Re-Gluing Wood Furniture
Basic to any repair project is gluing. The important factors in obtaining a long lasting gluing job are:
When you think about having someone glue your valued furniture items, you should know how they approach each of these factors. In addition, many people think adding screws, nails, braces or other supportive elements will yield a longer lasting glue job. We don't think this is a good idea and will tell you why at the end. In addition to this information page we have a gallery page illustrating our work of re-gluing.
Of the many glues available to be used on furniture, we have chosen three to use as our standard selection--hide glue, marine grade epoxy resin glue, and aliphatic resin glue.
Hide glue has been used at least since the time of the ancient Egyptians. Archaeologists have found items held together with hide glue in the pyramids. Hide glue is made literally with a substance found in the hides of horses and cattle. Hide glue is an excellent adhesive with great bonding strength and durability. However, it does have a fatal weakness. Hide glue is water soluble and needs to be heated to approximately 145• for its working temperature. As the adhesive cools, it begins to gel and develop its bonding strength. By the time room temperature is reached, it is extremely strong. However, if the temperature becomes elevated again, especially if it is also moist, the glue will weaken and the bonding strength can be lost. For that reason, we offer no warranty for any items glued with hide glue. A stint in a garage, attic or storage unit in hot summer, on a sun porch in spring, are all ways items held together with hide glue have failed before being brought to us for re-gluing. Another common reason for hide glue failure is the shrinkage of wood that is moved from a humid climate to a dry climate causing mating surfaces to pull away from each other. The hide glue will also shrink and the net result is a loose joint.
One might wonder, why continue to use hide glue? First, it is a good adhesive, it can hold up for decades to centuries in ideal environmental conditions. Second, for anyone who wishes to maintain the originality of valuable antiques, hide glue is an essential part of that originality. A third reason often given is that it is reversible. It's very weakest feature can be a benefit when taking apart a piece to replace a broken component. Enough people agree that hide glue is a good choice for their furniture that about half of the re-gluing we do is done with hide glue.
Marine grade epoxy resin glue is also an excellent adhesive with a very long life expectancy. Unlike hide glue, it is not reversible or subject to degradation through environmental effects. It is so strong and so lasting that we warranty for five years anything we glue with epoxy. It is also expensive and that is one factor that influences some to choose one of the other glue options. Again, about half of our customers choose this adhesive.
Aliphatic resin glue, better known to most as yellow woodworkers glue, is offered as an option for items made since the mid-1940s when modern synthetic adhesives replaced the hide and lignin glues used until then. It is less expensive than epoxy, but also not as versatile. It demands a clean, tightly fitting joint in order to hold well. Since tightly fitting joints are the exception when dealing with older furniture, yellow glue is usually not the best choice. The other option that works with some dowelled joints is to enlarge the sockets and use a larger dowel, thereby creating a tightly fitting joint. Aliphatic resin glues are potentially reversible, though not as easily as with hide glue. The primary advantage to aliphatic resin glues is their flexibility. Rather than shattering if a sharp jarring blow is sustained, yellow glue can (note that we did not say "will always") act as a bearing, absorbing the impact without the bond breaking. We warranty items glued with this adhesive for ninety days.
Many people wonder why we do not use polyurethane adhesives such as Gorilla Glue. These adhesives are also very strong, but like aliphatic resin glues, require tightly fitting joints to ensure a lasting bond. They also continue to expand long after you have set the clamps and left the piece. If you use too much glue the expanding glue can close wood pores making a finish applied later appear blotchy, or can stick to existing finishes removing them when the expanded glue is removed. Polyurethane glues are excellent for many applications in new fabrication, but we have found that their applications for furniture repair are very limited. Due to these limitations and the similar effectiveness to that of aliphatic resins, we have opted to use the yellow glues only.
It is necessary to remove any dirt or debris that might obstruct the spreading of the glue or the fit of the joint. Hide glue will adhere to itself, so when re-gluing with hide glue, the old glue does not need to be fully removed. For the other adhesives, fresh clean wood surfaces are necessary. This means drilling out mortises to remove dirt and surface glue, sanding or filing the old glue from tenons and mating surfaces, and replacing dowels with new. Any short cuts here will shorten the life of the gluing job.
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Our number one complaint with modern (since the 1940s) furniture manufacturing methods is that they rely on pin nails or screws to hold joints together rather than glue. We think we understand the practice, it is cheaper and that allows them to make their products more affordable for consumers. When those pin nailed or screwed together pieces get to us, they are usually broken at the nail or screw or are very wobbly--held together only by the nails or screws. It is always the metal supports that win the war of stress leaving the wood broken or badly worn. Properly gluing a piece is time consuming (expensive) but is the most sure way of creating solid feeling, lasting bonds at joining sections of wood.
With that introduction, we hope you are convinced that glue application is a topic worth attending to. The key idea in gluing is that the larger the surface area being glued, the stronger the glued joint has the potential to be. A dot of glue at the end of a dowel, which is what we often find when we take apart pieces made in the last two or three decades, only gives a dot's worth of strength to that joint. On the other hand, when all mating surfaces are held by the adhesive, a much larger surface area is involved and the joint will be held together with far greater strength. Consequently, our effort is to ensure that all mating surfaces are thoroughly covered with the selected glue. In addition, with epoxy, since it will penetrate the wood fibers, we pre-coat the surfaces, especially end grain, allow the glue to be absorbed by the wood, then re-coat the surfaces with epoxy to ensure that the joint is not glue-starved through absorption.
Whenever two parts are being glued, they need some form of clamping to hold the parts together while the glue sets. The clamping needs to be tight, but not over-tight or else the clamping can force too much glue out of the joint and it can become glue-starved through excess clamping pressure. Also important in clamping is that the correct angles be ensured. Special jigs are often needed to ensure this. Clamps can damage the wood surfaces if the clamping pressure is not distributed with clamping blocks. There are enough variables in clamping that our practice is to dry clamp items to be glued to ensure all those variables are accounted for before applying any adhesive.
A common and appropriate practice in gluing is to apply more glue than is needed with the expectation that the excess will be squeezed out by the clamping. Consequently, cleaning up the squeeze out is a necessary part of every gluing job and the time to do it is when it is still wet and easy to remove. A rag wetted with water works for hide and aliphatic resin glues but not with epoxy, denatured alcohol works best for epoxy, acetone can also be used on unfinished items.
Yucky Supportive Elements Are Bad
So, why do we not like the addition of supportive elements such as nails, screws, angle brackets and the like? All of these will help to hold the joint together, we grant that. However, a properly designed, tightly fitting and well glued joint does not need any of these supportive elements to hold up well for a long time. On the other hand, the use of these foreign elements initiates a war, metal against wood, which the metal will almost always win. Structural joints are points where the stresses of use are focused. When there are wood and metal components in those joints, the wood and metal work against each other and the metal will, over time, wear out the wood while still trying to hold the joint together. Very often we have items come in that have broken along the line of the (unnecessary) nail or screw. The strength of the wood consists in the continuous long wood fibers. When those are broken by the insertion of anything across the grain, the natural strength of the wood is reduced. Our practice is to remove nails and screws, glue properly and allow the glue to do its job without "helpers." We don't have the strong objection to nails and screws in non stress areas such as attaching back panels, hinges and other hardware, and so on.
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