Shagreen

Mark Vine <100436.3447 [at] compuserve__com> writes

>This enquiry is placed on behalf of a conservator without access to
>the list. He has been given two 20 x 30 inch panels which the
>custodian tells him are covered with shagreen (my understanding is
>that this is the skin from the underside of a shark).

The first time I saw shagreen it covered the surface of a tea caddy
case. It was labeled "imitation leather" and had me stumped. Once it
is known, however, shagreen has a very recognizable surface pattern
that is unmistakable under magnification. Shagreen is the term for a
coarse-surfaced hide; the original use for the name stems from the
seed-impregnated donkey, horse or camel hide found on Persian riding
boots and weapons from the 1600's. Generally, though, shagreen is
assumed to be the untanned skin of sharks or rays.

Shagreen has a long history of use. Early protective wear and
sand-paper applications aside, shagreen inlays for luxury items were
made by Jacques Galuchat for Louis XIV in the 1750's; Herman
Melville mentioned it 1851, in Moby Dick ("...Hosea Hussey had his
account books bound in superior old shark-skin"); and the 1920's saw
inspired designs by avant-garde furniture makers. The shagreen look
has been imitated with paper; more recently, new low-friction swim
suits which recreate the surface of sharkskin were used at the
Sydney Olympics. The best illustrations of artifacts with shagreen
components I know are in the catalogue of a retrospective exhibit
held in Paris in 1994. The book (Caunes, Lison de et Perfettini,
Jean. Galuchat. Les editions de l'amateur, Paris, France, 1994) is
out of print but can still be found through French book dealers.

Here are some identification notes:

Sharkskin has a rough outer surface due to deeply embedded dermal
denticles called "placoid scales". The placoid scales of sharks and
their close relatives, rays and skates, are composed of calcium
carbonate, calcium phosphate, dentine and enamel like teeth, while
the scales of all other fishes are composed of calcium salts and
collagen fibres, like bone.

Placoid scales are unique to sharks, rays and skates. They can range
in size from microscopic and closely spaced (over half of shark
species are less than one meter long) to prominent spikes. Depending
on the species, age and location on the body, placoid scales have
different shaped crowns with variable numbers of ridges, cusps,
blades or keels and furrows. On the body of the shark, the common
arrangement of denticles has the pointed cusp facing the tail. This
head-tail direction feels smooth to the touch, while in the opposite
direction it is sharp. On shagreen, the same directional difference
between smooth and rough can be expected to some degree.

Shagreen is prepared by scraping, stretching and drying. It is not a
true leather but a rawhide. It is processed into a useable material
by grinding, filing and polishing the surface denticles in order to
flatten their sharp points. The skin may also have been dyed, or
have  pearly white denticles interspaced with black varnish. (A
chemical process to decalcify and soften placoid scales for
producing commercially tanned leather did not exist prior to the
1920's). Black filler in-between closely spaced denticles is common,
as are natural, light-coloured skins with ground-down spikes, e.g.,
from the stingray. Copper salts and other dyes were used for
contrasting denticles and skin tones in shades of green, red, brown
and yellow.

The dermis of sharkskin is composed of a tight network of unusually
long connective fibres that may be seen along exposed edges due to,
for example, a distorted substrate or shrinkage. If necessary, a
fibre can be tested with a protein stain. Using the microscope is
advisable for this test since the shagreen (or its imitations) may
have been originally adhered to a substrate with hot hide glue. It
is also for this reason that I prefer an acrylic emulsion such as
Rhoplex N-560 for conservation treatment.

Finally, shagreen is susceptible to damage during RH fluctuations.
Under low RH, dermal denticles crack like teeth; the skin reacts by
tearing, distortion or lifting off its substrate. For the
conservation record, it is useful to describe the number of shagreen
sections that were applied over an object, and include a close-up
photograph to document the condition as well as the shape of it's
placoid scales.

Margot Brunn
Conservator
Provincial Museum of Alberta


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                  Conservation DistList Instance 14:55
                 Distributed: Thursday, April 19, 2001
                       Message Id: cdl-14-55-009
                                  ***