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THE CARE AND PRESERVATION OF
Historical Iron
Clara Deck, Senior Conservator, The Henry Ford
Historical iron can be maintained for years of use and enjoyment
provided that some basic care and attention is given to its preservation.
The conservation staff at the The Henry Ford have compiled the
information in this fact sheet to help individuals care for their
objects and collections. The first step in the care of collections
is to understand and minimize or eliminate conditions that can
cause damage. The second step is to follow basic guidelines for
care, handling and cleaning.
NOTE: This Fact Sheet will present a brief overview of the care
of iron objects, stressing good storage as the best method of
preservation. It does not address the serious problems of preserving
archaeological metals excavated from land or marine sites. People
who collect unconserved archaeological artifacts should be aware
that those types of objects are rarely stable if left untreated.
Contents:
Identifying Iron Artifacts
Causes of Damage
Corrosion
Caring for Iron Artifacts
Handling
Storage
Cleaning
and Care
Cleaning
Degreasing
Corrosion
Removal
Corrosion
Inhibition of Bare Metal
Polishing
Coating
Lacquering
Painting
Waxing
Bibliography
Suppliers
References
IDENTIFYING IRON ARTIFACTS
Iron is a very common metal in
historical collections. It is found in a variety of alloys, known
as "ferrous metals", comprising wrought iron, cast iron
and steel. Galvanized or tin-plated sheet is also a familiar material
in historical collections. Ferrous metals are magnetic so the presence
of iron can, therefore, be easily identified with the use of a magnet.
CAUSES OF DAMAGE
Corrosion, poor handling and inappropriate
storage are the major causes of damage to iron artifacts. By far
the most widely seen problem associated with metal artifacts is
corrosion. Active corrosion causes a continuous loss of metal from
the object. Mishandling, however, can result in breakage, bending
or cracking. Cast iron is usually a relatively brittle material
and will not bend as one might expect metals to do. Salts, oils
and moisture can lead to metal corrosion, so if you handle metal
artifacts with bare hands you risk damaging them.
CORROSION
Uncoated ferrous artifacts that have been kept clean and dry will
usually develop stable surfaces. Stable surfaces may appear blue-black
to brown, and are not scaling, flaking or pitting. This kind of
compact rust that does not progress may actually protect the object
if it is left intact. Hand-forged tools, for instance, often retain
a dark, rough finish. Not all iron or steel was polished, and
altering original surfaces may reduce the historical value of
an object. Dark, stable surfaces may be considered "patinas".
Collectors should be aware of any special finishes (i.e. tempering
or bluing on firearms) that may determine the correct color the
metal should be and the degree to which it should be cleaned.
As mentioned above, orange rusty iron is often seen but may not
be cause for alarm if the corrosion layer is continuous, relatively
even and does not flake off easily. If you notice ongoing rusting
and changes to the surface appearance, including paint loss, chances
are the iron is actively corroding. Problem corrosion in iron
artifacts usually becomes apparent as pits develop at active corrosion
sites. This active corrosion is of concern, especially if it develops
at joints between metal parts. Bright orange droplets or "sweating"
forming on the metal surface indicates advanced active corrosion
induced by high atmospheric humidity (above 70%) and the presence
of salts. Dust and grime left to accumulate on metal artifacts
will actually hold moisture to the surface and may induce corrosion
even where the humidity is not that high.
Painted metal artifacts can usually withstand corrosion as long
as the coating is not broken. Where there are paint losses, corrosion
will progress rapidly (as we notice on cars). If left untreated,
and in a poor storage environment, the corrosion in this case
will continue and eventually cause more severe paint losses.
CARING FOR IRON ARTIFACTS
Since iron artifacts are so varied,
it is not possible to cover all aspects of treatment in this document.
Your first line of defense will be good storage.
HANDLING
Most metal artifacts should not be handled with bare hands. Salts
and oils from your skin can etch into uncoated metals and may
even cause permanent damage. Handle your valuable collection with
gloves. Soft cotton gloves, or any clean glove or rag may be employed
for this purpose. Lift objects from their center of gravity, and
avoid lifting objects by limbs, handles, spouts or other extended
areas; the metal may have developed unseen weakness' over time
and could break if stressed further.
STORAGE
A simple way to preserve iron is to store it properly. Maintain
an even, low humidity where metal objects are kept, ideally below
55 % Relative Humidity (RH). Rapidly fluctuating temperatures
will cause coatings to fail as the metal expands and contracts;
this problem is most acute in the case of artifacts composed of
sheet metal. In most homes, an even environment is difficult to
ensure but, generally speaking, basements in our area are damp
in the summer and therefore should not be used for the storage
of metal artifacts. Humidity sensors are available through suppliers
listed on attached sheet for those who wish to check conditions
near their collections. Do not allow dust to accumulate on stored
objects. You can protect your collection by storing it on shelves
padded with inert foam (i.e. "Ethafoam"). You may choose
to drape plastic or cloth curtains around storage shelves, but
do not place iron artifacts in sealed plastic bags - the danger
of moisture condensation on the metal outweighs the benefit of
dust protection.
CLEANING AND CARE
If you choose to attempt cleaning your iron artifacts, and you
are sure of the surface appearance you wish to achieve, some of
the following suggestions may help:
Cleaning - Stable
or painted surfaces should be kept dust free. Vacuum clean all
stable artifacts regularly, using the nozzle attachment with
a brush. A bristle brush may help to raise dust from crevices.
Any wet cleaning should employ deionized or distilled water
only to avoid contaminating the metal with salts or other impurities.
Degreasing - The
presence of degraded oils and grime may promote corrosion. You
can degrease most uncoated metal artifacts with mineral spirits.
(Please consult the manufacturer or Material Safety Data Sheet
for complete safety requirements.) Wipe it over the surface
in a small, inconspicuous area first to test for discoloration.
After the solvent has evaporated, check for any undesirable
effects (usually caused by residual dust or an old finish).
Continue the cleaning process, using mineral spirits-dampened
cloths to lift the grime. You may find that sharpened bamboo
skewers, nylon bristle "parts brushes", craft stencil
brushes or even tooth brushes help you to get into crevices
and joint areas.
If straight mineral spirits does not seem to be raising the
grime, a conservation-recommended surfactant, "Vulpex",
may be used in a 1% solution in mineral spirits. Be very certain
to rinse with clean mineral spirits to remove residual detergent.
Corrosion Removal
- On objects such as cast iron stoves and rusty machinery, it
is sometimes possible to remove encrustations of corrosion products
by rubbing with steel wool pads or nylon "synthetic steel
wool" pads and a light lubricating oil, or mineral spirits.
Always start with the least aggressive method and work your
way up. Large sewing needles, scalpels or X-Acto knives can
also be used to chip up or lift encrustations. Heavily corroded
objects, original painted iron artifacts or those damaged by
salts may require the assistance of a trained conservator.
Corrosion Inhibition of Bare Metal
- If you are reasonably sure that there are no salt residues
on your artifacts and you are able to remove the worst of the
rust encrustations, it is possible to protect the surface with
a variety of corrosion inhibitors. The simplest inhibitors are
machine oils which inhibit rust by displacing water and moisture.
Oils and greasy coatings may hold dust and grime to surfaces,
so use sparingly, and check often. It also should be understood
that most "inhibitors" will darken the surface of
the artifact, and, therefore, are not appropriate for all cases.
Bright surfaces on machines and other objects that cannot be
stored indoors may benefit from the application of a thick,
waxy corrosion inhibiting oil such as "SP-400", made
by CRC, or similar brush-on or spray-on products.
"Rust converters" are another kind of commercial
product designed to work on rusty metal by converting unstable
corrosion into a stable, protective layer with the help of a
latex-based coating. Some conservators are now recommending
the following when a relatively even, lightly corroded surface
will be preserved as-is: "Rust-Oleum Rust Convert",
and "Extend".
Polishing - If you
wish to return a steel object to its original, polished appearance,
it is usually possible with a fair amount of elbow grease and
a good polishing compound. We recommend "Solvol Autosol"
for general purpose polishing. Test for the degree of polish
you wish to achieve on a small inconspicuous part of the object.
Buff on the polish with a clean rag or very fine steel wool
for added abrasiveness. The surface must be rinsed with mineral
spirits after polishing to remove any polish residues.
COATING
Choosing the appropriate surface finish is an important step in
the preservation of iron artifacts because iron is a very reactive
metal and usually needs the added protection of a coating, especially
if it has no natural protective layer. The inhibitors and converters
mentioned above may be considered as coatings. Sometimes traditional
coatings are recommended, such as "Stove Black" for
historical cast iron stoves. All surfaces must be carefully cleaned
before any coating is applied.
Lacquering - Polishing
exposes fresh, reactive metal to the atmosphere and, therefore,
to further corrosion. In rare cases, it may be appropriate to
lacquer iron or steel. Since lacquering requires the use of
volatile solvents and spraying equipment, we recommend that
this type of work be left to accomplished restoration professionals.
No coating is impervious to moisture, and badly applied lacquer
or paint can lead to worse corrosion.
Painting - It is
rarely appropriate to repaint an original artifact. Museums
and collectors recognize the value of preserving as many original
surfaces as possible, even on old tools and machines. Original
paint can tell a great deal about the use of objects and may
retain decorative detailing under darkened varnish layers. Conservators
can reveal and restore original paint in many cases.
Some objects that were originally painted may have lost the
majority of their finish (i.e.. a large outdoor machine). In
cases like these, painting may be an appropriate method of preservation.
Before preparing the metal for repainting, the first thing to
do is to check in crevices, behind handles, maker's plates or
doors for bits of the original paint. With even a small sample,
and a good eye, you may be able to match new paint to the authentic
color. If you choose to repaint and you have researched appropriate
paint schemes, consider the additional benefits of "rust
converters" explained above. If you do not use a product
such as this, you will need to completely strip and solvent-clean
your artifact to ensure any degree of paint adhesion. In either
case, it is best to use primers and paint specifically formulated
for use on metals (sometimes called "Rust Paints").
Waxing - For most
collectible iron artifacts, the best coating we can generally
recommend is wax. Wax provides a relatively flexible coating
that is easily applied and that can be renewed. In most cases,
The Henry Ford uses "Renaissance Wax", or other "microcrystalline"
waxes because it is inert and will not yellow over time. It
is simply applied with a clean cloth and buffed out with a rag
or bristle brush (shoe polish brushes are great for this purpose).
Again, wax is not an appropriate coating for all metal surfaces,
especially where it is impossible to cover the whole object,
or where the slightly glossy finish would be inappropriate.
BIBLIOGRAPHY
The National Trust Manual of Housekeeping
Sandwith & Stainton
Penguin Books Ltd.
536 Kings Rd
London, SW10 OUH, 1984
The Thames and Hudson Manual of Metalworking
Peter Scott
Thames and Hudson Ltd.
London, 1978
The Care of Antiquities and Historical Collections
A. Bruce MacLeish
American Association of State and Local History
AASLH Press
Nashville, TN, 1972
CCI Notes
Canadian Conservation Institute
Note 9/5
1030 Innes Road
Ottawa, Ontario, Canada
The Conservation of Antiquities and Works of Art
Plenderleith, H.J., & Werner. A.E.A.
Oxford University Press, London
2nd Edition, 1971
Corrosion and Metal Artifacts
Brown, Burnett, Chase, Eds.
U.S. Department of Commerce
Washington, D.C.
NBS Special Publication 479, 1977.
IRON CARE SUPPLIERS
Mineral Spirits, Lubricating Oils, Steel Wool, Nylon
Scrub Pads, Corrosion Inhibiting Coatings, Rust Converters:
Hardware Stores
Auto Parts Stores
Industrial Supply Companies (large orders only-i.e.: Grainger
TANNIC ACID, plus ingredients for preparing)
Chemical Supply companies, such as:
Aldrich Chemicals
940 W. St. Paul Ave.
Milwaukee, WI 53233
800-558-9160
Brushes, X-Acto Knives:
Arts/Crafts supply stores
Waxes, Vulpex Soap:
Conservation Resources International L.L.C.
8000-H Forbes Place
Springfield, VA 22151
800-634-6932
Fax: 703-321-0629
TALAS
568 Broadway
New York, NY 10012
(212) 219-0770
Polishes, "Solvol Autosol":
Good Hardware stores
Jewelry or Specialty Gift stores, such as:
C.R. Hill
2734 W. 11 Mile Rd.
Berkley, MI 48072
248-543-1555
Brit USA Imports
1378 Roslyn
Grosse Point, MI 48236
810-783-2271
Humidity Indicators:
University Products
517 Main Street
PO Box 101
Holyoke, MA
800-762-1165
"Ethafoam", Poly-ethylene Foam:
Great Lakes Packaging Supply
10650 N. End Ave.
Ferndale, MI 48220
248-548-1116
REFERENCES
For a listing of conservators in your area, please contact:
The American Institute for Conservation of Historic & Artistic
Works
1717 K Street NW
Suite 301
Washington, DC 20006
202-452-9545
http://aic.stanford.edu/guide/form.html
Last updated: 9/13/2001
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