About Lime Plasters

Lime plasters have a very long and proven history going back thousands of years.The qualities that made lime the literal cornerstone of many civilizations — its durability, strength, and permeability — make it well suited for straw bale walls. There is much to be relearned about working with lime plasters that has been forgotten in the past century as lime was replaced by the simpler-to-use portland cement.

There are two basic kinds of lime used in plasters: hydrated lime (the most common) and hydraulic lime, both available bagged similarly to cement and masonry cement. Hydrated lime powder is mixed with water and sand and does not start to cure until the wet mixture is exposed to the air, when it starts to cure by recombining with CO2 in the atmosphere.

Hydraulic lime powder begins to cure as soon as water is added, in a chemical reaction with the H2O, similar to portland cement. This is an important distinction to understand when researching lime plasters, as the two are handled quite differently because of their different curing styles.

The reason lime plasters are so suitable to straw bale walls is their excellent permeability. A lime plaster, though hard to the touch and able to withstand the elements, is porous enough to offer a rapid rate of transpiration for humidity in the bale walls.

Durability

Properly applied, lime plaster is very long-lasting, since it returns to being limestone. One only need to see some of the buildings made by the Romans to understand the durability of lime plasters. Lime plasters are suitable for exterior applications in any climate.

Cost and Availability

Type S hydrated lime is quite widely available and costs about the same as bagged cement. Hydraulic lime is less common, but there are distributors throughout North America. Both kinds are starting to make a slow comeback because of their importance to the restoration of historical buildings. Do not use agricultural lime; make sure you are buying a lime that is created for plastering or mortaring purposes. Even among plastering limes, there are noticeable differences in quality, so ask people with experience or try some samples of several brands before buying a large quantity. Expect to encounter raised eyebrows and skepticism when you let it be known you are trying to mix a lime plaster! Don't let a cement guy talk you out of using lime.

Mixing and Application

Hydrated lime should first be made into a putty by soaking it in water for at least 24 hours. Some brands of Type S can work if mixed right out of the bag and into the plaster, but the longer it hydrates, the more workable it becomes and the better and slower it cures. Hydrated lime plaster hardens first by drying (which can happen too quickly if not adequately hydrated), then has a long period of chemical reaction absorbing large amounts of CO2 to return to its natural state. It is slower to set into a hard surface than cement plaster, with a first coat taking two to ten days to harden to the touch. It is traditionally applied in successive thin coats, allowing each to have maximum exposure to the air. Modern additives — including portland cement and gypsum — have been used to speed up the curing process.

Hydraulic lime is not presoaked, since that would initiate its water-based curing process, causing it to harden. It is mixed directly into the plaster and used immediately, as with portland cement. Its curing process is longer than portland cement's, but shorter than hydrated lime's.

The highly pliable nature of both kinds of lime plaster makes them easy to work with; they stick well to the bales (even without the use of any reinforcing mesh) and are pleasurable to work with, offering longer working times than portland-based mixes.

Working with lime requires the same caution as cement. It is highly caustic to the skin, and correct breathing equipment must be used when you are mixing dry lime.

Code Compliance

Lime plasters are fairly rare in modern construction. This kind of plaster is unlikely to be covered by building codes, but familiarity with its historical precedent usually allows for its use.

Embodied Energy

The production of lime requires considerable energy to burn the limestone (although the temperatures are lower than required for making portland cement). The reason lime is seen as more environmentally sound than its portland cement cousin is that the CO2 that is driven off the limestone is almost entirely reabsorbed by the lime plaster as it cures, resulting in no net gain of CO2 in the atmosphere. Local small-scale lime production, now rare, is relatively sustainable (many homestead farms had a lime kiln in the back fifty).

Quality of Finish

Lime plasters have a softer feel than cement and are naturally a brighter white. Pigments will work well with lime. Any kind of finish — from a highly polished, glassy smoothness to a rough stipple — is possible to achieve with lime. It can also be mixed into a paint-like wash and applied in its natural white or with added pigments, making an easy color coat.

Other Factors

Lime is commercially packaged for many applications other than plaster making. Be sure to find a Type S lime that is meant specifically for plastering or masonry purposes — agricultural lime does not make good plaster. Hydrated lime putty can be mixed and stored indefinitely, since it will not cure as long as it's protected from exposure to air. This means you can always have some on hand for repairing cracks or creating additions. There are relatively few modern resources available to those who wish to use lime plasters, so expect to take some time finding the materials and experimenting with application.

Lime plasters, because of their high alkalinity, tend to discourage the growth of mold and mildew and can also repel many breeds of insects.

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