Pegs and ‘Tails:
Jack Plane’s mixture of wit, wisdom and first-hand experience will leave you in no doubt as to why your workshop is not complete without the miracle of nature that is animal glue.
Jack Plane is the author of Pegs and ‘Tails; a regular forensic discourse about the development and construction of British and Irish furniture during The Long Eighteenth-Century (1688 to 1820 – the pegs and ‘tails eras). Jack’s sharp eye for detail is matched by his even sharper wit, making a potentially dry subject an absolute joy to discover. In this extract, Jack considers the virtues of animal glue.
Glue, not adhesive
Cabinetmaker’s glue, also known variously as bone glue, hide glue, pearl glue, Scotch glue and most appropriately, animal glue, is all just collagen, rendered down from leftover bits of cattle and retired thoroughbreds. My preferred nomenclature is ‘horse sauce’. Horse sauce is the stuff of boys’ comics – it’s brown, viscid, highly tenacious and will stick your archenemy to the spot as required.
There is a plethora of sites on the internet covering the scientific particulars of animal glue and extolling its virtues, so I won’t venture too far down those paths, however I feel some discourse is warranted here.
It’s not the fact animal glue has held the few discovered pieces of ancient Egyptian furniture together for millennia that I champion the use of the stuff, rather it’s the convenience (when set up for frequent use), its two-stage setting and its reversibility that I applaud. Plus I’ve grown to relish its sweet smell upon opening the workshop door of a morning.
Horse sauce can do everything modern woodworking adhesives (they’re not glue – glue is sticky!) cannot achieve. Animal glue can be used to rub-joint carcase and drawer bottom boards together, corner blocks into long case clock cases and drawer stops into chests of drawers. Animal glue sticks on contact, can still be repositioned if desired and won’t creep when dry. There are few, if any, modern wood adhesives as strong as animal glue, yet it’s fully reversible with plain water and a little heat. My use of horse sauce will therefore spare me the hatred and expletives from the mouths of future generations of antique furniture restorers.
Modern wood adhesives have a shelf life. Horse sauce (in its dry form) will keep indefinitely. Even after activation with water, surplus glue can be kept refrigerated or frozen for… well, at least as long as I’ve owned a fridge!
If you’re still not sold on horse sauce, or even tempted to investigate it, then let me entice you further: horse sauce may be modified (using common household and horticultural substances) to be waterproof, infinitely more elastic (to create coriaceous canvas for making tambour doors, etc.) and slower setting (for those jobs that normally turn the air blue, like gluing up a Windsor chair in one go – glue waits for no man). Even the simple addition of a little extra water will slow down the set time. It’s that flexible.
If, for some reason, you don’t employ an unpromising eight-year-old boy to come in to work an hour ahead of you to prepare the day’s glue in a cast-iron pot on top of the workshop stove, then there are other, significantly more convenient ways in which to prepare animal glue.
During my career, I’ve had a selection of electric glue pots; some better than others. One brand (the one that seems to be currently available from most woodworker’s shops) lasted about a month – the spun aluminium inner pot developed numerous pinholes.
The glue pot I now use in my woodworking renaissance is nothing more than a perfect little thermostatically controlled wax pot, apparently used for bikini waxing – though why they don’t make the bikinis from waterproof fabric to begin with, I just don’t know.
There’s one final benefit of horse sauce I’ve found invaluable but has gone unmentioned elsewhere. When gluing up, say, a large set of dining chairs, rather than wasting time and effort cleaning up the glue squeeze-out from around all the joints, set the chairs on the floor and let Workshop Dog lick all the joints clean. The savings in expensive dog food can be considerable!
The efficacy of animal glue
‘Hot hide glue is all right, but it’s water soluble and won’t last.’ I have had it up to Pussy’s bow with the raft of misinformation regarding hide glue on internet fora and in newsletters, etc. from people with little to no experience of it, who perpetuate myths and untruths about the stuff.
Why should you listen to yet another blogger and his rhetoric? Well of course you don’t have to; though I believe my qualification (virtual daily use of animal glue for 40 years) affords me at least some credence.
Animal glue, whether asinine, bovine, caprine, equine (hence horse sauce – my preferred appellation), leporid, orcervine, ovine, piscine, porcine – bone, hide or skin, is indeed soluble in water and that is one of its greatest assets. Dry animal glue is first heated in sufficient water to make it brushable, however – and this is the noteworthy part – when the majority of water has evaporated from the glue (the glue is set and ‘dry’), it can, with a modicum of effort, be dehydrated/rehydrated, permitting the repair or repositioning of components.
I think when some people say ‘animal glue is water soluble’ they mean it’s not waterproof. That is true of unmodified animal glue (it can easily be made waterproof), but its most widely used competitor, polyvinyl acetate adhesive (PVA or ‘white glue’), is not waterproof either.
As any furniture restorer can attest, veneers or furniture glued with animal glue can be disassembled quite easily with steam/hot water and mechanical assistance, but even tepid water alone takes a considerable time to soften the glue to the point that the bond is compromised.
Although animal glue was known to the ancients, virtually every piece of furniture made since the mid-1600s was stuck together with animal glue and thousands of antiques dealers and their customers around the globe are quite happy with the results thank you very much!
Humid weather will not cause a room full of antique furniture to suddenly (or slowly for that matter) slump into a pile on the floor. Even roughly constructed 19th-century ‘country pine’ furniture survived the hot caustic stripping tanks of the 1970s intact (all right, a few drawers might not have survived the nightmare solution, but it was never fine cabinetwork to begin with).
‘…virtually every piece of furniture made since the mid-1600s was stuck together with animal glue and thousands of antiques dealers and their customers around the globe are quite happy with
the results thank you very much!’
The dismantling and reassembly of glued joints
Animal glue is mildly hydrophilic, which alone enables it to maintain its adhesive property. Glue that has been utterly deprived of humidity will become brittle and subsequently fail. Luckily for those who restore glued articles, this same action can be replicated chemically.
Alcohols are hydrophilic in varying degrees (methanol has the highest affinity for water, though ethanol rates a very satisfactory second) and restorers and furniture-makers normally have a supply of ‘dry’ ethanol on hand for making spirit varnishes.
Ethanol dehydration can be employed to reduce animal glue to a crystalline state, breaking its bond and thereby permitting dismantling of a loose or damaged joint. Ethanol is injected into the joint with the aid of a syringe whereupon the glue progressively relinquishes its moisture – often accompanied by a crackling sound – as the alcohol wicks its way in. The addition of a little tension and an audible crack will let you know the joint has been broken.
Further pulling, wiggling and possibly tapping of the joint is usually required to persuade the now granulated glue to crumble away. Larger chunks of crystalline glue can either be chipped or scraped from the open joint; however it’s not critical, as any residual glue will be rejuvenated with the application of fresh hot glue when the joint is reassembled.
PVA vs animal glue
I began a simple experiment and I took a 610 x 355mm piece of 2mm-thick mahogany veneer and glued it onto a 19mm-thick pine board (actually two boards rubbed together). I applied unadulterated animal glue to one half of the veneer/board and PVA to the other and then hammered the animal glue side and cramped the PVA side.
The unsealed veneered board lay outdoors on top of a stack of timber for 12 months, during which time it was baked in 42°C sunlight, drenched with rain and crapped on – not by me I might add! The PVA more or less gave up after a few months, but the animal glue held up quite well – at least it has largely kept the veneer in contact with the board.
Recently, following 12 hours of steady rain (after which I observed pooled water on the animal glue side), I checked the trial board and the exposed animal glue at the edges of the veneer was tumescent, but even pushing with some force, I couldn’t insert a pallet knife more than 5mm between the veneer and the board.
And while my tongue’s warm… I would like to clarify one point: hides and skins are initially boiled to release collagen, but at no time should animal glue be boiled or the protein chains that afford the glue its strength will break down. To prevent rapid degradation and to ensure long life, the working temperature of animal glue should never exceed 60°C.