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Lime;
Remembering an Old Friend
By Thomas Hirsch
Once
again, when examining our sustainability choices for buildings, it is
instructive to review the architectural legacy left by our ancestors.
What works, endures and poses little or no environmental hazards?
Last month we visited the topic of natural plasters for the
interior and exterior of buildings. One
ingredient often used in these plasters, or renders,
is lime.
Lime is produced by bringing calcium carbonate (CaCO
3
) in the form of limestone, shells or coral to a high temperature.
The heat drives off carbon dioxide (CO ) and produces calcium oxide
(CaO 2),
or lime, also known as quicklime or lump-lime.
The next process required to turn lime into useful building
products like mortars, plasters or paints is called slaking.
To slake here simply means adding water to the lime and make lime
putty or calcium hydroxide (Ca(OH)
2). Left to age, the
longer the better, lime putty
is then mixed with various components like sand, fiber (straw,
saw dust, hair), color pigments, oils and other additives to create
plaster, mortar, or paint. Once
applied to a building, the lime “product” begins to slowly absorb
carbon dioxide from the air (carbonizing) and transform itself back into
calcium carbonate or limestone, thereby completing the cycle (see chart).
The resulting finish is hard, weather resistant, inhibits mold and
mildew growth and repels insects. Leftover
lime products are easily recycled into future uses or put back into the
earth without toxic residue. Another
noteworthy quality about lime is its ability to “ breathe.” Lime’s
vapor permeability is what contributes to the longevity of wonderful historic buildings around the world.
Historic preservationists have learned the hard way that
substituting products with lower vapor permeability,
like cement based products for instance, has led to failures in the
restoration efforts or the building altogether.
Lime products are compatible with all earthen and clay- based
building methods and highly desirable in a healthy home environment.
The chemical
reaction that occurs when lime absorbs carbon dioxide from the atmosphere
produces crystals of calcite or calcium carbonate.
“These crystals are unusual in that they have a dual refractive
index; light entering each crystal is reflected back in duplicate.
This results in the wonderful surface glow that is characteristic
of lime washed surfaces, and
is not found in other decorative finishes.
When lime wash was discovered, man was not looking for a decorative
finish to impress his neighbors. He
had actually discovered a sacrificial treatment that protected his
dwelling against the worst weather. Early
mud structures and wattle and daub (sticks and earth) panels were very
vulnerable to the climate and lime wash still plays an important part in
protecting these surfaces.” (1.)
The use of lime as
a primary building material began to wane in 1824 with the patenting of
Portland Cement. The use of
Portland Cement gained
popularity because it is fast setting, hard and “ foolproof—in that
any fool can use it, whereas the use of lime requires an understanding of
the many processes involved, particularly in the slow carbonation back to
limestone in order to use it successfully.”
(2.) The decline in
lime usage meant a decline in the knowledge of how to use it. Typically, knowledge about lime was passed down from
generation to generation and a wealth of experience was preserved.
Thankfully, a renewed
interest and appreciation for lime is restoring its rightful place in our
building vernacular. Portland
Cement still has its place, but we have to come to grips with its high
total embodied energy content. It
has been said that the manufacture of Portland Cement alone is responsible
for approximately 8% of annual greenhouse gas production!
Disposal of “used” concrete can be problematic.
The low vapor permeability can also be an issue when used
incorrectly or with incompatible systems.
The use of
lime in not without its challenges, however.
While relatively nontoxic, there are some serious safety issues
that need to be observed. The first involves the slaking process in that the addition
of lime to water should be done carefully as a tremendous amount of heat
can produce steam and spit lime. The
second safety issue is the fact that lime is so alkaline that it can burn
the skin. I have had some
lime burns and they are extremely irritating.
Protective clothing, gloves and goggles should be worn when working
with lime. Portland cement
has a lime composition as well and continued exposure to wet concrete can
also cause skin irritation.
Proper curing
is imperative for lime’s successful use.
Freshly applied material should be kept moist by misting for a
period of days depending on weather conditions.
These surfaces should also be shielded from direct sunlight and
high winds during the initial cure time.
Success is also measured by the quality of the materials used, the
recipes employed and the quality and structural integrity of the surface
where the lime material is being applied.
Recipes and
advice abound in the natural building and historical preservationist
literature. I recommend
working with someone experienced with lime before using it.
At the very least do some solid research and experiment on a small
scale before embarking on a major project.
The investment in time is well worth the effort.
I expect the renewed interest in lime will produce a new generation
of skilled and happy craftspeople. My
appreciation for lime grows daily particularly as I gaze at the lime
plastered and lime washed walls in our home.
I chuckle to think that our home is turning to limestone! Happy liming!
References:
(1.)
The Use of Lime wash as a Decorative and Protective Coating; Bennett,
Bob; The Building Conservation Directory, U.K. 1997
(2.)
Working with Lime (from The
Art of Natural Building); Jones,
Barbara; New Society Publishers, 2002
(3.)
Building with Lime: A Practical Introduction;
Holmes, Stafford & Wingate, Michael. Intermediate
Technology Publications, 1997
(4.)
The Last Straw: The International Journal of Straw Bale and Natural
Building. 505-895-5400 or www.strawhomes.com.
Issue #29, spring 2000 was dedicated entirely to lime plaster.
(5.)
The Straw Bale House.
Steen , Athena, Steen, Bill, Bainbridge,
David and Eisenberg,David. Chelsea
Green Publishing Company, 1994
Harmony
Home Construction, LLC.
8122 Barney Road
Traverse City,
Michigan 49684
Ph: 1-877-45-GREEN Fx: 231-932-9193
info@harmonyhomeconstruction.com
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