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Subject: Ebonite


From: Scott Williams <scott_williams>
Date: Friday, March 23, 2001
Christina Hallden <christina [at] cfvh__kva__se> seeks info about crystals
on ebonite.

Crystalline deposits on ebonite/vulcanite objects and corrosion
deposits where brass or bronze has been in contact with ebonite,
have been analyzed at the Canadian Conservation Institute with the
following results.

    1.  Marconi Wireless Magnetic Detector No. 5267:

       Droplets on pulley: 2-4 mm diameter, acidic, water soluble
       sulfates. Crystals of the corrosion product on the brass
       bushings in the centre of the pulley: Pale green transparent
       crystals were identified as CuSO4.5H2O (chalcanthite) and
       ZnSO4.H2O (gunningite).  Colorless transparent crystals were
       substituted zinc and copper sulfate hexahydrates. (ARS Report
       1855, 1982/01/27).

    2.  Crystalline encrustation on black accumulator wheels of Van
        de Graaf type electrostatic voltage generator: ammonium
        sulfate with iron (ARS Report 2765, 1988/12/05)

    3.  Corrosion product from interface between copper conductor
        and ebonite of a wavemeter coil for a shortwave transmitter
        from Marconi Wireless Telegraph Co.: ammonium zinc sulfate
        hydrate, (NH4)2Zn(SO4-2-6H2O, with S, Zn, Cu, Sn, and traces
        of Fe, Si, and K. (ARS Report 3452, 1995/10/03)

Louise Bacon reported the presence of acidic materials and sulfate
salts on the surface of ebonite in "The Deterioration of four Giorgi
flutes made of ebonite and a possible method for their
conservation", Conservation today: papers presented at the UKIC 30th
Anniversary Conference, 1988, pp. 96-100.  The abstract states: "The
surface of ebonite (vulcanite or hard rubber) undergoes severe
decomposition through the action of light, air, and moisture. The
surface acid formed by this process can be wiped away, but in
extreme cases neutralization may be necessary. This can be done by
rinsing with a dilute solution of sodium bicarbonate, and quick
drying in industrial methylated spirit. For storage and display,
light levels should be reduced to a minimum and ultraviolet
eliminated. Relative humidity must be kept low. If possible, oxygen
should be eliminated; this can be achieved by keeping the objects in
special containers."

Since your crystals are white and you do not indicate they are
associated with corrosion, I suspect you have water soluble ammonium
sulfate or ammonium hydrogen sulfate on your ebonite.

Hard vulcanized rubbers like ebonite and vulcanite contain 30% or
more sulfur vulcanizing agent.  Oxidation in air of sulfur compounds
in the rubber produces sulfur oxides which react with atmospheric
moisture to eventually form sulfuric acid.  Sulfuric acid may by
present as droplets on the surface of the hard rubber.  It can react
with ammonia in the atmosphere to form ammonium hydrogen sulfate and
ammonium sulfate.  When in contact with metals, metal sulfate and
metal ammonium sulfate corrosion products can form.  All these types
of compounds have been detected at CCI. Many of the
metal/ammonium/hydrogen sulfates are deliquescent salts.  At high RH
these dissolve in their own water of hydration and are present as a
liquid droplets.  As RH drops the droplets dry and circular deposits
of crystalline material are formed, often showing a series of
tidelines created by successive wetting/drying cycles as RH

I have not treated any objects myself, but strategies that have been
reported to me include separating the components to allow cleaning
of each then reassembly with an isolating barrier of plastic such as
polyester, polyethylene/polypropylene or Teflon sheet between metal
and ebonite to prevent corrosion, and maintaining low RH to prevent
deliquescent salts that may be present from liquefying and
spreading.  Bacon, in the reference cited above discusses treatments
and storage conditions.

Your description of the ebonite as glossy black where not exposed
and light brown with pale spots where exposed is consistent with
hard rubber.  All hard rubbers I have examined give a positive test
for reducible sulfur using the azide test described by Daniels and
Ward in "A rapid test for the detection of substances which will
tarnish silver", Studies in Conservation, 27 1982-58-60.
Phenol-formaldehyde ("Bakelite"), a material commonly used as an
electrical insulator in a similar fashion to hard rubber, and
potentially difficult to distinguish for hard rubber, does not give
a positive reaction to this test.

R. Scott Williams
Senior Conservation Scientist (Chemist)
Canadian Conservation Institute

                  Conservation DistList Instance 14:49
                  Distributed: Friday, March 23, 2001
                       Message Id: cdl-14-49-001
Received on Friday, 23 March, 2001

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