1. Plaster Systems & Preservation
Adventures in Lime
Plaster Systems & Preservation

2. 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

3. A Short History of Plaster

4. 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

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

7. The Demise of Plaster Systems

8. Q & A

9. 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

10. 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

11. 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

12. 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

13. Q & A

14. How it Works—Lath & Plaster

15. Applying the scratch & brown coats

16. Finishing the float coat

17. Working with a screed
Pressing in the keys
Common plastering tools

18. How it Works—Plaster & Brick

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

20. Q & A

21. The Permanent Plaster Solution

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

23. Balloon Framing

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

25. 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

26. 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

27. 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

28. Q & A

29. Cracked & Detached Plaster

30. Step One—Fill Those Cracks

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

34. More Aggressive Strategies

35. Q & A

36. Decorative Elements

37. Conservator Corner

38. 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

39. Resources
Repairing Historic Flat Plaster—Preservation Brief 21
Walls and Molding: How to Care for Old and Historic Wood and Plaster. Natalie Shivers National Trust for Historic Preservation

40.
P.O. Box 14604
Knoxville, TN
Office: (865)
Cell: (865)