Down to Earth Design

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Down to Earth Design

LIME PLASTER

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WHAT IS LIME? Lime has been used for thousands of years as a binder in mortars, plasters, and paints. During the post-World War II housing boom, quick-setting cement products eclipsed lime in construction. Lime does cure more slowly than cement, but it holds many advantages as a workable, self-healing, breathable, nearly carbon neutral material, making it nicely suited to natural building. (Cement production creates 1.25 pounds of CO2 for each pound of cement produced.) Additionally, lime plaster is breathable - meaning it allows air-borne moisture to travel freely through - so does not trap moisture inside a wall system. Without moisture buildup, biodegradable materials, such as wood or straw, are protected from decomposition.

HOW IS LIME MADE?

1. kiln burn. First limestone, seashells, or other material containing calcium carbonate is heated in a kiln, driving off carbon dioxide and leaving calcium oxide, also known as quicklime.

2. hydration. Quicklime reacts with water in a volatile process called “slaking”, resulting in calcium hydroxide,

or hydrated lime. Calcium hydroxide can look like a powder or a thick, sour cream consistency putty. If purchased as a powder, it must be rehydrated in water for many weeks to create high-quality putty for plaster.

3. carbonation. Calcium hydroxide reacts with carbon dioxide in the air to create calcium carbonate…exactly what it started as!

THE INGREDIENTS IN LIME PLASTER 1. lime putty: sticky binder that adheres to the

wall/substrate and holds the sand intact 2. sand: aggregate, provides strength and cracking

control; use fine, angular mason’s sand for fine plasters, and course or fine sand for base coats

I use a mix of 3 parts sand to 1 part lime putty for all plasters, though you can increase the lime putty ratio to up to 1:1 where very fine finish surfaces are desired.

SAFETY

Note that lime is highly alkaline and can severely burn your skin. Unlike acid burns, you generally do not feel an alkali burn as it occurs. So please use full protective gear whenever working with lime, including elbow-length rubber gloves, long sleeves, eye protection, etc. If your clothes get lime putty or lime water on them, change so the lime is not in contact with your skin through your clothing. I always keep a bucket of vinegar water nearby to neutralize my tools, gloves, and hands throughout the day.

MIXING & APPLICATION Lime plaster can be mixed by hand, but this is labor-intensive, so I recommend using a mortar mixer (not a cement mixer!) for at least 20 to 30 minutes. The longer you mix lime plaster, the more plastic and workable it becomes. Allowing the mix to sit overnight also improves workability, but remember to remix before using. The plaster consistency should be thick but spreadable, like cream cheese.

To prepare your walls, first shape them exactly how you would like them to look once plastered. It is time-consuming to build up the lime plaster to fill in large voids (since it must be applied in thin coats). Next, install expanded lath to cover any slick surfaces, such as wood. Make sure your lath bridges across the wood and at least 6" into the straw so you don't get a crack right where the lath ends. I do NOT recommend using lath over all of the strawbale, unless you live in a seismic region and your code requires this, as it will impede your plaster from bonding fully to the straw.

I recommend 3 coats for lime stucco over strawbale walls. The first coat can be up to 5/8" thick if it is applied to strawbale, otherwise each coat should be a maximum of 3/8" to 1/2" thick. Any thicker and the lime cannot absorb carbon dioxide adequately for curing to fully take place. Score each coat except the finish coat to allow for maximum surface area to key two coats together. Allow at least 4 to 5 days between

coats to give each ample time to cure.

I do NOT recommend lime plaster over clay plasters for exteriors in wet climates. The clay substrate shrinks and swells depending on moisture content, causing the lime, which cannot shrink and swell, to crack. Lime can be used over solid clay walls, such as cob or adobe, because there so much more clay is present to absorb ambient air moisture without measurable swelling.

Soak all surfaces with water before applying each coat of lime plaster and keep the wall substrate damp as you work. Do not rework plaster once it has stuck to the wall, as this pulls lime to the surface and leaves what is underneath sandy with less binder. You can work the surface to create a desired texture once the lime is green hard. And you can buff or polish the lime as it is curing to create a very fine surface texture.

You want the lime to cure NOT dry out. If it dries out before it has cured by reacting with carbon dioxide in the air, the resulting plaster will be weak and possibly crumbly. So protect the plaster from wind and sun until it has cured, and it helps to dampen the wall daily as it is curing. Do not apply exterior lime stucco if there is any risk of freezing, otherwise moisture in the plaster can freeze, expand, and cause critical failure of the plaster. The temperature needs to be above 40 F for at least a week to keep the curing process going.

RESOURCES WEBSITES www.mikewye.co.uk en.wikipedia.org/wiki/Lime_plaster howardhallfarm.com/limeplaster.html www.nps.gov/history/hps/tps/briefs/brief21.htm www.buildnaturally.com/EDucate/Articles/Lime.htm

BOOKS Building with Lime: A Practical Introduction by Stafford Holmes and Michael Wingate This is my lime bible! Using Natural Finishes: Lime- & Earth-Based Plasters, Renders & Paints by Adam Weismann and Katy Bryce

HANDS-ON WORKSHOPS

Down to Earth Design teaches workshops on

plastering with hydrated lime putty on strawbale walls.

WORKSHOPS INCLUDE:

• overview of hydrated lime, including safety, surface preparation, troweling skills, and why calcium hydroxide outperforms cement and hydraulic lime

• guided hands-on experience mixing and applying hydrated lime plaster

Check our website www.buildnaturally.com for additional information.