Ó 2000-2001 Stephen A. Shepherd
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. See Using Hide Glue.
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. See Clamps and Clamping or Cramps and Cramping.
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. See The Nature 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.
There is a simple trick to determine if the glue joint between the ground work and the veneer is solid or not. Lightly rubbing your fingers over the veneer will indicate by sound if the glue is holding. If the glue is in good shape then no noise is generated by the fingers rubbing over the surface. If shrinkage, water or heat has damaged the glue bond between the ground work and veneer, then as the fingers pass over the affected area, they will produce a hissing sound. By moving the fingers over the entire surface all bad areas can be detected and subsequently repaired. The veneer will only hiss when the glue underneath has failed, so damaged areas are easily determined.
Repairs to veneer should only be done with animal hide glue to match the glue used on the original. Hide glue has a longer working time than white glues, is easy to clean up with water and hide glue also has fairly good void filling characteristics to provide a smooth veneer repair. Usually veneer bubbles or loose veneer (not on the edge) will have natural cracks occurring on the grain lines and these cracks can be enlarged with a fine knife to allow access to the glue joint. If dirt and accumulations have built up under the veneer it must be cleaned and most of the loose glue can be removed or at least smoothed under the area being repaired. The new glue will dissolve any old glue that has dried. The cleaning and smoothing can be done with a fine bladed painting knife or palette knife. The thin flexible blade can get under the veneer without causing damage. Once the area under the veneer has been cleaned and smoothed, the glue can be injected.
A knife is used to cut a slit in the wood so the needle can introduce the glue under the bubble. A clear plastic block is used to clamp it down, clamp omitted from illustration.
If the bubbles do not have any openings for glue insertion, then fine cuts can be made with a sharp knife. It is best for these cuts to follow the grain as they will not be apparent once the repair has been done. Any cracks that exist can be used for glue injection. In cases where the groundwork has shrunk or the veneer has expanded, as under a flower pot, it may be required to remove some veneer in order to make the veneer lie down flat once again. The veneer can be softened with water or heat or a combination of both. Cuts are made and the overlaps trimmed to butt tightly to each other. When veneer is missing, the repairs should first be done around the hole and allowed to dry. The hole is then cleaned up and the repair piece added in to complete the repairs prior to filling and finishing.
On edges the same process takes place, and its usually easier to find places where the glue can be injected. The ideal tool for injecting the hide glue is a hypodermic, discarded insulin syringes with micro-needles are the best, and the longer the needle the better. It is also handy to have a large commercial glue syringe filled with hide glue, to fill the smaller syringe as needed. Liquid hide glue available in bottles can be used instead of hot hide glue, whose temperature must be kept warm in order to flow through the fine needle. Some thinning must be done to both hot hide glue and liquid hide glue in order to work. The glue must be clean and fresh, fresh to hold better and clean to get through the needle (straining might be required). A drop of glycerin in the glue will help soften buckled veneer.
The needle tube has a fine opening down its center to allow the fluids to pass, and the tip is ground at an angle to provide a sharp point. This places the opening on one side of the needle tip and this opening should be placed down against the groundwork to force the glue down and outwards. The needle is inserted into the crack, worked along and underneath all accessible loose areas as the plunger is pressed, the injection continuing until glue starts coming out the crack. The needle is moved and the process continues until all loose veneer is re-glued. The painting/putty/pallet knife is used where possible to spread the glue evenly under the veneer. Make sure enough glue is used to provide adequate coverage, too much is better than too little. The damaged areas are then pressed with the fingers to force glue around in the voids between the ground work and veneer. If glue is not forced out the cracks then extra should be injected to insure a good glue joint.
Clamping the veneer is the most important step and must be done correctly in order to insure a smooth flat repair. Select a scrap of wood, very flat, large enough to cover the area being repaired. An intervening layer, wax paper or plastic food wrap, is placed between the block of wood and the repair to keep the block from being glued to the veneer. Clamps should be placed to provide uniform pressure over the repairs and tightened. Because hide glue is water based, the clamps should be re-tightened 15 minutes after the initial tightening to compensate for the water from the glue being absorbed into the ground work and veneer. Any excess dripping glue should be cleaned up with a wet cloth or sponge prior to clamping and any glue forced out the edges during clamping cleaned off . Once the repair has been wiped with a wet cloth or sponge, use a fresh cloth to remove any remaining moisture, making certain the finished surface is very dry prior to clamping. This prevents the finished surface from clouding and assists in the drying process if no water is trapped under the block of wood, slowing the drying process. I also use square or rectangular pieces of ¼ " or 3/8 " thick acrylic plastic instead of the wooden blocks. The advantage, besides not sticking to the work, is that you can see through the block and watch the glue being squeezed out, and if the repair is proper. They are also easy to clean. I apply the hide glue making sure that the surfaces are covered (and any surround loose but sound veneer has glue between it and the groundwork) then apply the veneer patch in place and smooth with a wet cloth. I then apply the blocks and tighten the clamps. I then remove the clamp and gently slide the block off and wipe the surface of the veneer and the block with a wet rag, then remove any moisture with a dry cloth. At this point I replace the block and tighten the clamp. Make sure to protect the underside of the clamp with a wooden block to avoid damage from the clamp.
Hide glue requires 24 hours to set up under normal conditions (60 - 70° Fahrenheit and 40 - 50% humidity). This time can not be rushed and is the only drawback, actually an advantage in the case of veneering, with this wonderful old-time adhesive. Heavy weights and go-bars can be used to free up clamps to be used for other purposes. After 24 hours the clamps and blocks are removed and the repairs checked. If for some reason the veneer needs further work, heat or water can be used to soften the glue and the repairs done again.
When making enclosed patches with solid veneer surround, I make an outline around the edge of the damage. I then cut a straight or more often curved line around the damage to give a crisp line for the patch. I always try and make the patch so there are no square lines across (perpendicular) the grain, as these are difficult to conceal. I then take a piece of paper and place it over the hole and rub the side of a pencil around the edge. Then cut out the paper until it fits the patch perfectly. I then place the paper over a scrap of veneer, align the grain properly to match the surrounding veneer and carefully cut out the patch to fit. It can then be glued into place.
Some areas, such as the butt joints between pieces of veneer used in cross banding, become loose due to shrinkage and exposure. These joints are opened by running a sharp knife through the veneer to the groundwork. Glue is inserted with a needle into the knife cut and clamped as described above. After the glue is dry, the clamps and blocks removed, the excess glue is removed with a damp cloth. Sometimes it is difficult to see the glue but any subsequent finishing or filling will be adversely affected by the excess glue. An Ultra Violet light can be used to make the glue visible and easier to remove. Any cracks or knife cuts are cleaned and filled with hot shellac. It is then finished smooth to the surrounding surfaces. See Using Hide Glue.
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