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Re: [BKARTS] Hot glue and gelatine (long)



By an odd coincidence hot glue also came up yesterday on another list
I follow, one concerned with hand woodworking tools, and I have spent
much of the last two day writing about it. On the BookArts list the
topic seems to be generating more heat than light (with the exception
of Jack Thompson's useful contribution), but some clarity might be
brought to conflicting statements by considering how glue and
gelatine are made and how they adhere. A start can be made by
considering that different people have been using the term "glue" to
refer to different products, while the difference between glue and
gelatine is one of degree, not binary nature.

Gelatine and hide glue are made by boiling hide scraps until the long
twisted triple-chain collagen molecules untwist, break shorter, and
go into solution. After the first boiling is made, the liquid is
decanted off, and the scraps are boiled again, more vigorously. The
second boiling untwists and slightly breaks up collagen that was to
stubborn to go into the first solution. A number of batches will be
boiled out of one load of hide scraps, each one containing
progressively shorter molecules. Late batches also tend to be acid,
because (if I remember correctly) there are so many choped-off tiny
bits of collagen floating around with acid ends. Bone glue contains
collagen also, but to extract it the bone must be heated much more
vigorusly, typically with live steam, and even the first batch of
untwisted collagen extracted from bone is chopped shorter than that
from the first batch extracted from hide. Also, the vigorous heating
of the bones will bring across salts as well as collagen.

All commercial gelatine glue and animal glue is a blend of various
batches of extract. The point of blending is to make the measurable
properties predictable. Gelatine has a very high proportion of the
longest molecules, which means it is pretty much pure hide.
High-quality glues like pearl glue will have a distinctly shorter
molecule length, but will still be very long. Low-quality glues like
so-called "rabbit skin" glue will be almost entirely bone and will be
very weak. Rabbit skin glue is also, by the way, highly acid.

It is customary to call hot glue "degraded" gelatine, but there is a
lot of emotional load to this word and not much objective meaning.
The basic difference being pointed to is shorter or longer molecules,
broken down by more boiling.

High-grade long-molecule gelatine differs from lower-grade glues in
strength, viscosity, and many other properties. Long molecules make
the adhesive stronger and tackier at a given concentration; and a
given weight of long-molecule glue will accept more water, making it
more economical and unobtrusive to use. Gelatine and glue are, like
paste, quite brittle in a cast film, but gelatine and paste typically
are put on paper in such a thin film that the paper remains flexible.
High-quality hot glue can be put on in a similarly thin film, but it
is very difficlult to get it thin enough, and unplasticized hot glue
tends to make a book spine much stiffer than does paste, gelatine, or
plasticized PVA.

Seems pretty clear, doesn't it? Gelatine is stronger and, in
practice, more flexible, so it can be substituted for hot glue with
no disadvantage and with improvement in quality. That's what I
thought too, until I had some working experience of glue as well as
gelatine. Its the working properties that alter things: gelatine is
cranky and difficult to use properly, hot glue is easier to use
properly, and the difference is enough to prevent simple
substitution. It is much harder to do good work with gelatine, and
easier to do poor work under the impression that the adhesive itself
is good enough to make up for mistakes.

Here is why: glue dries in three stages. While hot and wet it is
tacky and strong. Work will stick immediately without need for
weighting; it can be picked up and put down again, so that mistakes
can be corrected. This stage may last several minutes, somewhere
between one and ten depending on temperature of the room,
concentration of the glue, and so on. The second stage is while the
glue is cold and wet: in this stage it is weak, and if the two
workpieces are pulled apart they will not restick, or they will
restick only very partially. Finally, when the glue is completely dry
it develops its full strength but also its full rigidity. In general
a glued joint must be dried over night to reach the third stage. The
friendliness of glue as an adhesive is dependant on this three-stage
drying process: the strong initial tack of the first stage gives
plenty of time to get work into place and reposition it if necessary;
the weak second stage is not unduly weak. With hot glue you can work
sequentially on a book, going from one step to the next without
stopping, unlike paste where you must stop and weight work to let it
dry; but with hot glue you can work calmly and recover from mistakes,
unlike PVA where there is always pressure and a rush to do things
perfectly and quickly. In my experience hot glue is an uncommonly
friendly and easy adhesive to use, almost as pleasant as paste.

Gelatine has the same three-stage drying process as hot glue, but
exxagerated. The first hot-and-wet stage is uncommonly
short,sometimes just a few seconds; and the second, cold-and-wet,
stage is uncommonly weak, so that if you try to continue working on a
book with gelatine in this stage you are likely to pull apart the
earlier joints. If you linger over work with gelatine you may get a
joint that seems to hold together but which fails when stress is put
on it; when I was using gelatine I had spine linings come apart due
to slow work. This means that gelatine is far crankier and more
difficult to use than hot glue: you must work fast and precisely,
can't correct mistakes, and must weight work and allow it to dry
before going on to the next step. Of course, a lot of this is a
matter of degree, but as a practical matter if you are accustomed to
using gelatine and try glue it won't work right; and if you are
accustomed to using glue and try gelatine it --really-- won't work
right.

So the advantage of using gelatine is a meaningless "purity" while
the disadvantage is that it is slow and cranky to use; and the
advantage of high-grade hot glue like pearl glue is that it is very
easy and rapid to use, while the disadvantage is that it has somewhat
more rigidity on the spine. And hot glue has been in use for
centuries and has proved overall to be permanent, enduringly
reversable, and harmless to paper.

Low-grade hot glue like "rabbit skin" glue can be put right out of
the picture. It is acid; it is dark and penetrates paper deeply,
staining it deep brown; it is weak; it is brittle. No need to
consider it. And any hot glue will be converted into low-grade glue
by the heat of the glue pot if it gets too hot or is reheated too
often; remember, glue and gelatine are made by breaking down collagen
with heat and moisture, and more heat means more breakdown. The
visual signal that this has happened is that the glue turns much
darker in color. In good practice hot glue should be mixed up in
small batches, reheated every day to prevent the formation of mold,
and thrown away after a weak at most. Fresh glue should never be
mixed into a batch of old glue; after each batch the pot should be
scrubbed out. Hot glue is like beer or moonshine: you have to be
impeccably clean if you want it to be good. The need for time,
fussiness, and care is the main disadvantage of hot glues in general:
you cana't just leave them in the pot and expect to be able to use
them when you want them. You have to plan ahead at least enough to
start soaking the granules.

The main disadvantage of hot glue is its stiffness on the spine; and
about a century ago someone started mixing glycerine into the glue to
make at more flexible. This works because the glycerine holds water,
and keeps the spine permanently just a little bit wet. In theory this
should encourage mold and also make the glue weak, but pretty much
all publishers' bindings in the early and middle 20th century used
flexible glue and no disaster was caused. Nowadays commercial
flexible hot glue is more likely to be plasticized with sorbitol,
which works the way that glycerine does but is cheaper. Commercial
flexible hot glue is typically sold in large rubbery cakes which are
damp-feeling. Since these cakes don't develop mold it is obvious that
they have a lot of biocides in them. When mixing this stuff with
water it is more efficient to cut the glue into little cubes,
sugar-cube sized or smaller, so that it can be melted immediately
instead of waiting overnight; the smaller the cubes are cut, the
faster the glue will melt. I understand that there is now also
synthetic hot glue formulated to have the properties of plasticized
animal glue and, in the user's hands, undistinguishable from it. I
like some of the working properties of plasticized hot glue, but I
dislike others, and I don't trust the plasticizers and fungicides.

Some of the disagreements earlier in this thread obviously came
because different people were talking about different kinds of glue.
Although all gelatine and animal glue is denatured collagen, the
different degree of breakdown and different additives give rather
different properties, and there are tradeoffs involved in going from
one to another. Of the adhesives I have talked about above, I have
used and will use all for different parts of different books,
depending on the kind of work I am doing and the effect I want to
get. The only exception is "rabbit skin" glue, wich I regard as far
worse than useless for books and paper; and even this vile material
may for all I know be good to use in one of its main uses, preparing
gesso for gilding picture frames. At any rate I am prepared to
believe a frame-gilder if he tells me that rabbit-skin glue is
necessary to him.

Tom Conroy
Berkeley





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