Page 404 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Ormocer and other For outdoor bronzes today, Incralac and synthetic waxes remain
polymer coatings the most common choice, but recent research suggests that
some of the Ormocer coatings may also provide good protection.
The Ormocer family of coatings was developed by the Fraunhofer Institute in Munich,
Germany (Pilz and Römich 1997). The base material is a heteropolysiloxane, which is an
organic-inorganic copolymer made by a sol-gel process. The two-component Ormocer lacquer
GDiphenyl is synthesized from gamma-glycidoxypropyltrimethoxysilane and diphenylsilane-
diol hydrolyzed with a stoichiometric quantity of water. This produces a transparent lacquer
that can be stored safely for several months. It is cross-linked with an aminosilane hardener,
stabilized by the addition of some suitable organic oligomer, which results in the system being
reversible with organic solvents. The three most promising lacquers derived from this system
are designated ORÍ, ORIS, and ORI6; the last two make use of a top coat, which is an acrylic resin
applied over the polysiloxane system.
The polymer, mixed in butoxyethanol at high dilution, is applied using a spray gun, taking
the appropriate safety measures, such as using a breathing apparatus. In a series of trials with
this coating, monolayers - 8 μπι thick or bilayers 10 -12 μπι thick were applied to 90/10 bronze
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coupons. Adhesion properties to the bronze substrate were determined by the crosscut test
(DIN 53131) and the pull-off test (iso 4624). Good adhesion was achieved even after simulated
weathering for 336 hours in a "weatherometer," or accelerated weather chamber, in which the
relative humidity was periodically changed and the sulfur dioxide content maintained at a con­
stant and high level, over 60% RH.
The corrosion resistance of samples with monolayer coatings depended strongly on the
coating thickness. The monolayer treatments with ORÍ provided good protection for surfaces
with natural stable patinas. Some of the coupons that were not patinated before the experimen­
tal tests, however, and some of the bare metal coupons that were coated showed some corrosion
of the metal surfaces. This indicated that a coating thickness of - 8 μιη was not sufficient.
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Bilayer coatings with ORIS and RI6 about 10 μιη thick were very effective on both patinated
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and unpatinated cast bronze, by comparison.
Incralac was included in the general testing procedure, and it was found that coupons
coated with Incralac showed some loss of adhesion after testing. Compared with Incralac, the
Ormocer bilayer coatings showed no loss of adhesion after weathering. The natural patina color,
however, was darkened by the Ormocer treatment, which produced a "wet" appearance. Bilayer
coatings and Incralac both change the patina color, causing it to appear appreciably darker and
glossy, and this places aesthetic limitations on the use of these coatings. A suitable wax finish
over the Ormocer layer might improve the appearance.
The coatings in this study were also exposed to condensed moisture for 168 hours at 40 °C,
after which time both the bilayer polymers and Incralac had turned milky; the monolayer
Ormocer system remained in good condition.

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