First off, why go through all the trouble of making hide glue? Furniture made before World War II and many objects made since have been glued together with hide glue—not only the joins, but often the veneer. One wonderful property of hide glue is its ability to be reconstituted by reintroducing water. Usually failed glue joins still have crystals of hide glue in them, they simply are not attached to one surface making for a loose join. Water can be introduced and, once the glue has re-gelled, heat can re-liquify the glue. Once this has dried again, it will once again bond both surfaces. Often, during restoration, it is not possible to get enough heat in a joint to consolidate the old glue so more glue needs to be introduced into the join. Without having to remove the old glue, new glue can be applied and the joint clamped, but it must be more of the same—hide glue. The ability to reconstitute the glue will be preserved and future repair will be easy. In doing veneer repairs, this is of paramount importance because it is often impossible to remove other glues without doing considerable damage to the veneer, substrate or both.
The ingredients for making hide glue are quite simple: glue crystals, water, table salt, jars with a top, some type of scale to weigh the ingredients, and a way to heat the glue—a water bath using a cooking pot is easy. Nothing in this recipe is toxic and is in fact nutritious!
You can buy the dried crystals from online purveyors such as Woodcraft (www.woodcraft.com), Highland Hardware (www.highlandhardware.com), or Woodworkers Supply (www.woodworker.com). 192 - 251 gram strength glues are most common and wholly appropriate, 192 glue having a longer open time than 251. The crystals are soaked in a measured amount of cold, clean water for a few hours and subsequently heated until it is liquid. Some suppliers provide instructions and ratios but, in general, for 192 glue, the ratio of glue to water is 1:1. For 251 glue, the ratio of glue to water is 1:2 (twice as much water).
Hot glue presents issues with getting joins clamped fast enough as it gels very quickly—too quickly, usually. This is when you might wish to extend the gel time using table salt. If you add enough salt you will make cold, liquid glue similar to that which can be purchased. The only real advantage is knowing the glue is fresh. 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 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 140°F 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 make cold liquid hide glue, add about half as much table salt as dry glue (by weight) to the mix. 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 (140°F), just don’t cook it all day as the glue breaks down.
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 hot glue, fresh for the day it is going to be used. Cold glue can last much longer, maybe a month or two. 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.