2.5 I. A Critical Review of Organic and Inorganic Protective Coatings for Stained Glass, and II. Protection of The Last Judgement Mosaic of Prague

University of California, Los Angeles

Mary W. Colby
Ting J. Yuen
John D. Mackenzie
Dusan Stulik
Eric Bescher
S. S. Park
Period of Activity: 6/87 to Present

Project Abstract
(The first part of this project involved a literature review of organic and inorganic coatings for stained glass and represents the state of the art as it was in 1987.)

The protection of stained glass and painted glass against water-initiated chemical corrosion is a grave concern to the conservators of stained glass windows. Presently, organic polymers and inorganically modified polymers are being used to protect these treasured works of art. However, the service life and durability of these coatings are not as good as desired. This report investigated the possible use of oxide films, prepared by vapor deposition or sol-gel processing, to be used as a durable moisture barrier for stained glass windows.

Recommendations from the First Part of the Project
The water permeability through an oxide film is ten orders of magnitude smaller than through a polymer. This coupled with the tremendous hardness and thus scratch resistance (two orders of magnitude greater than that of polymers) makes oxide coatings a promising alternative for the protection of stained glass. Although many processing techniques have been proposed, the sol-gel technique appears to be the most effective because of its adaptability and inherent processing versatility. Sol-gel thin films have been shown to be preparable at low temperatures approximately 90 C, however, the chemical and mechanical properties of these films have not been thoroughly studied. Thus the factors that govern the relationship between processing temperatures and film properties should be investigated.

By choosing a binary oxide system with good corrosion resistance, like ZrO2-SiO2, both the refractive index and thermal expansion can be graded, thus yielding a stronger coating with little reflectivity.

(The second part of this project seeks to apply, ten years later, what was learned in the first part combined with the intervening years of product research and development for the proposed treatment of The Last Judgment mosaic in Prague.)

In late 1994, the Getty Conservation Institute approached the Department of Materials Science at UCLA about on-going degradation of the mosaic of the Last Judgment in Prague, Czech Republic. Created during the fourteenth century, this monumental medieval mosaic, located in the St. Vitus cathedral, is affected by a white efflorescence which veils its surface. Although this degradation mechanism has affected the mosaic for centuries, it is believed that it is compounded by the current level of atmospheric pollution in Prague. Over the years, attempts at protecting the mosaic failed either because wrong protective materials were chosen, or through lack of proper maintenance. At present the Getty Conservation Institute is engaged in the mosaic's stabilization and cleaning.

Primary Publications
Colby, M. W., T. J. Yuen, and J. D. Mackenzie, "A Critical Review of Organic and Inorganic Protective Coatings for Stained Glass," Final Report to the Getty Conservation Institute, June 1987.

ABSTRACT-Same as Project Abstract

Colby, M. W., T. J. Yuen, and J. D. Mackenzie, "Protective Coatings for Stained Glass: A Critical Evaluation of Materials and Processes," Materials Issues in Art and Archaeology, Vol. 123, 1988, Proceedings of the Materials Research Society, Spring Meeting, Reno, Nevada, 1988, pp. 305—310.

ABSTRACT-The protection of glass against water-initiated chemical corrosion is an important problem. Presently, organic polymers nd inorganically modified polymers are being used to protect art glass. However, the service life and durability of these coatings are not entirely satisfactory. Oxide coatings would offer much greater resistance to water penetration. The water permeability through a polymer coating is some ten orders of magnitude larger than through an oxide coating. This coupled with the greater scratch resistance makes oxide coatings a promising candidate for the protective treatment of stained glass. Methods to deposit an oxide on glass include sol-gel processing sol-gel processing ;and vapor deposition. However, such oide coatings must be deposited at relatively low temperatures. This paper compares the performance of different oxide and non oxide coatings and presents new concepts of the deposition of oxide coatings at low temperatures.

Bescher, E. P., S. S. Park and J. D. Mackenzie, "The Last Judgment Mosaic of Prague: Possibilities of Long Term Protection", Report to the Getty Conservation Institute, February 1995, Department of Materials Science and Engineering, University of California Los Angeles, CA

ABSTRACT- This report identified the chemical composition of the mosaic and its degradation mechanisms. It reviews oxide, polymer, and organic-inorganic hybrid coatings. Six plans are suggested for treatment including cost estimates.