jdowning - 5-22-2012 at 06:23 AM
I use PVA glues for general wood working applications but not for instrument work these days - although many luthiers do use PVA (and other modern
adhesives) for its convenience.
Hot hide glue has been successfully used for centuries for wood working (Ouds, Lutes , Violins included) - examples still surviving to this day. PVA
and other modern adhesives do not have the same historical 'track record' as they were first introduced commercially in the mid 20th C.
PVA glue does not set rock hard so is subject to 'creep' where a glued joint is subject to stress. A PVA joint may be undone with the application of
heat and moisture but - as PVA glue does not stick to itself it cannot be used to reliably repair a joint unless the old glue is completely removed
down to the wood surface. Some PVA glues also stain the wood if allowed to remain on the surface after finishing which shows underneath a varnish
coating as a white stain. Modern glues also have a finite shelf life - often as short as one year - before spoiling.
Hide glue comes in a dry powder or granular form and will keep indefinitely without spoiling if kept in a dry, sealed container. Once mixed with water
to make glue it has a shelf life of only a few days (but may be kept in a refrigerator to extend its use for a little longer) - so only mix sufficient
glue at a time for each application. The glue must be made liquid after adding water by heating on a water bath to a temperature no greater than about
140°F (60°C) as it will spoil and weaken at higher temperatures.
The glue gels quickly so unless the joint is uncomplicated and can be assembled in a matter of seconds, joints such as rib joints or sound board to
bowl joints require that the glue be reheated with a hot spatula blade or hot iron to remelt the glue bit by bit followed by immediate 'clamping' with
glued paper (better) or (more conveniently) self adhesive 'masking' tape. As hot hide glue hardens it shrinks pulling the joint surfaces tightly
together (PVA glue does not).
Hide glue joints are easily repaired after opening with a spatula blade or moisture and heat by recoating with hide glue - so hide glue is used
exclusively (or should be) for repair of old instruments or other antique artifacts that were originally constructed with hot hide glue.
Some claim that hide glue joints being harder than PVA joints transfer sound vibrations better.
Recommended strength for hide glue used for instrument work is about 256 Bloom. Hide glue is readily available from many online sources such as Lee
Valley Tools of Ottawa. Lee Valley stock two grades as "Antique Restorer's Veneer Hide Glue" - Granular at Bloom 260± and Pearl at a slightly lower
strength. I use both - Granular for neck, pegbox and bridge joints and slightly diluted Pearl for sound board joints, fingerboard etc.
So - use of hot hide glue requires some easily learned experience - but can be used exclusively for instrument construction and repair. For more
information about glue preparation, application techniques etc. check out the Oud Projects forum or the many other web sites dealing with the same
subject matter.
The attached article may be of general interest in demonstrating the relative extreme properties of hide glue compared to PVA. Note that it is claimed
that a hide glue joint can take up to ten years to achieve full strength!
(The mixture of hide and isinglass (fish swim bladder glue) mentioned in the article - often used in the past for ultimate strength joints - is likely
not an option for most people these days due to the very high cost of isinglass glue coming from the now endangered Sturgeon fish).
Good luck!
Attachment: Microsoft Word - FoMRHI Comm Glue - More Information..pdf (185kB)
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