it with a clove of fresh garlic. I wipe off any residue then it is ready for hot hide glue. This etching also works on brass or other metals, prior to gluing it down to wood. Extremely hard woods such as snakewood or ebony need to be toothed before gluing and wood such as lignum vitae, which has unique lubricating properties, the joints should be keyed and etched with garlic. Of course the best cut for wood that is to be joined is quartersawn, if possible as this is the most stable cut of wood.
There are two types of glue ups when it comes to wood; edge joining for panels and tops and face joining for large posts or turnings. I will first discuss edge joining of boards to form flat panels for cabinet sides or table tops and you should notice immediately that this goes against the rule about the outside of the tree always to the outside of the project.
Boards to be used for glued up panels should be quartersawn if possible, they also must be flat and the edges joined square. The joined edges should be toothed or keyed and then they are ready for layout. This is the crucial step in the construction process and in order to produce the best results this step must be followed. Look at the ends of the boards and place the cups in the boards opposite to each other. One board should have the cup down, the next the cup up, next cup down, etc. The cup is the curve in the grain of the wood formed by the annual growth rings. Now of course when you use quartersawn material there is no cup and the pieces can be positioned for best grain match and glued up. The curved growth rings want to straighten out when they age and by having opposing curved rings or cups they will tend to ameliorate any movement from board to board. If all of the cups are put in the same direction the whole panel will move in the other direction causing warping. When gluing up very wide panels, sometimes it is better to glue up a couple of sets of boards, allow them to dry then glue the sets together to the final size. If some of the boards have bows from end to end, this ‘set’ method can eliminate having to deal with several stubborn boards at one time. You may have to move the boards in different positions to get the right sequence and to get a good grain match, just make sure to keep the curved rings in opposite directions on adjoining boards. Once you are happy with the layout place ‘witness marks’ on the board to indicate their location in the glue up and it is also a good idea to number your boards in sequence to keep them in order. Make sure the joints are square and have been toothed, clean off any dust or debris and quickly apply the hot hide glue to the boards and clamp them up. It is helpful to warm the boards prior to gluing to prevent the glue from setting up too fast. I will sometimes wipe the joints down with hot water prior to adding the glue; this prevents the wood from absorbing the water from the glue causing it to thicken too quickly.
These directions are for butt joints but there are methods of joinery that add strength to the joint. A simple lap joint of two boards with a rabbet down the edges will increase the glue surface and does add strength to the joint. Also tongue and groove or matched joints works well to strengthen an edge joint. The groove should be slightly deeper than the tongue is long to leave a little space for glue and for any expansion that may occur in the future. A loose tongue or spline can also be used to align and strengthen joints with matching grooved into which the loose tongue is glued. Some prefer to have the grain of the spline or loose tongue running cross grain to provide the greatest strength. This will require that the spline be made of several pieces and will not cause the cross grain problem that occurs in other joint treatments. Dowels can also be used to strengthen a butt joint and can be very helpful for alignment of particularly stubborn boards or to just line up the boards to make finish work easier. The dowel holes should be perfectly aligned and the dowels should be slightly shorter than the total length of both dowel holes.
When face-gluing boards, many of the same methods for edge glue ups are used but the grain orientation is different. Face glue ups require that the faces of the adjoining pieces be perfectly flat and toothed prior to any gluing. These are usually squared up after they have been glued together, but it is helpful to have the pieces all the same size and square when you start. Now for the grain orientation, some feel that any cupping grain should be opposite to counteract any movement. While this is important on panels it can present problems with face glue ups. When gluing up pieces for large posts or turnings, I always have the cupping grain going in the same direction. This means that when the piece moves with age, and it will move, all of the movement will work together rather than having opposing internal forces that can cause cracking and delaminating of the joints. Quartersawn wood will not have any other problems other than normal shrinkage due to age. The matched cup grain of the wood produces a more uniform look when squared up or turned and finished. You also want to orient the grain so that the tops of the boards (as they grew in the tree) are all in the same direction and oriented pointing up if possible. You can usually determine the top of a board by the peak of any arched grain pattern in the board. These peaks should point up, whether on panels or posts. This has been a tradition for centuries and a common mistake made by woodworkers today.
There are special considerations when gluing up cross grain work such as a breadboard end for a panel. The cross-grained piece will not shrink along its length but there will be shrinkage or movement along the other boards. The ends of the boards are rabbeted on both sides to produce a tenon along the ends of the joined boards. The cross-grain piece is usually cut with a groove that fits on the tenons and helps hold the panel flat. Problems occur when there is movement of the wood, if the cross grained end cap prevents movement, the panel will split, at the joint if you are lucky and in the middle of a board if you are not. Some ends are secured with pegs or nails in the joint and this will prevent any movement and will cause damage. By adding a drop or two of glycerin to the hide glue will make it slightly flexible and hide glue is also thermoplastic and will adjust and actually slide and if this is not too fast will adjust without splitting. If you use a peg or nail only use one or two near the center of the panel to allow for movement on either side of the nail or peg. Rely on the glue to hold the pieces together. This also applies to cross grain pieces in dados or slip dovetail in solid panels. Fasten them with a mechanical fastener only in the center of the piece, use the hide glue and glycerin for these joints and always be cognizant of the fact that there will invariably be some movement. This also applies to molding that is applied cross grain such as the crown molding around the top of a cabinet or cock beading around a drawer front. If you use modern glues you will have failure, while hide glue can go a long way to prevent this problem. These are usually secured with nails and in many cases there is no evidence that glue is used at all. Many old pieces will have moldings that are longer than the cabinet is wide, the cabinet has shrunk cross-grain and the molding that is long grain doesn’t shrink as much. These usually stick slightly out the back of the cabinet or in the case of cock beading will push the miter joint open as the board in the drawer front shrinks. The more stable the panel the less movement and the fewer problems. Quartersawn panels will shrink less than panels with flat sawn boards but there will still be some movement.
In most instances you want the wood to stay together but there is one particular application where you only want a temporary bond. This is the technique used when making split turnings. This has the advantage of being easier and safer than sawing the turning in half. Cutting a round turning in half can be a difficult task but this method makes it easy to produce half turnings. The pieces of wood should be slightly oversized; they can be worked down after the glue dries. The pieces should be half as thick as it is wide, placing the joint in the center of the finished piece. The surfaces are prepared as with other glue ups, planed flat and toothed to hold the glue. The toothing is not that critical but I do it out of habit and it does produce a smooth albeit toothed surface. Place glue on both pieces to be glued together and place brown craft paper in between the two pieces of wood. I cover the entire joint with the brown paper and smooth down any air bubbles, align the pieces and clamp them together. These I always allow to dry overnight as you want a good secure joint while the piece is being worked on the lathe. I then square up the piece making sure that the joint remains in the exact center of the piece. Use caution when securing to the lathe, avoid excessive tailstock pressure as it may prematurely split the center joint. After the piece is turned to the final shape it is removed from the lathe and a thin sharp wide blade chisel is placed directly on the paper joint and lightly tapped. This will split the two pieces of wood apart at the seam producing two identical half or split turnings. The paper can be moistened and easily scraped off and is ready to attach being already toothed.