Plaster is an interior finish that aids in noise reduction and fireproofing and can be an integral aspect of a historic interior. A variety of plaster types have been used throughout history, characterized by composition and application. The most common constituents are gypsum, lime, or cement materials to form the matrix, blended with fine aggregates and fibers, depending on the application.
For flat and detailed surfaces like walls, ceilings and cornices, plaster is commonly mixed and formed onsite or pre-cast and installed. In these applications, the material is applied wet and finished when hardened as an interior finish primarily for walls and ceilings. Historically, plaster would serve as a surface finish or as a substrate for paint or wallpaper. Ornamental elements are typically cast into molds and applied to an architectural surface after curing.
Specialty ornamental plasters like Scagliola and Composition Ornament flourished between the 17th and 19th centuries and were often created to imitate stone and carved woodwork. These plasters used a variety of recipes and additives like animal glue and pigments to augment properties like moldability, strength, color, and aesthetics.
Prior to repairing, replacing, or restoring existing plaster, sources of deterioration should be addressed and mitigated. Moisture infiltration is one of the major and most common sources of deterioration for plaster. Therefore, maintaining the building structure around existing plaster is one of the primary ways to protect it from major deterioration and the need for replacement.
Refer to our restoration best practices page for additional information, details, and resources. Below, find plaster repair and restoration options for consideration on your project.
Flat or plain plastering is the application of 1 to 3 coats of plaster to create a smooth, neat finish. Ornamental plastering is the art of creating decorative profiles and sculptural shapes in relief like cornices, moldings, and medallions. Plaster is directly applied to rough masonry or to lath over furring or framework. Lime plaster, consisting of non-hydraulic lime, sand, and hair was commonly used until the 19th century. Gypsum plaster, consisting of gypsum, non-hydraulic lime, and sand became popular after. Lime plaster sets gradually and takes time to reach full hardness but can resist occasional moisture after fully cured. Gypsum plaster sets faster and harder but requires total protection from moisture. Portland cement plaster was also used, as well as specialty and proprietary types of plaster like Keene’s, Martin’s, and Parian cement. These cements were used for their increased strength, impact resistance, and fire-proofing properties.
Wholesale replacement of flat plaster is done when most of the plaster is severely damaged, or if the plaster support is significantly compromised. New plaster should be matched as closely as possible with the existing composition and system. Depending on the condition of the existing lath, new plaster can be installed on it or it can be replaced.
Ornamental plaster elements can be replicated in-kind or replaced with alternative materials. Replacing elements requires making molds that can be taken from existing ornamentation and used to recast new elements. For run cornices and moldings, a sheet metal template can be made from the existing to run new units for replacement. Newer technology like laser-scanning can also be used to replicate elements in modern alternative materials.
IMI’s free project support, technical assistance, and education is here to help you at any stage in your building’s lifecycle.
Our multidisciplinary team draws on decades of experience developing solutions for high-performing masonry and tile projects.
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