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Re: dry rotted suede :: maintenance



Richard Minsky writes:

>Thanks Sumner Zacks for the anatomical response from the M.D. viewpoint.
It's >wonderful how many fields come together in bookbinding. My grey cells
are >getting greyer, but if I recall correctly, when I was studying leather
tanning >at National Leasthersellers' Center in Northampton, England (1978),
the grain >of the leather was the epidermis and the corium was the dermis.

As I understand it, Mr. Zacks is correct.  What is normally called the
'grain' of leather/parchment is actually the boundary between the epidermis
and the underlying corium.  The bond between epidermis and corium is dissolved
in the unhairing process, and the epidermis is sloughed off when the hair is
removed.  Good thing, as the epidermis does not react to tanning agents the
way the corium does, and any traces of it would be unsightly on the finished
leather.  And as I was just doing this very operation not an hour ago, I can
verify that the epidermis does in fact come off.
The corium itself is composed of different layers.  The grain layer
(at the corium/epidermis interface) has a different structure than the
underlying layers.  But this is mostly a matter of how the collagen fibers
are arranged, and the relative density.  In some skins, the grain layer can
be easily separated due to a layer of fat running under it (eg. sheep).

> A suede had most of the grain sanded off, creating the nap, as
distinguished >from a split, in which the grain layer was sliced off,
usually by machine with >a rotating blade. Perhaps one of our listmembers in
the leather trade would be >kind enough to post a correction or
corroboration. I donated my books on >tanning to cba, so can't reference
them at the moment.
>

A 'split' means just that - the hide was split into two or more layers. This
is done with a machine which I believe is called a 'band knife'.  Not sure
exactly how the suede nap is raised, but I don't see why they couldn't make
suede from a split.  Has nothing to do with epidermis/corium at any rate.

>
>When tanning I tend to keep the fatliquoring down, as it makes the binding
and tooling of the leather easier, and I oil the books when they're finished.
>

This makes sense.

>Dr. Zacks also brings up an interesting point in citing living cows. There
is a >great difference in the structure of skins between cows, calfs, goats,
sheep, >and pigs, and, of course, the "novelty" leathers (reptiles, birds,
snakes, >etc.).

Yes, the structure varies between the different animals, but he was talking
about the composition of the dermis as a network of collagen fibers.  I think
that holds true as a generalization for all the herbivorous quadrupeds whose
skins are usually used in leather manufacture.  Variations in gross morphology
aside.

>The relative thickness and strength of the grain and corium is of
>interest to binders. The strength in goatskin is near the surface of the
grain, >and in cow deeper in the corium.

The goatskin has a very pronounced/thick grain layer compared to cow/calf, and
the underlyeing layers are less dense.  There isn't such a dichotomy with calf.

>A cowhide split can be quite strong-- the
>layer without the grain can be stronger than the grain layer.

I've only worked with very young calf in my parchment making, so I can't
speak directly about cowhide.  Calf is very dense (compared to goat)
throughout it's thickness, whereas goat/kid is densest at the grain layer.
This is why a parchment made from the full thickness of a calfskin will
be stiffer than a comparably thick goatskin parchment.  Deer seems to be
somewhere in the middle as far as density goes, with more of a distinct
grain layer than calf, but not so much as goat/kid.

>Splitting and
>Paring cowhide and using the grain layer on a binding does not make as
strong a >binding as goatskin or even calf, which is thinner originally,
coming from a >smaller (younger) animal and has the strength layer closer to
the grain layer. >Again, I hope one of our professional leather people will
correct this if I'm >in error.

The impression I have is that the younger the animal, the finer the fibre
structure, and so, the greater the strength/thickness ratio.  Also, I've
seen it recommended that skins be used as close to their full thickness
as possible.  For this reason, when I prepare parchment/vellum/tawed skins
for binding, I prepare them full-thickness.

Cheers, Rick Cavasin     parchmenter and occasional whittawer
email: cav@storm.ca     http://www.niagara.com/~acavasin/rick/rcav.html

>


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