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Re: Crack or wood grain?

Author: triplereed
Date: 2008-07-21 13:35

Elizabeth wrote:

"[...] my repairman recommends against that, as superglue does not expand and contract with the wood and would not hold the sides of the crack together when the wood moves. Some people use a black epoxy to fill the crack, I think more for aesthetic reasons than to stop the crack from growing."

Sorry, I disagree with the repairman who told you so. Success with such repairs depends on various factors and circumstances, choice of one technique over another, and these must be planned beforehand. Keeping this into account, he shouldn't have made a marble statement in the first place. I'm mystified from the fact that most repairpeople retain a «trash it or pin it» approach toward oboe cracks, maybe just because they saw many a superglued one open again upon time because a previous careless repair had been done with the haste to bring instrument in playing condition rather that bringing it back in pristine conditions.

I had repaired with full success many a cracked woodwind (clarinets, recorders, oboes, and Irish concert flutes) with superglue. Of course this did not mean a simple fill-in job with the kind of superglue you find in hardware stores but rather involved a careful preparation of the crack and an adequate choice of both glue and glue primer if the repair is going to last indefinitely.

First off, the actual reason of a given crack must be investigated thoroughly because a good repair that doesn't depreciate the instrument is not always possible. It must be conceded that, if a given instrument «blows out» by developing a deep crack through the bore all of a sudden, chances are that the wood itself was so defective that it probably would go on behaving wildly even with multiple retaining rings, maybe shifting its slow movements toward pillars and toneholes as I saw once in a badly cracked clarinet belonging to a friend. Those joints should be replaced, not repaired. On the other hand most minor cracks, even long and ugly looking ones, can be caught at the beginning and fixed perfectly without the need to drill holes through the joint wall. Moreover, pins are almost always metal so I don't see any rationale about why a severe crack in the upper joint should grip without further creeping to a metal pin whose application involves the added trauma of removing wood to make room for it, with the side effect of hollowing out the mass of the cracked zone: it would be much wiser to devise a way to key-in one or two hardwood dowel-shaped insets, not necessarily of the same wood of the oboe, if they're supposed to act as a cast.

Second, all of the hardwoods employed in woodwind manufacturing contain natural oils. These would impede any glue -not only cyanoacrilates- a good, effective tack thus rendering the job useless at the first attempt of prolonged use or at next season change. The damaged zone must be first vacuum dusted, then dehydrated with 95° alcohol so that the wood can shrink a little more exposing the crack's walls to the last crevice. Exposure to citrus fruits' rinds inside a paper sack also helps to shrink the wood a bit. Every trace of oil, dust, even finger grease must be cleaned with great care with repeated applications of dry deoxidizer or lighter fuel, avoiding lacquer thinner, the last pass always made with fresh acetone since it must must be compatible with the primer. Then follows the primer, a substance that increases the adhesive strenght of the glue. Too little and it won't work at all; too much and the glue will polimerize abruptly with the net effect of drying brittle, barely filling the crack without actyally repairing it. While the preparation is done once for all, the filling must be done in stages with just a droplet added each stage - the less the better. Evidently this is not the outcome of too many amateur glue jobs performed by otherwise clever professional repairpeople who on the other hand don't know inside out all there's to know about modern adhesives with their useful application field. Cyanoacrilate come in three basic forms and its behavior can be oriented in many ways with other substances that interact with it. For woodwinds, along with the glue I always use a mixture of very fine seasoned wood dust (ebony, rosewood or whatever a given instrument requires) and silicon carbide dust, at times with cotton linters dust or paper dust, all of them previously treated with the appropriate dye.

Cyanoacrilates aren't elastic at all; but if an oboe developed a split under temporary instrument abuse and/or a temperature shock, once poured its tensions out the wood should not move furthermore. The matter is different with epoxy, as strong as cyanoacrilate if carefully prepared but maybe a little less rigid. Yet the only serious epoxy to be employed is the one that mixes in 1.2 - 1 ratio and cures in about twenty hours (even better if under an infrared lamp set to mild heat), than the common "5-minute" type which has no place in such jobs: it is far more elastic that the long-cure one but doesn't have the same strong bonding properties. Even worse, polimerization begins as soon as you start to mix the binder with its hardener, thus impeding a really good blend of the components. Conversely the 'real' epoxies give plenty mixing time (during which the mixture becomes thinner than the separated components, and it's not uncommon to spend more than 15 minutes over the mixing glass) and equally ample working time before they start to cure, namely at least 4 hours. And once for all, regardless of what the producers claim, NO dyed glue retains even half of the bonding strenght of undyed ones. The percentage of pigments allowed to epoxies before they lose effectiveness (I'm talking about a real glue bond, not to a mere fill-in) is too little to give the hardened surface a convincing look. If epoxy is chosen, the best thing is to mix the last layer with wood dust. Once cured, the hardened filling can be brought flush to the body with a chisel. After wet-sanding and polishing with almond or linseed oil, the job is virtually invisible.

If music was an apple, I'd be the snake

Post Edited (2008-11-28 10:08)

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