Monday, October 03, 2005

A Glimpse at Glue: White, Yellow & Hide

A non exhaustive glimpse.

The white and yellow glues, poly vinyl acetates and aliphatic resins respectively, (more details another day about those specifics), are generically known as, surprise to none, wood glues. The former often gets tagged with the name Elmer’s glue , a confusion of nomenclature with brand. It’s even mistaken for the awful 3rd grade white wheat paste which, seemingly, stuck to nothing but fingers. Yellow glues carry along with them the term, carpenter’s glue. I think this term is accidental and is not clarifying. These glues are the progeny of chemistry and plastics.
Hide glues are protein glues; the famous" send the horse to the glue factory" glues. Overcook fish, it turns tough and rubbery. It’s sort of on its way to becoming a glue instead of a dinner. Hide glues traditionally dwell in hot glue pots, but some now live in plastic squeeze bottles, like ketchup, and thrive at room temperature. Don’t confuse the two. (My wife though, does like ketchup on fish.)

Glues work either by adhesion or cohesion. Hide glue works by adhesion, stickiness. Of these three glues, it is the only one that will stick to itself. New hide glue, hot or room temperature, can be applied over old and still stick. Now you see why your old chair, pumped full of yellow glue, then epoxy and finally nails still isn’t tight. (Note: because the room temperature stuff becomes so with the addition of urea, I’d be suspicious of using it anywhere near things like million dollar violins. Stick with the traditional hot hide glues there. I’ve used it with confidence repairing chairs though.)

The poly vinyl acetates and aliphatic resins, the white and yellow glues work by cohesion, molecular attraction. They require greater clamping pressure than hot hide glue. (Some hot hide glue joints are just rubbed and pressed together for a minute or two.) And because they work by cohesion rather that "stickiness", the term "open time" comes into play here. Yellow glue has an open time of about eight minutes, white about 10 or 15 minutes. Beyond these time limits the glue begins to set. Tighten your clamps too late and the joints may fail. Gluing up things like desks or dressers, with their multiple mortise and tenon joints, floating panels, and what have you can be warfare with theminutes.

One other contrast. Hide glue is reversible, both mechanically and chemically. It is not stronger than the wood and can be broken if done carefully. I once saw a Luthier take a thin broad blade to a Guarnieri violin, slide it into the joint and pop $600,000 worth of back off a $1.2 mil violin. My heart almost stopped. The repairman seemed unfazed. Hide glue also yields to steam, that is, to heat. And denatured alcohol turns it into a very un-sticky jelly. The cohesion bond of yellow glue, like a weld, is stronger than wood. Undoing these joints requires blocks of wood, big hammers and violence. The joints must be broken apart. They quickly need more repair than when you started. Mercifully, table top joints at least can be sawn apart, re-jointed and re-glued. God forbid a sturdy mortise and tenon joint needs undoing
The aliphatic resins are great glues, to be sure. They command the field, and have swept away from common use casein glue, (coming attraction), white glue, and, except for Luthiers, musical instrument repair and , probably period reproduction furniture makers, the hot hide glues. I wonder if some future furniture repair person, in frustration at smashing a digit will be swearing at us for abandoning reversible, traditional hot hide glue for chemistry?

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