One of the oldest painting techniques, despite its demanding processing, is still one of the most important today, especially in the field of monument preservation, because of its many positive properties.
History of lime paint
Whitewash, calcimine, kalsomine or calsomine
Lime-based paints have been found since the Stone Age; even in Mesopotamia, their cooling, moisture-regulating properties were appreciated. Variations on the use of lime-based paints, such as lime casein and fresco, have existed since ancient times. Long after, lime paint remained almost the only commonly available paint and still characterizes the urban landscape in many historic places, especially in the Mediterranean region.
Components and production
The lime cycle: limestone - quicklime - lime putty
To produce lime paint, naturally occurring limestone is first burned at approx. 1000° C - this removes impurities and changes the chemical composition to quicklime or unslaked lime, a highly reactive alkaline compound. This is then ground and mixed with water (slaked), an exothermic chemical reaction in which water molecules bind to the lime and so-called hydrated lime or slaked lime is formed. In the past, this was done in earth pits, where the lime was soaked for weeks and months for optimum quality and purity, which is why this processing stage is also called sump lime, or fat lime because of its loamy, bile-like consistency. If it is strongly diluted with water, it becomes a very liquid color, the milk of lime. It is applied to the surface in several thin layers with a tassel. In a humid environment, it reacts with carbon dioxide from the air to form limestone, which forms an abrasion-resistant layer on the wall.
Properties and opacity
The paint is transparent when wet, but dries opaque. Various factors - such as irregularities in the substrate or direct sunlight - can affect the drying speed of the paint and thus also the final color shade, since the crystals arrange themselves differently when drying slowly than when drying quickly. This can result in the desired lively, irregular color appearance, but can also have undesirable consequences, such as the scaffolding on a facade showing up as a "shadow" on the finished painted wall. For this reason, exterior work is traditionally carried out under cloudy skies. Since not all lime is converted into limestone if it dries too quickly, abrasion resistance also suffers - but the paint remains weakly alkaline and thus antibacterial and fungicidal. Irregularities can be evened out by applying additional coats.
Pigments, binders, solvents
Lime paint is very special in that the lime acts as both a binder and a pigment; water serves as a solvent. If pigments are added, they should be alkali-resistant - for example, stable pigments such as ocher and iron oxides. Because of the weak binding power of the paint, they must not exceed a percentage of 5%.
Application and techniques
What is lime casein paint?
It is not uncommon for lime paint to contain an addition of casein - coagulated milk protein with strong adhesive power - as an additional binder. This increases the abrasion resistance, viscosity and thus the possible layer thickness of the paint and improves several other of its properties. Lime casein is one of the oldest binders in mural painting. Casein is traditionally extracted from skimmed milk or curd and digested in an alkaline solution; in the case of lime casein paint, this is the as yet unset marsh lime.
A much higher volume percentage of pigments and fillers can be added to the paint; moreover, it does not require the long, constant dampening in the drying process and is waterproof to a certain degree. Due to its strong adhesive power, it also adheres to very smooth substrates. However, care should be taken when using too high a concentration of casein: the extremely strong tensions created during drying can cause weak or porous substrates to flake. Another disadvantage of the paint is its low durability: when open, the casein provides an ideal breeding ground for bacteria and fungi and must therefore be used up in a few days.
Pure lime paint adheres to a variety of weakly absorbent grounds, for example, clay, wood and various plasters. Highly absorbent grounds should be pretreated with deep primer; for very smooth, non-absorbent grounds, such as metal, lime casein paint is more suitable.
Restoration and preservation of monuments
Because of its porosity, lime paint carries moisture away to the outside; components that have not completely set have a weak alkaline effect and are therefore antibacterial and fungicidal. Therefore, the paint is ideal for rooms with high humidity or with strong fluctuations in the degree of humidity and helps to keep moisture-sensitive substrates (wood, clay or similar) dry and prevent mold growth. In addition, many listed buildings require the use of the original paint technique anyway.
The positive properties already mentioned - the moisture-regulating, disinfecting and cooling effect as well as the lively appearance - are offset by the fact that the paint is very demanding and error-prone in processing, so that it is not always resistant to abrasion and water. Especially outdoors, it is quickly washed off facades by rain. In areas with a high concentration of sulfur in the air (for example, with high traffic volumes), the calcium reacts with the sulfur to form gypsum; this is water-soluble and washes off even faster.