Lime wash versus lime paint

In general we talk about lime paint without making a distinction between “lime wash” and “lime paint”.
Lime wash is hydrated lime mixed with water in a ratio of 1:3.
Lime paint is a mixture of water, rock powder and hydrated lime. The rock powder, usually marble powder, is the filler that forms the framework for the connection to the lime binder. In this respect, the rock powder is as important as coarse aggregates in concrete for its strength. On the other hand, the addition of rock dust may create a slightly sandy texture and quickly leads to brush marks remaining visible.

It should come as no surprise that the soup that we paint on the wall with “only with water” diluted hydrated lime and we call “lime wash” has no resistance because the lime, well diluted, cannot form a stable structure without the stone powder to connect with. As well as ciment milk easily can be wiped away from the wall…
In outdoor areas, lime wash quickly absorbs moisture and swells. This volume formation leads to the separation of the individual layers, a typical appearance of continuously repeated lime wash coats.

The difference between the two variants also has consequences when it comes to painting.

Lime wash should be massaged into the surface as liquidly as possible and rapid drying must be avoided under all circumstances. Air lime can only carbonate (solidify) with the help of H2O. If it dries too quickly, it has no strength.
Lime paint with marble powder, on the other hand, is deliberately processed thicker, often with the addition of cellulose in order to keep the moisture in the (thicker) coating from drying out for longer. KREIDEZEIT has a proven product for this purpose.

Pure lime paint or lime wash cannot be painted on surfaces that are not 100% mineral, at least outdoors. This excludes use on one-component silicate paints. On the other hand, almost any paint can be used to paint on top of a firmly stuck lime coat, possibly with a fixing as an intermediate coat.

You often hear about the idea of mixing the lime with oil or other additives. Naturally, the addition of oil or fat to lime has a water-repellent effect by forming “lime-soap” (saponification) and reduces the swelling of the hydrated lime and as a result of the detachment of individual layers, a typical picture of old, multi-layered lime washes. But at the same time, it can be assumed that the ability to breathe is reduced by adding fat. It is also discussed whether the strength does not suffer because the fat binds calcium, which is no longer available for carbonation = solidification of the lime.

An additional consideration adding fat is that oil or fat is often added at high temperatures when the burnt lime is mixed the first time with water and the lime slaking is starting with an explosive chemical reaction in order to enable the two materials to be mixed better. When mixed cold, fats and lime only combine at extremely high mixing frequencies without losing their (small-format) individual particle size. Now, lime putty (slaked hydrated lime) should rest for at least three months after started slaking before it is used for painting. Why? Because the explosive reaction of the burnt lime after entering in contact with water does not end after a few days but only begins to calm down after weeks – barely noticeable to our eyes. For church painting, for example, only lime that has matured for more than three years is used. In Japan and Germany’s lime craft, lime putty is inherited. The problem with the oil is that lime is very alkaline and mixed with oil and stored three months attacks and decomposes the oil.

I am not aware of any scientific studies on the influence of the addition of fat on the strength and durability of a limewash or on the breathing and adhesion behavior under everyday conditions of repeated lime wash coatings. The idea with the addition of fat (including pomace/trester) has its logic, but is counterproductive to the natural properties of the “magical” hydrated lime. If it actually had the desired effect, it should also be taken into account that any lime renovation painting needed later would have problems with adhesion.

Now we hear and read that the addition of fats to the lime (when hot) has been practiced since ancient times. That doesn’t mean that it is technically reasonable. So much of what is held up today and revered as a sentimental, sacred building tradition was born out of necessity and lack and does not remotely correspond to today’s state of the art.

A different approach prevails in the paint industry. In order to meet the fashion of lime wash for trendy interior painting and today’s quality expectations in terms of abrasion and durability, additional additives are added. A popular additive in ancient times was casein, for example the paintings in the Sistine Chapel were made with lime casein and that is the main reason why they delight us with their long durability. Unfortunately, casein cannot be delivered to customers pre-mixed in a bucket because in its liquid state it is quickly eaten by the aggressively alkaline lime. Here we are back to the unwelcome story of hydraulic and synthetic additives in pre-mixed paints, as already described in the blog “eco-paints“: emulsifiers, additional binding agents, preservatives, anti-foaming agents and so on. Lime paint with nano technology simply uses a different form of adding additives, the exact origin of which often remains obscure.

Conclusion: if you want to use lime for painting, you either have to have experience and prepare the lime paint or lime wash yourself on site at short notice with well-matured lime, best with casein (available lime quality would be worth another blog), or have to read the ingredients of ready-mixed products in the technical data sheet carefully, and know that according to the law, 0% VOC is not 0% VOC but rather a tolerance is permitted – which is not controlled by anyone neutral, just as many air limes actually have hydraulic properties. The decision is once again a question of where individual priorities are set. My doubts are based on my many years of application experience and experiencing the complexity of the factors that influence the result of working with air lime.