Plaster Systems & Preservation Adventures in Lime Plaster Systems & Preservation

plaster, plä´stėr, n. [L. emplastrum] A pasty composition , as of lime, sand, water, and often hair, used for covering walls and ceilings, where it hardens in drying; v.t. To apply a plaster to, to cover with a plaster, or to daub or fill with plaster. (3) i. An expletive commonly used by owners of old houses

A Short History of Plaster

Application over stone or brick France, 675 C.E. America, c. 1700 England, 1550 C.E. Application over stone or brick Half-timbering using wattle interior and exterior Framing and timbering using chinking

Wood lath systems—riven & cut Metal lath & screen systems Drywall systems

The Demise of Plaster Systems

Q & A

Types of Plaster—Lime Plaster and Stucco materials are often interchangeable—the difference is where they are applied Lime plaster begins with raw limestone—which is first burnt, then slaked, and finally prepared for application Whitewash is slaked lime and chalk—Limewash is pure slaked lime in water

Making Lime Putty Lime must be slaked and aged for at least 3 months Up to 1 year is common Finer grades of finish putty may be stored for 3-10 years Longer storage makes plaster stronger and more durable

Mixing Plaster Scratch and brown coat plasters are generally 1 part lime putty, 2-3 parts aggregate, and ample binder Float or finish coat plaster is usually a 1:1 or 1:2 part mix of putty and fine sand. Chalk, oils, or egg white may be added for additional finish appearance

Types of Plaster—Cement & Gypsum Portland cement began appearing in quantity (USA) in the late 1880s Although related to limestone, Portland contains calcium silicates, aluminum and iron compounds, and at least 5% magnesium oxide. It sets faster and harder than lime putty’s Portland cement was often mixed with lime putty or gypsum and began to replace conventional scratch and brown coat applications with a single layer finished out in lime putty or plaster of Paris Although common since the 18th Century, it did not make inroads to America until the mid 19th Century PoP is made by heating gypsum to form calcium sulfate hemihydrate—and when water is added it converts back to gypsum By the 1870s, PoP and other gypsum based plasters were quickly replacing lime plasters Gypsum based plasters are the foundation for decorative elements on historic woodwork and other architectural features

Q & A

How it Works—Lath & Plaster

Applying the scratch & brown coats

Finishing the float coat

Working with a screed Pressing in the keys Common plastering tools

How it Works—Plaster & Brick

Decorative Plaster Usually composed of plaster of Paris, or a PoP and gypsum mix.

Q & A

The Permanent Plaster Solution

Major Failure Problems Foundation settling Water and Humidity Mechanical shock/vibration Thermal problems Material defects Gremlins Old Age

Balloon Framing

Water & Moisture Damage What Goes Wrong and Why Water & Moisture Damage

Identifying the Problem Bad roofing materials—shingles, tiles, etcetera Flashing stock around chimneys and stacks Plumbing leaks Spills and cleaning hazards Rising damp from basement/crawl space Cracked or loose siding Soffit or window casement issues HVAC system condensation Attic ventilator blow-through Drainage and wicking

Solve the Problem ALWAYS FIX THE CAUSE BEFORE MAKING REPAIRS Allow the area to thoroughly dry before repairing Correct any humidity problems under the house Improve drainage characteristics of house perimeter Block off interior wall drafts that create “cold spots” Control ventilation flow into attic spaces Have your HVAC system assessed for leakage and efficiency

Repairing Water Issues Determine if area is soundly attached Scale off any loose material or efflorescence Anchor plaster to lath Remove plaster if necessary Use multiple coat filling—or block fill with drywall Apply finish coat and sand finish Apply primer or Kilz Apply final finish

Q & A

Cracked & Detached Plaster

Step One—Fill Those Cracks

Simple Repair Open…Clean…Dampen…Mud…Sand Mud…Tape…Mud…Sand

More Aggressive Strategies

Q & A

Decorative Elements

Conservator Corner

Never use harsh or untested chemicals to clean plaster Control heat and humidity from extremes Only tackle projects that you are comfortable with Delicate surfaces and finishes require a conservator Never paint lime putty plaster with oil based paint Correct problems with settling gradually Replace only when necessary

Resources Repairing Historic Flat Plaster—Preservation Brief 21 http://www.nps.gov/history/hps/tps/briefs/brief21.htm Walls and Molding: How to Care for Old and Historic Wood and Plaster. Natalie Shivers. 1990. National Trust for Historic Preservation

http://www.timescapeconsulting.com P.O. Box 14604 Knoxville, TN 37914 Office: (865) 223-6421 Cell: (865) 223-9772 patrick@knoxbeat.com