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Federally Compliant Protective Clear Coating for Metals

In the near future, the United States and Europe could run out of options for protective clear coating systems for some outdoor monuments, because of environmental regulations for many solvents that have been commonly used. Learn how the museum’s conservation research helps to identify new ways to protect outdoor sculptures and architecture.

Bronze sculpture of Adam by Auguste Rodin outside of the Rodin Museum


IMLS- National Leadership Grant to the Conservation Department

Philadelphia Museum of Art

Federally compliant protective clear coatings for metals

October 1, 2008 – September 30, 2011

The conservation and preservation professions in the United States and in Europe face the real prospect that in the near future there will be no viable clear coating systems to protect outdoor monuments, sculptures, buildings, and other significant artifacts made of copper or iron alloys against corrosion and degradation. If regulations outlawing the use of solvents common to the formulation and application of such coatings are expanded in the near future, the only options available may be short-lived wax pastes that typically require reapplication every one to three years and contain some percentage of solvents that are also likely to be restricted. Agencies regulating air quality in the United States include the EPA, OSHA, state and regional air resources boards, ozone transport groups/commissions and departments of the environment; the controlling federal documents are now 40CFR51.100 and 40CFR.part60.AppendixA.TestMethod 24/24a. Proven, acceptable alternatives to traditional solvent-based coatings (including Paraloid™ Incralac and Permalac) have not been addressed nationally or internationally by the conservation field, except in the work undertaken by Clare and Lins at the Philadelphia Museum of Art over the past two years.

This research is not only aimed at meeting the urgent need for alternative efforts in the United States, but also to advance professional practice globally by evaluating high-performance coatings with careful consideration of the practical cost savings and benefits, and to develop better standards for testing and modifying coatings in the conservation field.


Research to meet these goals will be undertaken with the participation of scientists from two of the best resin manufacturing companies in the United States, Rohm and Haas and Arkema.

Rohm and Haas is the manufacturer of Acryloid/Paraloid™ series of resins (B-72, an ethyl methacrylate copolymer), which have been shown to have very good UV resistance with little yellowing, cracking, or chain scission on long-term exposure. Whether in solvent or water-based formulations, these resins have a long and proven record of excellent performance in the conservation field. We will engage specialty coating firms when we encounter suitable products for comparative testing.

Participants will also include significant members of the target audience—conservation professionals—who will evaluate prospective coatings and lab results and perform independent tests in their own facilities, with an outer limit set at twenty-five participants. The selection of participants for independent field-testing will be based on the demonstrable skill and familiarity with coating applications and will include:

  • Conservators whose practice centers on the treatment and restoration of outdoor metals, especially monuments, sculptures, and decorative elements from private firms
  • Architects and engineers, whose work includes large monuments and buildings with metal components
  • Metalworking firms with substantial restoration businesses
  • Coating scientists

The research will include a comparison of high-performance coatings (KYNAR® Aquatec) with traditional and improved coatings such as Incralac, Permalac, and Syncralac. An evaluation of basecoats will include the study of base coat-to-substrate performance and basecoat-to-topcoat adhesion, as well as the effectiveness of pretreatments, such as inhibitors and passivation/conversion coatings. The role of additives to increase performance will be explored including corrosion inhibitors, organic UV inhibitors, and inorganic UV inhibitors such as nanoparticles.

Work Plan

Work Plan: Experimental Methods

Phase I. Accelerated Weathering

Standard coupons of both patinated and unpatinated bronze and iron will be used to test promising coatings. These samples will be tested first in an accelerated weathering or condensation. The performance of these samples, after determining exposure periods, will be analyzed using several methods:

  • Their permeability to water will be measured using Electrochemical Impedance Spectroscopy (EIS); funds for purchasing this equipment are requested
  • Chemical changes within the coatings will be analyzed using Fourier Transform Infrared Spectroscopy (FTIR)
  • The susceptibility of coatings to discolor or whiten with age will be documented and measured using optical photography (according to ASTM D 1729-96)
  • The resistance of the coatings to degradation by ultraviolet light will be monitored by recording mass changes of the coated samples during accelerated weathering exposure
  • Corrosion products will be analyzed using XRPDA and FTIR

From these initial accelerated screening methods, high-performing candidate coatings will be selected for outdoor exposure tests. The samples will be expanded to include corroded substrates and corroded substrates that have been treated to remove active corrosion.

Phase I. Accelerated Weathering

Phase II. Outdoor Exposure Tests

Coated metal coupons will be tested in standard outdoor south Florida exposure tests. This

Phase includes testing on weathered and weathered/cleaned samples of bronze and iron in addition to the cold/rolled metal coupon types used in Phase I. The color, whiteness, and mass changes over time of each sample will be measured. Periodically, scientists at the museum will examine the samples in situ (in South Florida). At given intervals, the samples will be analyzed by EIS, FTIR, and other instrumentation as required; for these analyses, the samples will be shipped to the museum.

Phase II. Outdoor Exposure Tests

Phase III. Field Testing and Double-Blind Study

High-performing coatings and traditional coatings will be provided to experienced metals conservators, the Participants (Controlled Substance Ordering System [], Milner Carr, Historic Arts & Castings, Moorland Studios, Pincus Associates, Vitetta, et al.) in a double-blind study. The Participants will be provided with application instructions for each coating (if necessary), but they will not be told product names, nor will they be informed whether the coating is novel or traditional. Feedback will be solicited from the Participants who will respond in a formal survey and also provide anecdotal feedback. The survey will assess a variety of physical characteristics of the coatings including the ease of application/ sprayability, ease of use, and appearance, and will ask the Participants to rank the coatings based on their overall satisfaction. At the end of the survey, the coatings will be identified (to the Principal Investigator and to the Participants) and the results will be disseminated in a report. Based on the results of the Double-Blind Study, additional samples coated with the highest-ranked coatings and traditional coatings may be made and exposed outdoors in South Florida.

Phase III. Field Testing and Double Blind Study