As you’ve imagined, there’s a lot of ways to do things. We can go ‘old fashioned’ using animals parts, ie animal glue or hide glue that consists mostly of collagen from skins, bones, tendons, and other tissues, similar to gelatin.
It doesn’t smell great, and handling it usually involves hot plates and vessels, so be prepared.
Another venue is to go ‘modern’ and employ chemical compounds.
Both approaches have merits and disadvantages: hide glue needs heating and fast application whereas industrial glues may be applied at almost any temperature. The former would allow repairs without damaging the pieces whilst the latter would not ‘leg go’ unless you saw it or break it.
Animal based glues
You might encounter different names for animal based glues, like:
- Rabbit skin
- Fish glue
- Hide glue
- Pearl glue
Of course, there are several types of animal glue in the market, made from different source materials. Very basically, what you want to look for is hide glue. On AliExpress, you can find 100g for £3 (as of October/2023), listed usually as “Hide glue animal glue violin viola making furniture repair”.
The Wood Knight has a nice video on why to use hide glue instead of PVA based glue.
Industrial glues
Polyvinyl acetate (PVA), aka, wood glue, PVA glue, white glue, carpenter’s glue, school glue is a chemical compound used to join or laminate wood.
It has a long set up time, meaning that we don’t need to worry about hushing to clamp the wood together as well as room temperature concerns.
The material is very strong when dry; it will stick to the wood and YouTube tests (highly non-scientifically) prepared by Mathias Wandel show us different PVA glue types and how it might break after application (ie, right beside the glued part).
Preparation and tips
First of all, you should use a 1/1 mix, ie, 1 part water, 1 part hide glue flakes let it rest to absorb the water for one hour (do whatever it works for you).
IMPORTANT
- If you used less water than you should, it will become very dense, then you should add more water. It should be rather consistent, ie, not like water and not like PVA glue when put it on a brush. It should run off like honey. You will learn the right amount in time.
- If you used more water than you should, do not boil it down, add more glue flakes and wait.
The material is very forgiving, meaning that you can ‘abuse’ it and it will still glue things together; the only thing I read that really matters is to never let its temperature rise beyond 85C, point where the adhesive properties might disappear altogether.
You should strive for a temperature of between 60C and 70C for ‘optimal’ results. Maestronet has a good discussion on this.
Well, that’s consistent with violin making in the 16th hundreds, where I believe ‘industrial controls’ were not the norm.
I digress, let’s return to preparation and tips.
It might be useful to use a Crock Pot, something to heat up your hide glue not to the point of boil it but to keep it warm. There’s some fancy items for sale, usually expensive, tailored for hide glue, however, it’s quite unnecessary really, unless you want to spend money needlessly or you want to show off (Stradivari and Guarneri would have used a candle, I’d imagine).
I’ve read people suggesting to use distilled water: of course you can, but it’s not necessary. Again, do you really think they’d use distilled water in the Middle Ages to make violins that stood the test of time?
I digress again.
Fine Woodworking magazine has fired up a nice discussion on hide glue on Crock pots in this link.
After use it, cover it, and ‘treat like food’, ie, keep it in the refrigerator (or freezer). After it’s cold, stick your finger in it, and see how consistent it is. This might help you understand the right amounts of water to add to the mix whilst preparing.
One issue about hide glue is that it sets very fast, do don’t waste any time using it, you must be fast. Your wood and environment should be warm enough to help you in this task (you could use a heating lamp to help you set this up).
Other ideas
You could add urea (not urine!) to the mix.
“Add 15% urea or table salt by weight. It will extend the working time of 251-grade glue from 1 minute to 5 minutes. Adding even more salt or Urea will convert the glue to ‘Liquid Hide Glue’ and will work at room temperatures without heating.” Perhaps that’s the method they use for commercial Hide Glue (Titebond has a product on the market).
More information on this link.
On-line resources
Davide Sora’s video is very interesting: Giunta centrale delle tavole di violino – Incollaggio. It details the proportions and how he applies to wood (he uses a 1 to 4 proportion, but his glue might be stronger than yours – the one I bought on AliExpress had no information as to its strength and the vendor was clueless why you would need such information):
“I prefer a glue with a fairly high Bloom degree (gel strength between 300 and 350) which needs more dilution than one with a lower Bloom degree (e.g. 192) and a slightly higher operating temperature (between 65° and 70°C). To glue the top and back joints, soak 5.5g of glue in 22g of distilled water (proportion 1:4) and let it absorb until it is completely swollen. Depending on the size of the granules, it will take from 3/4 hours up to 8/10 hours.”
Test it on wood. I see that it takes longer in the clamp to actually glue it, so, if you are used to the response time of any PVA glue, take a seat, rest and wait, because hide glue takes much longer to settle and join pieces together.
The advantage is that with the right pressure, it ‘disconnects’, which is a good property if you ever want to repair your instruments.
That’s more or less it, all you need to start working with hide glue right away.
Knacker and knackeries
“A knacker (/ˈnækər/), knackerman or knacker man is a person who removes and clears animal carcasses (dead, dying, injured) from private farms or public highways and renders the collected carcasses into by-products such as fats, tallow (yellow grease), glue, gelatin, bone meal, bone char, sal ammoniac,[1] soap, bleach and animal feed.”
Wikipedia entry on “Knacker“
There you have it. And remind yourself that:
“Nothing is lost, nothing is created, everything is transformed.”
Antoine Lavoisier
You should be wary of knackers though, see this: “Since the Middle Ages, the occupation of ‘knacker man’ was frequently considered a disreputable occupation. Knackers were often also commissioned by the courts as public executioners”.