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Lime

[professional translation]
French text

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The natural process by which lime becomes calcite again is partly comparable to the massive degasification that enabled earth's atmosphere to become breathable through the fixing of colossal quantities of atmospheric carbonic gas to the soil (marine) at the beginning of our planet. The carbon contained in this gas, associated with the calcium of the first coral (**) was thus fixed in the ancient compacted reefs, which became huge limestone plateaux.

Still to this day, coral, or what is left of it, continues to recycle (more than forests) the carbonic gas of the air, carried by countless marine microorganisms swept along by the current. A wall painted with lime fulfils the same function - of course in infinitely less decisive proportions and unfortunately for the quality of our air and that of our climates.

Characteristics of "lime binding agents"

Lime is a very useful product that has been known since time immemorial (over 6000 years or more). It has the particularity of being able to be used as a protecting filler, as a binding agent for frescos, as a complement to plaster and as a major component of mortars. It allows humidity to pass through and for this reason, ages well, naturally, without trapping water within walls (see tadelakt).

Lime and auxiliary binders

Lime must often be mixed with another binding agent (particularly casein, but also acrylic, vinyl, etc. - even suet is mentioned, but we do not recommend this), otherwise it can reveal itself to be as pulverulent as a dry pastel. Certain authors suggest that it needs only a very small quantity of auxiliary binder, or none at all, except in difficult climates: the process of carbon fixation being generally hardening enough on its own, they say. One can however object that this process is so slow that the lime will have had ample time to blow away before giving its results!

Apparently we are not talking about the same lime. As this stage of the investigation, it seems that the more fat they are, the more fragile they are - this being prior to carbon fixation having produced its hardening effects, after which things could reverse (but this is still just an hypothesis).

Putting lime into a solution, application and drying

Water makes it possible to put lime into a solution in order to apply it to a support. The quality of this water is important. Water that is too acid can cause the undesirable formation of salts.

Drying conditions are likewise decisive: if the temperature of the workspace or worksite is too high, if there is sun or wind or a draught, the water evaporates before the lime starts to harden and so it crumbles into powder.

Pulverulence, tadelakt and smoothing

First of all, for all of these reasons it is advisable to conduct a few tests. Pass your finger over the dried surface and you will immediately know the state of the pulverulence (dustiness or powdery quality) of the lime that you have applied. However, one must take into consideration the fact that certain auxiliary binding agents take longer to harden after evaporation. It is therefore prudent to keep the test for a few days or even a few weeks (depending on the nature and proportion of the auxiliary binding agent) to form a reliable opinion.

In addition, pulverulence is not necessarily a handicap in so far as a practice called 'smoothing' is often practiced after drying.

In Europe, this practice is carried out with a metallic tool.

In Morocco, a smooth stone is used. The technique is called tadelakt (read the glossary article).

Lime and alum

We progressively hear of the use of alum in lime. It seems to act as a hardener and to increase the "permanence" of the paint, just as it does in the domain of dying.

Virtues in the plastic arts

Fat or even a little quiet, dry lime is transparent, compared to plaster for example.  This property of transparency is one of the keys that has made it as popular as "good taste" for thousands of years: it does not impose itself heavily, it modifies pictorial surfaces and that is all. Certain authors go as far, perhaps exaggerating a little, as to say that a good lime whitewash having aged well (having fixed atmospheric carbon well) ends up having the appearance of a glaze.

Lime presents a relative elasticity (compared to plaster) and especially, as we have said, an aptitude for allowing water to escape that puts it more or less beyond certain pictorial accidents like cracking and blistering. 

Quick or slaked lime, danger

All forms of lime are rather strongly alkaline.
For this reason, they are often associated with casein, at times right from their initial stages of preparation.

With respect to the chemistry of pure lime (as distinguished from calcite and other siliceous limes) there exist two principal categories:

* quicklime, calcium monoxide, CaO

* slaked lime, hydraulic or non-hydraulic (air-slaked). It is a hydroxide with a calcium base Ca(OH)₂.

Quicklime is transformed into slaked lime through a violent and dangerous process that produces boiling, foaming and heating (400°C). Quicklime, greedy for water and burning is an extremely dangerous product. The extinction process is long. Three months of soaking is really a minimum. The Ancients believed that three years were necessary. A badly slaked lime is dangerous, but its use is very specialized. It practically only concerns certain limes destined for liming.

As we have said, slaked lime maintains a rather high pH. That means that without necessarily being really caustic, it is not totally harmless. The use of gloves and protective gear is not frivolous for massive or repeated use. Avoid repeated contact with the skin and especially with the eyes. Certain authors compare its dangers to that of bleach in the sense that it is manageable without too many problems if handled with precaution.

Limes that whiten a lot in drying are often brought up. In fact, everything depends upon the purity of the lime. A quiet lime (see below) loaded with silica, will obviously whiten more than a fat lime. The presence of linseed oil can equally influence the appearance of lime (brown colouring, transparency).

 Fat or quiet, air-slaked or hydraulic lime

Depending on its original, more or less pure clay content (concentration of silica, an element that is sometimes added artificially during its initial firing), lime is:

* fat (without clay, and therefore with a proportional abundance of binding agents, which corresponds to the definition of fat).
It augments in contact with water, not combining with it nor retaining it in any manner; clay, which plays the role of a linking agent, is here absent. Fat lime only combines with air, whence the expression "air-slaked lime". It reduces a little in setting. It can in certain cases be used in painting, notably in very small quantities in an emulsion with oil paints, but especially and very commonly with casein and in whitewashes.

* or quiet (presence of clay or silica).
It is used diluted with water like a filler (limewash) for skins and in agriculture, but also of course in the field of construction (see cement). It is treated by water action on a particular variety of quicklime containing fired clay (therefore quiet). It reduces less in setting than fat lime. On the other hand, a white colouring is said to appear during drying.

Quiet lime is called hydraulic because it is capable of hardening even in the presence of water. It is nonetheless no more resistant to bad weather than fat lime. The first studies conducted on hydraulic lime only date from the 19^th century (Vicat and Johnson).

For the use in painting, it is often preferable to employ air-slaked lime.

Use in oil paint

The addition of oil in or on lime is well known in the world of decorative painting.  Nothing prevents the artist painter from doing the same thing, but at their risk and peril.

Lime - in significant amount - put in contact with oil becomes brownish. We have noted that this phenomenon increases greatly over time (see opposite a mixture of oil & lime that is completely botched: after eighteen months, the oil has browned considerably and has been concentrated into a kind of crust - the lime was too water imbibed - but happily, the overall aspect evokes that of a good cake). One must not forget that a gentle chemical reaction, saponification, takes place when an ester (oil) is combined with a base (lime). Another reason to use this powerful product in minimal doses.

There are two ways of combining lime with oil:

* by throwing the powder as is

* by mixing it with a little water to make a kind of emulsion.

The second solution is only viable if the proportion of water is feeble. The water is essential (without it the lime plays the role of a monstrous siccative), but in too great a quantity, its presence provokes cracks and crumbling (observed experimentally). It is an astonishing phenomenon when one considers that the marriage works perfectly when the oil is, on the contrary, in a very small quantity. It is therefore certitude that the dosage is decisive.

In the light of our experiments on the mixture of oil and lime, it seems that this should be considered more as an additive than an emulsifier and of little interest in artistic painting. It nevertheless has a few anecdotal attributes:  

* it homogenizes mixtures through its viscosity, rather like stand oil, to the point of destroying relief when it is added in quantities that are too strong;

* it slows the loss of crystalloid materials such as hydrate of alumina or marble powder. Why and how? We can suggest several hypotheses. First, lime is highly viscous. It has binding qualities. It therefore slows this movement by associating more rapidly than oil with the other elements that are present. In addition, it reacts not only through saponification with the oil but also by association with the carbon found in the paste (see also sulfur and other elements). In the course of these exchanges it is also possible that it gives off a little "assimilated" oxygen through the oil in the context of a normal siccativation reaction.
These are really only hypotheses, but the drying time did seem terribly short to us, even with small quantities and even with impasto techniques;

* the transparency of the lime put into the oil is quite remarkable. From this point of view, it behaves infinitely better than whiting, which also contains calcium. But look out for possible developments in the coming months.

Available in certain manufactured mediums, it should not be disregarded in a decisive and univocal manner, but its use necessitates great parsimony.

Whitewash, milk of lime and liming

Milk of lime, used in the technique of whitewash, is typically made up in the following manner (a Sennelier recipe that we are passing on as is, or practically, for reference):

* 1 volume of slaked lime in solution to 2 to 3 volumes of water

* 10 to 25% volume of pigments (10% for the pigments that cover the best like ferrous oxides)

* 10 to 15% volume of a binding agent like casein, acrylic or vinyl - the auxiliary binder plays a decisive role, as it is it that prevents the milk of lime from becoming too pulverulent in drying. Certain authors however, suggest much weaker quantities (a tablespoon for a bucket!) We advise you to make a few tests before.

* 5% liquid soap or washing up liquid.

"Milk of lime" can have a highly variable composition.

A thicker whitewash, intended for "liming", can be prepared with the same ingredients by using only one volume of water for a volume of lime.

ATTENTION: certain limes intended for liming are not entirely slaked. They must therefore be handled with protective gear for the hands and eyes or even for the whole body. They should not be applied when it is raining, when the sun is too hot or when it is windy. They are sometimes referred to as "natural white limes".

If liming is a rather heavy treatment, whitewash should be applied in thin, rather diluted coats, allowing full benefit to the transparency of the lime.

The surface should be porous, clean, and free of grease. They should be sprayed with water and often "wetted again" during the application.

Other uses of lime

For the moment, let us look at the use of lime as a flux in glass manufacturing (read the passage in Glass).

See also the page devoted to: casein, glossary article on whitewash.

See without fail: stucco, limestone, chalk, marl, Other linseed oils and other applications in Linseed oil.

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(*) This rather brutal operation, is carried out in an electric arc furnace, heated to 1700, 1800, or even 2000°C:

CaO + 3C + 108,300 calories   =   CaC₂ + CO

Where CaC₂ is calcium carbide. It has been used for a long time to produce an important substance - for the visual arts as well -, acetylene, which it gives off through contact with water:

CaC₂ + 2 H₂O   =   Ca(OH)₂ + C₂H₂ + heat (31 kcal)

Where Ca(OH)₂ is slaked lime and C₂H₂ is acetylene.

Even though acetylene is mainly produced by the petro-chemical industry nowadays, calcium carbide is still being produced. One of its uses involves... the ingenious acetylene lamps of speleologists. 

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(**) Here is a short quote from a text by Hervé Morin.

"[the] coral reefs [are] the fruit of patient work by little polyps that resemble miniscule jelly fish who build a limestone skeleton in which they make refuge.  They live in symbiosis with single-celled micro algae, the zooxanthellae. These life forms benefit from certain organic compounds produced by the polyps and supply, in exchange, a supplementary supply of oxygen through photosynthesis. The zooxanthellae likewise protect the calcification by breaking down certain acids produced by the polyps. But all that is needed is a stress, caused for example by a perceptible rise in water temperature for the zooxanthellae to be expelled, which initiates a process - at times irreversible - of a whitening of the coral."

The first coral reefs (and today's coral reefs) seem to have played a role as patient fixatives of atmospheric carbon, with the help of marine calcium and intermediary micro-organisms (notably plankton).  

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