One question I am often asked by do-it-yourselfers who are faced with regluing a chair or reattaching some veneer is, “What glue should I use?” While the variety of glue is dizzying—Elmer’s® White Glue, Carpenter’s Glue, epoxy, cyanoacrylate (Crazy Glue®), contact cement, polyurethane glue (Gorilla®), hot melt glue, hide glue, urea formaldehyde glue—the list goes on, the choice for regluing original glue joins depends on many factors: What is the age of the object and what glue was originally used? Is there a possibility that the join will need to be reglued in the future? And what properties do we wish the adhesive to have? These are just a few questions to pose before deciding what adhesive is appropriate. However, in the restoration trade, a few adhesives come to the fore in making repairs to antiques and the list gets smaller for the do-it-yourselfer as we want first and foremost to be able to undo a poor first attempt, a common occurrence.
Most antiques (pre-1900) were glued together with hide glue, the glue we refer to when we suggest sending a horse to the glue factory. This is the gelatin part when you cook down bones and skin, pulling out the collagen to form a water-soluble adhesive. For restoration this is a blessing as old glue lines can be reactivated with hot water or thin hide glue to re-attach a failed glue joint. Even more helpful, it is relatively easy to remove during the repair process, yet doesn't need complete removal as new hide glue will stick just fine. Many veneered surfaces have cross-grain issues that will require periodic treatment of the glue line. Use of other adhesives will make this future treatment difficult if not impossible without serious damage to the surface. Yet hide glue will always be able to be made liquid again and, with adequate clamping, the veneer reattached to the substrate. So from a preservation standpoint, hide glue is perfect for regluing joins previously glued with hide glue. Even a bad attempt will still afford the future restorer or conservator the chance at making it right again.
The only real problem with using hide glue is that you have to make it yourself. Yes, you can buy cold, liquid hide glue (Titebond’s Liquid Hide Glue® or Old Brown Glue®) at some hardware stores, lumber yards and paint stores (it's used in creating crackle finishes), but you will have trouble determining its freshness and whether the glue has already broken down. You can easily make hot hide glue which is no different than making Jello®, or you can make cold, liquid hide glue, which is what I recommend. Hot hide glue gels very quickly so any assembly has to be done quickly before the glue gels. If you use cold glue, you will have plenty of time to assemble the joints and put clamps on as the glue no longer has a gel phase. Sometimes there is an advantage to hot glue. Once the join has been made and the glue gelled, the clamps can come off. You will have to limit your usage of hot glue to things which can be put together quickly, e.g. replacing a piece of wood that has fallen off, otherwise, use cold, liquid hide glue.
To make hot hide glue, soak the glue granules in the appropriate amount of cold water (the amount depends on the gram strength of the glue—instructions come with the glue) and set it aside for a few hours until the granules have absorbed all the water they can. The glue is then heated in a water bath at 140oF until all the glue is completely liquid. Some practitioners take the glue off the heat and refrigerate the glue and rewarm the mixture the next day. While not strictly necessary, you will avoid lumps in your glue by doing this. The glue is applied hot to one surface and the join is brought together before the glue has gelled. Depending on the situation, it may need to be clamped for a short while. You only have seconds to get the wood assembled so if you need more time, use cold liquid hide glue. To maker cold liquid hide glue, add about half as much table salt as dry glue (by weight) to the mix. Cook twice and that’s it. Put it in a small mason jar or other clean dispenser and you have fresh liquid hide glue that you can be confident in. If a little too viscous, heat it in the water bath (140oF), just don’t cook it all day as the glue breaks down.
Because making hide glue requires measuring and cooking, you won’t generally find it on the hardware store shelves but it is easily purchased as granules from online suppliers. It is sold to the woodworking trade in 192 and 236 gram strengths. Since hide glue is a protein-based glue, once made it will grow mold and will undergo hydrolysis (protein breakdown by water). Most restorers make their own, fresh for the day it is going to be used. Yes, it seems like a lot of work when you can buy a bottle of yellow glue in any hardware store. But the next time your precious antique needs work, you (and your restorer!) will be happy you didn’t make a mess of things with those other glues.
EVA Glue (otherwise known as hot-melt glue)
Another glue often used in the restoration trade is hot melt adhesive, the craft-person's go-to glue. Conservators appreciate this glue because it can also be reactivated, with heat alone, and it is soluble in mineral spirit, posing no danger to most original materials found on antique furniture. It comes as a solid cylinder that is fed into a hot-melt glue gun. Plug it in, pull the trigger and off you go. EVA is generally not used for gluing as much as for filling gaps. It will not expand or contract with the humidity and offers some flexibility. Squirted into a stripped screw hole, it will afford a good purchase. Heat the screw with a soldering iron, push it in, and the EVA will melt and solidify around it. Conservators will have no difficulty in removing or retreating this adhesive. It isn’t appropriate for regluing chairs or woodworking joins as it gels too fast.
Yellow Carpenter's Glue
This is made by many manufacturers–Titebond, Elmer's–and is probably the most popular… and maybe the most unsuitable for regluing antiques. It is fine for new work and splicing in new wood to old, but in old joins it makes future repairs much more difficult. Yellow glue (aliphatic resin) sets up very quickly, often too quickly before clamps can be put in position and tightened. For regluing, use the liquid hide glue described above.
There are places for all the other adhesives on the market, but for the amateur reading this commentary, I suggest using those that will not make things difficult for the future repairman. I frequently get furniture poorly glued by the owner (or their handyman) that requires removing gobs of glue and sometimes original material just to make a sound repair. These adhesives have their place and I, myself, use a wide variety of adhesives appropriate for the task, but from a preservation standpoint, hide glue and EVA will rarely pose a problem should the repair need to be redone.