I know I am going to get a lot of crap from some die hard hide glue only folks (I’m looking at you Joshua Klein), and I don’t necessarily disagree, but there are a lot of situations at home where you don’t need the most conservatorally accurate glue to keep your furniture lasting for many years to come.
Here’s the thing. An incredible amount of furniture repairs involve gluing. Almost 95 percent of the repairs I have done either involve gluing at some stage or consist entirely of gluing.
In furniture conservation and restoration there are a number of glues that may be used, although the big two are probably animal hide glue and epoxy resin glues.
Animal hide glue is one of the most preferred options for historic pieces because it is the most prominent glue in furniture history. If your piece of furniture was made before the mid 1900s, it was probably made with an animal hide glue (unless it is made with no glue at all). The other major benefit of animal hide glue is that it is reversible, which those of us who work in repair really really like.
That said, the easiest and cheapest glue for a lot of people to get their hands on (if it isn’t already in their homes somewhere) is some kind of white PVA glue. So here are 6 things I think are key to understanding when using PVA to repair timber furniture.
NOTE: Ideally, if you are using PVA it should be reserved for places where the timber has split or broken along the grain. Try to use hide glue (or maybe some other reversible glue) in the joinery of the furniture. As always, if you don’t feel confident, consult your local furniture restorer/repairer/conservator.
PVA Glue is Damn Strong
Firstly! Don’t do what these people did.
If a glue joint is done well, it is damn strong, stronger even than the rest of the wood in some cases (though not always for as long). This means that there is no need to mar the piece with extra nails or screws that weren’t in the original piece!
I used to work with a guy who would demonstrate the strength of PVA by snapping a plank of wood, gluing it back together, then trying to snap it again. It would snap, but never where the glue joint was. It was a very cool demonstration. I recommend trying it at home.
Think of PVA as Something that Weaves Through the Grain
Wood is a collection of straws bundled together via magic (Note: not scientifically accurate), and most breakage of timber is a split between the strands where the magic is weak and has given way. In other words, wood snaps with the grain more often than not.
Glue is a liquid that eventually becomes a solid, often bonding with nearby elements in some manner. A great way to think about how it works (for me at least) is that it soaks into the two pieces of timber and solidifies into one solid structure that weaves its way through the grain of the timbers holding them together.
Glue Joints Need to be Tight!
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Continuing along this line of thinking, what makes the glue joint strong is how it weaves between the two timbers joining them together. A lump of glue itself is brittle and easily broken, so if there is a gap between your timbers and you try to just fill it with glue, you will end up with a lump of hardened glue which can easily be broken again.
Similarly, don’t use too much glue. If you put too much glue between the timbers, it might make it really difficult to get the actual wood to touch. You again end up with a piece of wood, a chunk of glue, and another piece of wood.
Please stop doing this! It makes it very difficult to re-glue the piece once it’s inevitably broken again because:
The Timber Needs to be Prepared
If the PVA is going to work well, it needs to be able to bond to the surface of the timber. If the timber is painted, or finished in some way, the glue will not be able to penetrate the surface, thus not happily weaving between the grain.
If a person ignored rule number 3, and the joint has broken leaving big collections of hardened glue on both pieces of timber, just adding new glue on top will not work. You will need to remove all traces of glue from before and perhaps sand the surface down until the new glue can actually penetrate the wood.
Glue Joints Work Best Long Grain to Long Grain
If you don’t know what long grain is, I’ll explain. If again you imagine wood as a bundle of straws, long grain is the sides of the straws, whereas end grain is the end of the straws. Glue weaves easily in between long grain, creating a nice strong join. Gluing end grain has two fundamental problems.
First, the weaving image doesn’t work. The join is largely relying on the adhesive or bonding element of the glue and not also on it’s being knotted threw the grain.
Secondly, many end grains will just suck the glue up into it’s depths and not leave enough at the surface to make a solid joint. Frame makers (who glue end grain all the time), often deal with this by “Sizing” the glue.
It Will Prevent Finishes From Soaking in
Lastly, A more aesthetic concern. If you have glue drips on the outside surface of your piece, and you let it dry, if you then try to apply some kind of oil or resin, or stain, it will not penetrate the timber where the glue is, often leaving lighter patches where the glue has dried. Try to quickly wipe away any glue that gets on the surface of your piece with a warm damp rag.
That is all for now. Hopefully, you’ll feel a little more confident next time you have to glue your chair together again.