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Care
of Leather
With
any leather object, a time comes when its historic value exceeds
its utility. At that point, the type of care required to preserve
it becomes very different from that which was applied when
the artifact was in regular use. Indeed, continuing some practices
(for example, saddle soaping, oiling) on aged leather actually
will shorten the life span of an antique object made of leather.
This
article gives a general overview of the structure of leather,
the different methods of tanning, the causes of deterioration
in leather, and optimal conditions and preferred methods for
handling, storage, and display of historic leather artifacts.
Artifacts
made of leather, or, more properly, of cured animal hide (from
mammals, reptiles, birds, even some fish), most conveniently
fall into four categories of tannage. Non-tanned leathers
include rawhide, parchment, and vellum. Semi-tanned leathers
are oil-tanned or alum-tawed, wherein the skin is simply soaked
in either oil or potash alum. Native-tanned leathers include
smoke-tanned and brain-tanned, and fully-tanned leathers are
tanned with extracts of plants (vegetable tannage) or with
salts of metals (mineral tannage). The purpose of tanning
a skin is to make it impervious to rot and to render it useable;
the type of tannage will determine a leathers resistance
to moisture, its flexibility, appearance, and longevity, and
will vary with intended use.
It
should be noted that this article is intended to be a guide
for preserving leather artifacts that are in a collection
of historic objects, rather than those articles that are still
in use.
Structure
of leather
Leathers
are made from the dermal layer of skins. The epidermis, the
outermost layer of any skin, carries surface structures such
as hair, feathers, or scales and generally is removed during
leather making (except with furs). The dermis, or corium layer
of skin, consists of connective tissue fibers (primarily collagen
fibers), the cells that produce the dermal structures, and
the ground substance (a viscous fluid surrounding the fibers,
removed during tanning). The dermis also will contain some
fatty tissue. The dermis has two layers: the papillary layer
is the outermost; the distinctive grain pattern that one sees
on a given piece of leather in the papillary layer is due
to the way hair grows on different animals. The majority of
the thickness and strength of a piece of leather is in the
fiber network layer, which consists of long wavy bundles of
collagen fibrils, arranged in a network perpendicular to the
surface. Tanning is directed primarily at the dermal layer,
specifically at the collagen fibrils: chemically, it fixes
the ionizable side groups of the collagen fibrils by increasing
hydrogen bonding between collagen molecules. This links the
open network of fibers, leaving the leather pliable, and occupies
all sites that otherwise would allow the leather to rot.
After
tanning, leathers are frequently dressed, or treated with
fatty substances, to improve their flexibility and resistance
to water and wear. Heavy-weight vegetable-tanned leather is
curried with cod oil and tallow, worked in mechanically. Alum-tawed
leather is stuffed with flour, egg yolk, or oils. Chrome-tanned
leather is filled by fat-liquoring with an emulsion of sulfated
oil and water. As a result of tanning and dressing, leathers
are acidic, the degree to which being determined by the specific
tannage and dressing. Achieving the correct pH in the finished
product is necessary to maintain the stability of the tannage
and the collagen.
Deterioration
of leather
Leathers,
being hygroscopic (readily absorb and retain moisture), are
capable of being damaged by moisture loss and absorption.
Control of the environment in which a leather artifact is
being stored or displayed is essential. At a relative humidity
(RH) level of 35% or below, leather becomes desiccated and
can crack when handled. At RHs of 70% or above, mold growth
can occur; molds will break down the very structure of leather
as they grow and feed off the proteins in the leather and
on the fatty acids in dressings. Additionally, absorption
of excessive moisture can promote hydrolysis of the protein
chains that form the collagen fibrils, resulting in shrinkage
and the ultimate embrittlement of the object. (Hydrolysis
is the decomposition of a chemical compound or structure by
reaction with water.) Finally, leather may be sewn or riveted
to other materials that will respond to changes in humidity
at a different rate than the leather, leading to cracking
or splitting of the leather. Therefore, a range of 35% to
70% RH is optimal. Monitoring and maintaining stable humidity
levels is essential for promoting long-term preservation of
leather artifacts.
Elevated
temperatures also can have severe effects on leather artifacts.
Besides drying the leather, temperatures above 68° F (optimal)
can accelerate the chemical reactions just mentioned, causing
deterioration to progress much faster. When a leather object
becomes embrittled, the bundles of collagen fibrils stick
together, resulting in hardening and loss of tensile strength.
If
we add airborne pollution to the mixture of high RH and temperature,
resulting deterioration can be noticeable in a few years.
Airborne sulfuric acid, formed when sulfur dioxide (generated
by manufacturing plants and burned fossil fuels) combines
in the atmosphere with oxygen and moisture, will attack vegetable-tanned
leather in a reaction known as red rot. Initially the leather
becomes softened and swollen, followed by cleavage of the
protein chains in fibrils, the leather ultimately becoming
dry and powdery and easily crumbled by handling. This most
often is seen (for example, on leather bookbinding's) as reddish,
raw areas of loss. Red rot is not reversible; damage from
it is permanent. Additionally, all acids, whether atmospheric
or residual acids left in the leather from the tanning process,
will accelerate the hydrolysis of collagen. It goes without
saying that liquid water will have the same effect on partially
degraded vegetable-tanned leather. Water is also detrimental
to untanned hides, such as parchment, vellum, and rawhide.
Contact with water will cause these materials to cockle and/or
shrink, and the deformation is for all purposes permanent.
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Red-rotted
leather
Alkaline
conditions (relative to the natural acidity of leather) also
can be damaging to leather. A pH* greater than 6.0 will result
in hardening and embrittlement of vegetable- and chrome-tanned
leathers, whose normal pH varies from 3.0 to 5.0. As an example,
buffered tissue (as opposed to acid-free or unbuffered) is
impregnated with an alkaline salt and should never be used
to wrap a leather artifact.
*pH is a logarithmic scale to measure percent Hydrogen, as
an indicator of acidity or alkalinity. 1.0 is the most acidic,
14.0 is the most alkaline; water is neutral at 7.0.
Light
can fade dyes and pigments on or in leather and, given sufficient
time, will break down virtually any organic material. In leather,
strong light accelerates oxidation of organic materials (collagen)
and causes the formation of peroxides from tannins and oils
in the leather, which catalyze further hydrolysis. Damage
from exposure to excessive levels of light is cumulative and
cannot be reversed. Avoid exposing leather artifacts to direct
sunlight, spotlights, fluorescent lights, and even bright
indirect sunlight for prolonged periods.
Mold
growth, as mentioned above, can permanently damage leather,
but can be controlled by lowering the humidity in which the
artifact resides. Mold can appear as a white, gray, or green
powdery deposit, or as black spots on leather. Mold can be
removed by vacuuming the artifact or by gently brushing the
mold off, but the artifact should be removed to a well-ventilated
area away from other objects during cleaning to avoid depositing
spores on them. If the artifact is fragile, the vacuum nozzle
should be covered with screening, and the nozzle should never
touch the leather to prevent pulling off sections of the surface.
A dust mask should be worn during mold removal, as many different
mold spores have been proven to be a respiratory hazard. Removal
of the "fruiting bodies" (the visible portion of the mold)
from the surface still can leave the tendrils and spores inside
the leather. Solutions designed to kill the remainder of the
mold can discolor the leather, and some are toxic to humans.
It is safer for the artifact if the owner takes advantage
of the fact that mold remains dormant at lower humidity and
keeps it suppressed by denying mold the humidity it needs
to grow. Additionally, improving air circulation around an
artifact will reduce the chances of deposition of new spores.
Finally, it is possible to confuse fatty bloom from leather
dressings and salts from perspiration with white molds; bloom
will have a waxy feel, and salts will be crystalline.
There
are certain insects for which leather is a feast. Some beetles,
such as the hide beetle, the carpet beetle, the leather eater,
and the museum beetle, will eat the leather itself. Other
insects are attracted to the oils in the leather or to materials
attached to or painted on the leather. Artifacts should be
inspected regularly for signs of insect activity, which may
include droppings, larval cases, holes in the leather, or
damaged surfaces, especially on the darker or hidden surfaces
of a displayed object.
Dust
can be very difficult to remove without damaging a fragile
or decorated leather surface. It is a fact of todays world
that airborne pollutants are found everywhere. Dust, in addition
to being unsightly, is hygroscopic and contains pollutants;
the combination contributes to the degradation of leather.
Storage and display enclosures for artifacts should be designed
to exclude dust.
Incorporated
materials, especially metals (rivets, brass eyelets, buckles)
and their salts, can be corroded by leather and can be corrosive
to leather. Iron and its compounds seem to be most reactive
with leather, followed by copper, brass, and tin. Copper and
brass also react with the fatty acids in oils used as leather
dressing, forming waxy green accretions of organometallic
salts, generally copper stearate, at all areas where leather
contacts metal. Wherever it is possible without damage to
the leather (such as inside the loop around an iron buckle,
behind a brass button), a barrier of thin mylarŪ sheeting
should be inserted between metal and leather, preventing contact.
Ample
studies have proven that leather dressings and saddle soaps,
rather than preserving aged leather artifacts, actually hasten
their deterioration. Oils in dressings are intended to provide
internal lubrication for leather that is still in use; the
oil allows the bundles of fibrils to slip over each other
as leather is flexed, keeping it supple. Historic leather
artifacts in a collection no longer need to be flexible, since
they are no longer functional objects. Research has shown
that many oils and fats used in leather dressings (neatsfoot
oil, mink oil, lanolin) oxidize and harden over time, causing
the leather to become even stiffer and brittle; oils also
will darken with time, thus darkening the leather. Saddle
soap originally was developed as an emulsified dressing for
leather. Its ability to clean a surface is dubious, as the
"soap" in it is employed to emulsify the oil/water mixture,
leaving little reserve cleaning power. Saddle soap is also
alkaline and leaves residues that cause degradation of the
leather.
Obviously,
one should never attempt to use a historic artifact, or attempt
to reverse the effects of age with the intent of restoring
it to usable condition.
Care
& prevention
The
greatest danger to leather from improper handling, especially
with aged, brittle leather, is from insufficient support,
both when moving it and in storage. If a brittle section is
allowed to bend far enough when it is lifted, it may crack;
thin leather straps and belts (for example, a sword belt),
are most susceptible. Artifacts being lifted should be completely
supported by a tray or stiff board that has been covered with
polyethylene foam (EthafoamŪ) or unbuffered, acid-free tissue.
One will want to provide extra support for heavier, attached
elements to take strain off the point of attachment. Cotton
or nitrile gloves (many people are allergic to the latex in
examining gloves) should be worn when handling objects to
avoid leaving dirt, oil, or perspiration on and in the leather.
When in storage, artifacts should be wrapped in cotton fabric
or acid-free tissue. Leather artifacts that are still flexible
(baby shoes, dress gloves) can be stuffed with polyester batting
or unbuffered tissue paper to maintain their shape and to
provide internal support. Inner corners of sharp folds should
be padded to prevent cracking and splitting as the leather
settles over time (leather pouches). Artifacts that already
are hardened and brittle may be damaged in trying to regain
their shape and should never be overstuffed to try to restore
lost shape. Wrapped and padded artifacts should be kept in
cabinets or in unbuffered, acid-free cardboard or corrugated
polyethylene or polyester boxes to exclude light and dust.
(Never use any type of vinyl or PVC.) Storage containers should
not be completely air-tight to reduce the chances of mold
growth within a sealed enclosure. Storage locations should
avoid high temperatures (attics) and high humidity (basements),
for reasons already enumerated.
Similarly,
artifacts being exhibited should be subject to the same limits
of light exposure, temperature, humidity, and support as those
in storage. A schedule of regular monitoring for insect activity
should be followed.
Conservation
Special
problems with a historic leather artifact may be beyond the
ability of the owner or collector to repair, such as a cockled
parchment document; an exceptionally dirty, painted buckskin
shirt; or a torn and red-rotted childs shoe. In such special
instances, the owner may wish to contact a professional conservator,
who is trained to perform treatments that neither damage the
artifact nor contribute to its long-term deterioration. The
conservator will examine and analyze an object to determine
original materials used and cause of deterioration. The conservator
will propose a minimally intrusive treatment and will document
the treatment performed. For help in locating a conservator
in your area, contact the American Institute for Conservation
of Historic and Artistic Works (AIC), 1717 K Street, N.W.,
Suite 301, Washington, D.C., 20006; (202) 452-9545.
Disclaimer:
Due to the potential fragility of leather artifacts, aged
leather can be damaged easily by incorrect treatment. It is
not recommend that the owners of leather artifacts perform
their own treatments as we assume no responsibility for damage
incurred by owners based on information provided in this article. |
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