Why use hide glue? Hide glue is a protein-type adhesive that was used on all clocks and furniture until the 1950’s, and in some cases, even to the present day. Most are made from processed cow hides. You can buy it in two different forms. The dry form comes as crystals or pearls that you have to mix yourself in a heated container. The liquid version comes pre-mixed in a squeeze bottle. Either of these is reversible, which is a primary reason restorers and conservators of antiques exclusively use hide glue. Historically, animal hide glue has been successfully used for millennia. Many examples of furniture dating back hundreds of years testify to the holding power of this adhesive. Many industries, including piano and luthier manufacturing and repair, and antique conservationists, will use nothing else but hot hide glue, as it has some wonderful properties that make it especially well suited to woodworking. 1) As previously stated, hide glue is reversible. This is very important because all wood is affected by seasonal humidity. As time goes on, wood will contract and expand and occasionally these forces can loosen the glue bond line that holds them together. So the repair made today on a 100 year old clock will likely have to be repaired again. The fact that the glue can be redissolved lends itself well to the fact that new hide glue can be applied directly over old hide glue and is 100% compatible. 2) Hot hide glue will quickly transition to a gel. This initial tack is one of the benefits in the industry that makes hot hide glue so well suited for wood working. It greatly speeds up assembly time. Glue blocks and veneering are both good examples of why hot hide glue should be used. Hot hide glue has the advantage of gelling quickly and holding glue blocks in place almost immediately. By simply using hot hide glue, you can apply glue to a block of wood, known as a glue block, and hold it in place until the glue gels. As the glue quickly cools it gels and holds the glue block in place. No clamping is necessary, although the glued pieces should be set aside to dry. 3) Hot hide glue doesn’t shrink when the water content dissipates by evaporation. This is a common problem with other water-based adhesives that shrink when they evaporate. 4) New hide glue is compatible with old hide glue. This is a wonderful characteristic of hide glue. Although it is wise to clean off any excess old glue when repairing a joint, the new hide glue will reactivate and mix with the old hide glue for a sound bond line. This is not the case with modern adhesives. To loosen an old hide glue joint, use some warm water, or a hair dryer and the glue will release its hold. It can take a few hours for it to release with water, so be patient. A hot steam iron can be used to loosen hide glue under veneer. Once the joint has been debonded, an application of 140 degree F water will reactivate the joints and the pieces can be rejoined. Removing the shellac finish from the veneer will hasten the reactivation of the hide glue when a steam iron is used. To make hot hide glue, you’ll need glue, water, and a heat source to keep the mixture warm. If you decide to use hot hide glue, you’ll need a means to keep it hot. Glue pots specifically made for this application sell for approximately $100. However, a thermostatically-controlled electric kettle will do the same for about $15 and are available at the grocery store. The electric kettle makes for a double-boiler set up and is very efficient. Dry hide glue granules can be purchased in varying gram strengths ranging from 30 to 600. The most popular gram strengths for typical wood working range from 150 to 250. The higher gram strength glues will gel faster. I prefer the granules over the hide glue pearls, because the granules dissolve much quicker. To make hot hide glue, mix equal parts of dry hide glue granules with water in a glass container. Allow the glue to soak up the water. This may take 10-15 minutes if warm water is used. Heat the glue to 140°F to 145°F, while stirring it occasionally. It should be a thin creamy consistency like hot maple syrup. Heat it until all the clumps have dissolved, and the liquid is homogenous. Use a brush such as a 1/2” wide cheap paint brush to spread the glue. Since hide glue gels within a couple of minutes, clamps and cauls should be pre-set and ready for use. When clamping, the squeeze-out should be evident on all sides of the joint. If a little extra working time is required, mix in about 10% urea in the glue mixture. Also, the wood to be glued can be pre-warmed to delay the liquid-to-gel conversion. When gluing small parts, apply the glue, mate the pieces together, and hold the joint until the glue gels. Depending on conditions, the glue should gel in one to two minutes. Clamping is usually required, but not a necessity in all cases. Remember, once the hot hide glue gels, it is too late to try to bond the two pieces of wood together. If gelling occurs scrape it off, or just add a little more hot glue to the gelled glue to reheat the entire bond line. Excess glue can be removed after a couple of minutes after it gels, so squeeze-out is easy to clean up. Allow the bond line to cure overnight. An electric kettle as shown in the attached picture is an economic route to make hot hide glue. After the water in the electric kettle has been heated, monitor the temperature with a cooking thermometer to sustain a 145°F temperature. Modern Glues Are Not That Bad Are They? I must admit that it’s easy to pick up a bottle of carpenter’s glue when I need to glue a couple pieces of wood together. But the problem with modern glues such as carpenter’s glue, contact cement, white glue (polyvinyl acetate), yellow glue (aliphatic resin), polyurethane glues, etc. is that they will not adhere to themselves or to old hide glue. (When I refer to modern glues, liquid hide glue is not a part of that group, even though it was invented in the 1950’s). Modern glues have more disadvantages for both short term and long term use. 1) For modern glues to bond the joint back together, the old hide glue must be scraped away, and usually the wood needs to be dampened and heavily scraped to remove any residual hide glue from the surface. In these circumstances, the newly bonded joint will not be nearly as strong as before, because residual incompatible glues don't bond together well. Further, it is difficult to get all of the old hide glue out of mortise and tenon joints or holes, making a sound bond line nearly impossible. 2) Modern glues will not take a stain. When modern glues, even a thin film, are left on the surface of the wood they will show through on subsequent finishes as a lighter spot. 3) Modern glues take longer to set up than hot hide glue. They need to cure overnight with a clamping force applied to the joint to hold it together. Hot hide glue gels within a few minutes, but modern glues do not gel like this. 4) It cannot be emphasized enough that any of the modern glues, which are just a cured plastic, are not considered reversible. This attribute makes subsequent repairs very well near impossible without damaging the wood itself. These types of glues were never used on antiques. For example, I see a lot of repairs to kitchen clock tops that have been glued together with carpenter’s glue. Most have poorly aligned joints with a visible glue line at the repair. A misalignment could occur during the glue process, or as a result of humidity effects on wood, resulting in a need for another repair. The down side to the irreversible characteristics of modern glues means that all of the previous plastic adhesive must be removed for a subsequent repair to take hold of the wood. Most glues dry by evaporation. White and yellow glues cure by a combination of evaporation and oxidation when exposed to the air. Once cured, they are no longer fully soluble by their solvent, which is water. When white or yellow glue is used on a joint, it puts a plastic coating across the surface of the wood, which also blocks the wood pores. A subsequent repair using any type of adhesive that relies on absorption into the wood and evaporation will be impeded from being effective because it cannot penetrate the plastic coating, which is not dissolved by the adhesive solvent. The old adhesive must be thoroughly cleaned off by scraping down to clean wood in order for the joint to stay together. 5) Modern glues are not good for the long term. All wood movement occurs as a result of seasonal changes. Eventually a glue bond line will weaken or fail. Modern glues applied to a failed glue joint will not last as long as one where hide glue is used on a joint previously bonded with hide glue.