The beauty of Ocher is fully realized with the use of lime. Lime has a long history of use, serving both functional and decorative purposes. A limewash is a thin coating that serves as a protective layer but also lends itself wonderfully to artistic expression. When applied, the lime within the wash begins to react with carbon dioxide in the atmosphere in a process called carbonation, gradually returning it to its original state—limestone.
As it reverts, the crystals within the lime expand and adhere to the surface below. In addition, these crystals offer an incredible light
refraction that enhances the beauty of Ochers. The process of carbonation is critical for limewash to succeed. In this article we present the basic principles
of preparing and applying limewash, introducing the foundational rules for achieving success.
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For the purposes of this article, lime powder refers to Hydrated Lime Type S. This is lime that has been industrially slaked and is ready for immediate use. All ratios in this instruction have been carefully calculated by weight using this lime powder to ensure proper proportions.
Not all lime is created equal. Be sure to purchase from a quality building supply store. The lime should not be more than 2 years of age and should be a striking white. Do not accept lime that is gray in color.
Lime washes have traditionally been made using lime putty, lime that has been slaked or soaked and comes in a paste form suspended under water to prevent carbonation. Before use in washes it must be sufficiently thinned to a suitable fluidity. Thus the water content when using lime putty can vary considerably. When following this article's weight ratio guidelines for pigments and additives, it is up to the end user to make the proper conversions to determine ratios when tinting washes made from putty.
For lime to properly adhere to a surface the two must be:
When it is chemically compatible, lime will not adversely react with the surface, causing the two to separate, (the lime flakes or falls off). When mechanically compatible there is sufficient porosity and texture — referred to as "key" — to allow the lime crystals to expand, "lock" onto and penetrate the surface.
A limewash has more rigid requirements when it comes to suitability with a surface because it is a thin coating. As with lime plasters,
the supporting surface must be able to withstand a pH of 12 or more.
Surfaces suitable for limewash without a primer:
The surface must be fully cured. Very smooth surfaces that don't have enough tooth (roughness) for the lime to bond with may require texturing with medium abrasives or a wire brush. Dust and loose debris should be removed by brushing or dusting, and then washing.
Next, the surface must be prewet (humidified) before each layer of wash is applied. Prewetting can easily be accomplished with a fine mist of water using such tools as a plastic spray bottle, pump sprayer or a garden hose with a suitable mist nozzle.
Prewetting the surface prevents it from absorbing water from the limewash itself, which could impair the critical
carbonation process. A light dampness is desired — too much water on the surface will
interfere with cling and coverage. Depending on the surface, humidity and temperature, it may be necessary to wait
several hours (up to over-night) after you prewet before you begin the limewash application.
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Each surface must be assessed for both chemical and mechanical suitability, and must be fully cured. The unnatural denseness of some modern coatings such as plasters, paints and cements can prove particularly difficult for traditional limewashes to adhere to because they can have both chemical and mechanical incompatibilities. Readymade primers specifically made for use with traditional limewash contain modern additives that alleviate some of these obstacles.
Conditions for application are the same as for lime renderings. Proper temperature and humidity are required for the carbonation process to be successful.
The limewash effect (visual appearance) is determined by the water to lime volume ratio, and can be divided into four distinct types. Each type corresponds to a unique recipe:
Water to Lime Ratios by Volume:
Pigments have traditionally been added to lime coatings according to local availability. Today, artists and decorators have a huge palette to choose from. Ochers and earths produce warm shades and are unaffected by UV radiation. Oxides produce cool shades and are often used in small quantities with Ochers to expand the range of colors. Many of these enhanced colors are already available in our Mineral Pigment line, (which are 90% Ocher based).
For limewash, pigments are added after the water and lime are thoroughly mixed. Slake (wet) the pigment in an equal amount of water to form a paste. This paste can be left to soak overnight if desired. Add the pigment paste to the limewash and stir — an electric paint paddle is recommended for complete dispersion. How much pigment you add depends upon individual pigment density, intensity of color desired, and chosen recipe.
Here when referring to a maximum percentage of pigment, we mean the saturation limit. Beyond this percentage, the pigment no
longer adds intensity of color but instead begins to thicken the mixture, increasing the load that is to be fixed by the
lime. Therefore, too high a percentage of pigment will result in mechanical failure of the limewash. The chart below lists
limits for Earths, Ochers and Oxides. Note that as the volume of water in your limewash increases so can the weight of
pigment before saturation occurs. Use this as a guide — these are upper limits; you can use less than indicated.
Maximum percentage for Earths/Ochers/Mineral Pigment in relation to lime powder by weight (For Natural Yellow, Natural black and Natural red use instead the maximum percentages shown for Oxides.)
Maximum percentage for Oxides in relation to lime powder by weight
Additives have traditionally been used with limewash to enhance its adhesion to the support surface. They generally fall within three categories:
Linseed Oil is inexpensive and natural but it will alter the matt (flat) quality of a limewash, (it will also yellow over time).
Casein is inexpensive, natural and will not alter the matt quality of the coating. Prepare by soaking in hot water at a ratio of 2.5 pounds casein per gallon of water. Allow mixture to stand overnight.
Methyl Cellulose is a plant-based glue. It is inexpensive, and has the additional benefit of enhancing water retention. This slowing of the drying process is especially important in dry, hot or windy conditions where drying too quickly in the first two or three days can interfere with the important carbonation process. Do not use in wet conditions or on overly wet supports. Preparation: 20 grams per gallon of water. Allow mixture to stand overnight. It is the weakest of the binder additives, and really serves more to extend the "open time" of the limewash so that it does not dry too quickly in dry or hot conditions.
PVA, (polyvinyl acetate) is strong and inexpensive. We recommend our Acrylic Binder - a superior product that, unlike other PVA products, dries crystal clear. Dilute the proper amount with some of the water used from the chosen recipe then add to the remaining limewash mixture.
Additives are added after the initial mixing of the lime and water at a ratio of 1 to 10% of the volume of prepared limewash, before adding pigment. These additives can enhance binding strength and thus help to overcome less than perfect surface and application conditions. They can also assist in stabilizing the pigments within the limewash mixture. Since these are positive effects, their use is fairly common. However, these additives will also affect the porosity of the final lime coating, and a decision on whether to use them or not should take this fact into account.
One important rule: always add the same ratio of additive binder for each successive coat. Also, never exceed this 10%
volume ratio as your limewash will no longer be a lime coating, but will primarily become a distemper paint.
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It is best to test your mixture to be sure ratios are correct and that, when dry, the desired color and effect has been achieved. Dry color will be lighter than when wet. Limewashes also will dry to be more opaque than when wet, so do not make the mistake of applying too much in one coating.
Several thin coats are the key to limewash success — never apply too thick. For exteriors, 5 coats is recommended. For interiors, 3 coats should suffice. Lime is hard on brushes so choose a longhair masonry brush, (or sponges for patina applications). For mixing, the use of an electric paint-mixing paddle is recommended, along with rubber buckets, rubber gloves and eye protection. Mix only the amount of limewash to be used in one day.
Secco or "dry application" for cured surfaces: prewet the porous surface and allow it to sit several hours or overnight. When applying, do not spread the wash out as you might do with paint — apply with an even stroke and pressure to work the wash into the surface. Stir the wash between strokes. Water can be added to dilute the thick layer of wash that will be found at the bottom of the bucket as the wash is used. Apply your strokes vertically, then horizontally, finishing with a vertical stroke. When applying successive layers, allow the limewash to dry for 1 to 3 days between coats. Mist the surface with water and use slightly higher ratios of water with each successive coat — this ensures good adhesion and carbonation.
Fresco or "wet application" for partially cured, damp lime plasters and renders: in the Fresco technique, pure lime water with pigment is painted onto fresh, set, wet plaster using similar application techniques as with Secco. This technique requires that the surface be hardened enough so as not to deteriorate as it is being brushed, but not be excessively dry as to inhibit proper adhesion of pigments with the lime in the surface. Technique and climatic conditions play a major factor in application time and success. For a detailed discussion of the Fresco Technique please see the article Fresco & Patina Application for Lime Plasters.
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A thicker limewash is often used as a maintenance coat on interior or exterior masonry. Since it is thicker and more opaque than a wash, brush strokes will be apparent. Be cautious about using this recipe — although at first it may seem to cover better and be successful without all the work of several thinner coats, liming coats can easily be applied too thick which will result in failure due to poor application technique.
A wash has less texture than a liming coat, but still masks the surface. To obtain the best result, stir between every stroke.
Lime Tempera is like a watercolor in its color intensity, utilizing both round and flat brushes. It is applied in both dry and fresco techniques.
Patina coats give great transparency that will enhance the texture of a surface using either brush or a sponge. Used with both the dry and fresco techniques, it also acts to even out areas.
The subject of lime coatings is more extensive than is presented in this short article but we hope this information will
help you understand the basics. Application techniques, variations in lime types and the many visual effects possible make
limewash a fascinating topic for further research and experimentation.
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