Stain Reduction Techniques for Ceramics
Seminar taught by Lauren Fair, Associate Objects Conservator and Affiliated Assistant Professor at the Winterthur/University of Delaware Program in Art Conservation, on stain reduction techniques developed by herself, Bruno Pouliot, and Richard Wolbers.
The seminar covered reducing staining on ceramics in three primary steps: chelating, reductive bleaching, and rinsing.
The first step in stain reduction utilizes a chelator, a chemical compound which complexes with divalent metal ions that bind the stain to the surface. This step necessitates careful control of the pH so three of its groups are ionized. This means that the aqueous solution needs to include a buffer to hold the pH within the correct pH range.
Sodium citrate is the most common chelator used as it complexes with calcium ions, the primary inorganic component of common stains (e.g. solid and hard water). An added benefit of using citrate is that it can be cleared by rinsing with water. EDTA is another common chelator that is used, most often to capture copper ions and Fe2+ (not Fe3+), that may be left behind from dirt from archaeological or outdoor settings or from metallic components (e.g. metal rivets).
Care needs to be taken in this initial step if there is gilding, metallic luster, enamel colors, etc. present or if the ceramic is not stable (e.g. poorly fired).
The second step often utilized an oxidative bleach, such as carbamide peroxide. This decolorizes stains and makes them more polar so that they can be more easily rinsed away with an aqueous solution. Certain bleaches, like carbamide peroxide, also function as a first rinse. Rinsing is the final, critical stage of stain reduction. Bleaches should be which should be delivered in a poultice to minimize depth of penetration. Ideally, the conservator will use as low of concentrations as possible, and rinse multiple times with water.
Poulticing is often used in stain reduction as it is the most effective means removing stains not at the surface of the ceramic. Poultices are effective only if used appropriately and chosen with the following in mind:
- What is the degree of absorbency/porosity?
- How does it hold the reagent?
- How well does it stay in contact with ceramic throughout drying?
- What is the ease of removability?
The seminar included a half day of testing various buffered chelating solutions, oxidative bleaches, and poulticing materials and included a discussion on variations for archaeological vessels, objects with gilding or paint, and different types of stains (e.g. local and concentrated versus overall and diffused).
- Location of the stain
- How it is bound to the ceramic
- pH of surface
- pH of solution
- Nature of the stain (e.g. localized or concentrated, associated with a crack, crazing or both)
- Poultice type
- Type of rinse solution
Stain reduction was carried out on Bust of a Woman (c. 1525) from the Cleveland Museum of Art.