Paddy Handscombe's attempt to correct Tim Crake's comments regarding
the use of hide, PVA, PVC-E and RTV adhesives itself contained several
statements that should be corrected.  He wrote:

> In MMD 110308 Tim Crake says, "Patrick's practice of 'sealing stack
> and pneumatic boards first with water-soluble PVA to achieve a better
> RTV bond and to completely protect the original wood of the deck' has
> a drawback.  By sealing the original wood with PVA, the repairer
> confines future repair of the player to these modern adhesives only."
>
> This is incorrect.  PVA is entirely water soluble and can be easily
> and completely removed with the same techniques as animal glues are
> removed.  This is why PVA is invariably used in relining valuable
> canvases, which have in the past suffered much damage from animal glue.
> PVA is anyway compatible with animal glue and will not prevent it
> adhering.
>
> It seems that few on MMD have bothered to grasp that PVA is absolutely
> _not_ the same as PVC-E which becomes water insoluble when set.  I do
> not know how much clearer I can be about this important distinction.

I have hesitated so far to join the fray in the vigorous debate on glues.
But I know a bit about PVC-E and PVA glues, as my formal training and
former employment was as a Ph.D. polymer chemist, with a specialization
in latex/emulsion materials and industrial gelatin (effectively hide
glue).

Both PVA and PVC-E glues are emulsion/latex glues which are milky in
appearance until they dry, because they are a water-based suspension of
very fine polymer particles that scatter light just like the suspended
fat/protein droplets in milk.  Both can be thinned with water, and both
can penetrate into wood pores and fibers.  The particles in both glues
coalesce irreversibly when dried into a layer of water-insoluble plastic.
Neither PVA or PVC-E glue is water-soluble after that point, and in my
experience, neither can be easily cleaned off of wood surfaces.

The properties of the dried glues are very different.  PVC-E (poly
(vinyl chloride)) is softer and more rubbery, and more hydrophobic
or water repellent, while PVA (poly (vinyl acetate)) is harder and
stiffer, with a more wettable surface and a greater tendency to absorb
a little water and become a little softer.  Thick bulk layers of PVC-E
can sometimes be peeled or scraped off, because it is softer.  And
thick bulk layers of PVA glue can be swollen and softened by soaking
in water (PVC-E with long! soaking) to aid removal.  But neither is
"soluble" in the same way hide glues are, and a thin surface layer or
the material impregnated in the pores of wood or fabric is effectively
impossible to remove.

Most woodworkers who have used both PVA and hide glues have seen how
a spilled or oozed PVA glue usually creates a later finish defect, even
if it is wiped off the surface immediately with a wet cloth, or even if
the surface was lightly sanded after drying.  Hide glue drips that are
wiped off are much less likely to create a defect.  Because the PVA
penetrates quickly and dries irreversibly, it remains in the wood pores
and shows up during finishing, while hide glue is much more removable
and more forgiving.

Hot hide glue will 'wet' cured PVA glue surfaces better than cured PVC-E
surfaces, and will adhere to some degree, but in my experience not as
well as to a wood surface that hasn't been contaminated with PVA glue.

Cured RTV (room temperature vulcanization) is a cross-linked silicone
rubber usually based on the polymer PDMS (poly(dimethylsiloxane)).  It
is extremely hydrophobic (water repellent) both before and after curing,
so it bonds better to polymer surfaces like PVA-sealed wood than to
bare wood.  And it is entirely insoluble after curing, so it is also
almost impossible to remove entirely from a rough surface, and wherever
a little remains, the contaminated surface will repel most water-based
glues including PVA, PVC-E or hide glues.

It is probably good that different rebuilders have different styles and
use different materials.  Maybe 20 years ago, I used lots of PVA glue
in general carpentry (never in stacks), but I have since stopped using
it almost entirely except where I'm working with a large joint where
repositioning and longer working time with the cold glue is important.

I have found that hot hide glue is almost always superior for wood
joints, in ease of use, "grab", clamping and positioning, heat response
in veneering, avoiding finish defects, better bond strength and
properties, and reversibility or renewability either next week or in
80 years.  Because it gels on cooling, hide glue is also far superior
for joining fabric, felt, or leather to wood without saturating and
hardening them.  Unlike PVA glue joints, hide glue joints can usually
be convinced to break along the joint with little or no damage to the
wood, a tendency that increases somewhat when the joints are 40-80
years old.

I also use some PVC-E glue ( for cloth-to-cloth bonding in some
pneumatics, and the resilience is nice with some plastic keytops),
CA glue (as a binder for ivory powder in keytop chip repair, for
example) and epoxies (where deep penetration and space filling are
needed, like some bridge repair).

I also keep my hot glue in a cheap plastic flip-top container with
a plastic toothbrush and plastic paddle in it as applicators, so
I can microwave it to ready in 45 seconds when I need it quickly,
with no risk of burning it.  The plastic container can be set in a
thermostatted pot of hot water during longer jobs, and can be frozen
indefinitely to avoid fungal growth, if it isn't needed for a few weeks
(all tricks from the industrial gelatin world).  And despite propaganda
to the contrary, neither momentary boiling in the microwave or long-term
freezing affects its properties at all.

I also find hide glue superior in stacks for positive airtight sealing.
To me it seems apparent that early manufacturers specifically did design
their pneumatic elements for reversible disassembly and rebuilding.
Use of hide glue, often along with leather, buckram, or cork gaskets
that aid removal of pneumatics from decks all indicate that, along with
service manuals from the 1920s that instruct the technician to recover
striker or expression pneumatics if there is a hint of fabric stiffness
or damage.  While it is a secondary concern, it is interesting to note
that gelatin or hide glue has the lowest gas permeability of almost any
polymer, while silicones like RTV have almost the highest.

For my part, I prefer the superior performance, reversibility, and
time-tested performance of hide glue and traditional materials for most
wood-joining applications, and especially in stacks.  And I see no
advantage in using insoluble, non-removable and irreversible materials
like PVA glue or RTV based on claims that they solve problems I simply
don't have with hide glue.

Ralph Nielsen
http://www.historicpianos.com/