A Study of the Role of Heterogeneities in the Initial Stages of Corrosion of Glazes Using Dynamic Imaging Microellipsome
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(DIM) B.E. MCCARTHY*, P.B.VANDIVER**, and J. KRUGER*** *The Getty Conservation Institute,1200 Getty Center Dr. Suite 700, Los Angeles, CA 90049, [email protected] **Smithsonian Institution, CAL-MSC Washington DC 20560,***Dept. of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218 ABSTRACT Common processes used in glazed ceramic production often result in a glaze with a heterogeneous microstructure. Heterogeneities may be due to residual batch materials, intentionally added colorants and opacifiers and/or the products of devitrification and phase separation. To study the effect of heterogeneities in the corrosion process, dynamic imaging microellipsometry was used in-situ in aqueous solutions to measure spatially and temporally resolved changes in the surface of glasses (model glazes). The measurements showed increased durability near inclusions. Residual stress fields surrounding the heterogeneities influenced the results. Decoupling of chemical and mechanical factors causing this increased durability was not possible. INTRODUCTION In the past, attempts have been made to link various types of weathering of glass to differences in chemical composition, but it has been shown that there is no simple correlation'. Often a single object exhibits variations in the amount and types of corrosion which can not be explained solely by the effect of microenvironments. In some cases, localized corrosion is caused
by physical defects: cracks, surface accretions or pits; in others, it may follow the weak link in a phase separated structure, or on a larger scale, a structure of heterogeneities such as cord, striae, or crystals and their exsolution/dissolution rims. Heterogeneities in a glaze may be crystalline or noncrystalline areas of compositions differing from that of the bulk glass. Their sources can be residual batch materials, contaminants or purposefully added opacifiers/colorants in the raw materials, or precipitates caused by devitrification 2. Some heterogeneities result from nucleation and crystal growth during a slow cooling or undercooling step that promotes devitrification and phase separation. Heterogeneities also act as stress concentrators, aiding in the formation of microcracks, which disrupt surface layers. The presence of these microstructural features determines the path that corrosion will follow3 . Glasses increase in durability with increases in silica content in cases where phase separation or crystal formation do not change the composition of the glass phase 4 . Dissolution of a quartz particle into the matrix glass during firing forms a glass with a slightly higher silica content in the region of the quartz particle. This creates a local area of greater durability than that of the bulk glass. A glass with silica inclusions was chosen for initial corrosion studies because in addition to having the well understood effect on corrosion chemistry described above, residual (unmelted) quartz is often found in glazes after firing.
Glass corrosion often involves the developm
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