Frequently Asked Questions (FAQ's)

by Francisco Uviña, a.k.a. the Adobe Doctor

Snap to Previous Topics: Adobe vs. Cement

This Month's Topic: Lime Plaster

Are there any alternatives to mud plaster – compatible with adobe, but requiring less maintenance?

Yes. Lime plaster was prevalent in Mexico and New Mexico (mostly in the southern region of the state) before the introduction of Portland cement. It is very compatible with adobe because it is permeable and, if applied correctly, self-healing. Lime is more durable than mud, and though it requires maintenance, it is not as frequent.

What exactly is “lime” plaster?

Basically, it is limestone that has gone through a chemical cycle to break it down and make it workable. The cycle is somewhat complicated, but it boils down to this:

Lime plaster starts as limestone, which is quarried out of the ground and burnt in lime kilns at very high temperatures. This drives the carbon dioxide from the calcium carbonate. What’s left is calcium oxide, or “quicklime.” These lumps of lightweight calcium oxide are then mixed with water (“slaked”) to become a thick, white putty of calcium hydroxide, or “hydrated lime,” which is its most common form. This putty is then mixed with two to three parts sand to make a plaster.

This calcium hydroxide combines with the carbon dioxide in the air, replacing that which was driven off in the kiln, and becomes calcium carbonate again. It is the completion of this chemical process – from calcium carbonate to calcium oxide to calcium hydroxide and back to calcium carbonate – that makes it hard.

Is it safe to work with?

Only if proper safety measures are taken. Always wear goggles and gloves when working with lime, since it is caustic and can harm eyes and skin. It is especially dangerous during the slaking stage, since the calcium oxide is “desperate” to replace the water it has lost. It is best to leave the slaking to professionals, and buy already hydrated putty.

Specific methods for application and maintenance are illustrated in the Adobe Handbook (below), or you can direct your questions to me, Francisco, at adobedoctor@cstones.org. Until then, remember that a dry wall is a happy wall!

Past Topics:

Adobe vs. Cement

Portland cement has replaced traditional plasters as an interior and exterior finish for many earthen buildings, new and old. However, there are great differences between old construction methods and new, and what is right for one is not always right for the other.

Why is cement plaster bad for adobe buildings?

There are several disadvantages to applying a cement plaster to an earthen structure, based primarily on the fact that cement plaster is a rigid material and adobe is a soft one: Adobe and cement plasters expand and contract differently. Cement does not “give” like mud does and therefore cracks. Cement does not naturally adhere to an adobe surface and therefore it needs the aid of wire lath that is nailed to the surface of the wall. Nailing the lath to the walls may cause damage to the adobe fabric, especially when the old plaster is removed in order to apply a new one.

Experience has indicated that after about 30 years, adobe walls cased in concrete show damage that is frequently irreparable. Though cement became very popular during the 50’s and has been applied with all the best intentions, it is not a sustainable substitute for traditional materials

Is cement compatible with adobe?

No. Cement used as plaster on historic earthen structures may eventually cause irreversible damage. It is advisable to use compatible, permeable and traditional plasters even if they need cyclical maintenance. It is inaccurate to think that if an earthen wall is not protected by an impervious substrate, such as cement, that the building will disintegrate when exposed to weather.

How does cement cause damage?

Moisture Retention - When adobes have a permeable, “breathable” plaster such as mud or lime, walls are in a constant cycle of getting wet and drying out. As long as the traditional plaster is maintained, the building can last for hundreds (even thousands) of years. When a cement plaster, elastomeric coating or foam insulation covered with a cement plaster is applied to many of these historic buildings, problems may start to appear due to the moisture that accumulates in the walls.

There are two main ways that water invades a cement-covered adobe wall:

1.Capillary Action - Many old adobes were constructed with stone footings and, in some cases, no footings whatsoever. Walls that are built straight up from the ground and then covered by a cement coating are susceptible to capillary action. Like a drinking straw, moisture is “grabbed” and “pulled” up the wall.

2.Cracks - Moisture also enters through minute cracks in the cement. When this happens, the water tends to rise up, looking for a way out. Because the cement is not permeable, water is trapped in the walls.

Once saturated, a wall begins to erode. However, damage goes unnoticed because it hidden behind the rigid cement shell. It is not until the plaster is removed that large gaps and cavities are noticed on the wall surface. When the moisture exceeds 12%, the bottom portion of the wall loses stability and cohesiveness to support its upper loads. The lower part of the wall not being able to support its upper load will begin settling, bulging and slumping.

Freeze/Thaw Cycles - During cold weather walls may suffer from freeze/thaw cycles, which will eventually cause the cement plaster to delaminate and in some cases the adobes to crack.

Salt Accumulation - Another common, yet not so noticeable, problem is the accumulation of salts on a wall surface. These salts, which are present in cementitous products and also seep up along with water from the ground, crystallize on the surface of a wall. Salt deposits can cause the plaster and adobe surfaces to delaminate and fall apart, and the wire mesh used to adhere the plaster to rust.

The enormously popular Adobe Architecture Conservation Handbook, written in large part by Francisco, can be purchased at our online store.