Page 407 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The phospho-titanate coupling agent Lica 38 is being tested—along with the performance
of conventional corrosion inhibitors, such as BTA—as part of the outdoor exposure trials of
coatings being conducted at the J. Paul Getty Museum site in Malibu, California; at the Swedish
Corrosion Institute in Stockholm; and at a site in Tîrgu Jiu, Romania. About five hundred coated
brass (80Cu20Zn) and bronze (87Cul2Sn, 0.3Fe, 0.6P, O.lAs) test panels were made, together
with a series of controls of untreated or partially treated blanks of copper, bronze, and brass.
Accelerated chamber testing with exposure to N 0 2 and S0 2 , "scab testing," and outdoor tri­
4 0
als were initiated in November 1994 and were still in progress in 2000.
The abraded, polished, and cleaned panels were first treated with a variety of inhibitors:
0.25% (v/v) Lica 38 in ethanol; 3% (w/v) benzotriazole in ethanol; and 3% (w/v) tolyltriazole
in ethanol. This was followed by two coats, using both brush and spray application, of
the following:

1. 5% (w/v) Paraloid Β 72 in acetone
2. a wax mixture of 13 g Polywax 2000, 71 g Victory white, and
18 g Multiwax w-445 in white spirits, applied warm by brush
3. titanium nitride, a clear inorganic coating used commercially for brass
4. Autoclear (an automobile finishing top coat) based on a stabilized acrylic
system with supposedly good uv stability
5. Ormocer A and Ormocer Β (similar to ORÍ, described earlier) from
the Fraunhofer Institute
6. Incralac in toluene 4 1
7. a polyurethane-isocyanate used for coating boats that is supposedly uv stable

After one year of outdoor exposure, the Incralac, Autoclear, and Ormocer A coatings were
still protecting the brass and bronze substrates from corrosion very well, but the others were
beginning to fail. No noticeable beneficial effect was found when any of the inhibitors or cou­
pling agents were used with the coatings compared with using the polymer coatings alone. After
i
two years, the bronze panels were practically unchanged n color. Because of the greater pro­
pensity for copper-zinc alloys to corrode, however, the brass substrates had discolored, or they
were beginning to discolor around the edges because moisture and oxygen were infiltrating
under the coating and detaching it from the substrate metal. After three years of outdoor expo­
sure, all of the coatings showed some signs of either deterioration or failure to protect the under­
lying brass panels from tarnishing.
At this stage, the best coatings appear to be Incralac and Ormocer A, with Ormocer A
outperforming Incralac in both outdoor exposure trials and indoor accelerated weathering tri­
als. Only tiny, light green spots had formed on the samples coated with Oromocer A after three
years of exposure in Malibu, compared with some delamination and patchy discoloration of the
samples coated with Incralac.

C H A P T E R T W E L V E
390