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.
For larger cracks, voids, and damaged areas of flat plaster, more complex repairs are needed. Old lath may need resecuring or to be replaced if it’s been compromised structurally or water damaged. Installing new lath over the existing may be required to aid in keying of the new patch material. Small batches of plaster are mixed and installed in the prepared patch area. If the damaged plaster penetrates multiple layers, the patching material is applied in the same number of layers that are present and finished to match the adjacent finished surface. For run ornamental elements, plaster can be installed in the patch area and a sheet metal template can be used to replicate the profile.
Care should be given to matching the plaster patching material to the aesthetic, texture, and material composition of each layer being repaired. Inappropriate repair materials may have a negative impact on the historic character and aesthetic qualities.
Plaster’s brittle nature means that cracks aren’t uncommon. Though cracks aren’t always a sign of significant deterioration, cracks can be an indication of structural movement or damage. Before a crack is repaired, it’s best to fully understand and address the root cause. Fine cracks can be filled with a compatible plaster patching compound or limewash.
Plaster can be carefully dusted or vacuumed to prevent soil buildup over time. Uncoated plaster is more porous and moisture sensitive than coated plaster. Avoid water cleaning and chemical cleaning treatments that can damage and even dissolve an uncoated plaster surface, leaving it at risk of accelerated deterioration and lower resiliency.
With repair and restoration projects it’s important to understand the properties of the existing plaster, composition, layers, and condition prior to selecting new repair materials. The following field and laboratory testing may be needed for interior plaster evaluation.
FIELD TESTING:
- Tensile and Bond Strength
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LABORATORY TESTING:
