Critical Concrete

Wood Protection: Flour Paint


How paint affects the environment

In general all paints are prepared according to a simple recipe featuring 5 basic ingredients:
Paint is composed of liquid, solvents, resins, pigments and various additives. Pigments give the paint colour; solvents make it easier to apply; resins help it dry; and additives serve as everything from fillers to anti fungicidal agents.

General Compounds of Paint

There are many different types of paint, yet we can broadly distinguish paints by their composition. It’s crucial to pay attention to whether they have natural ingredients or artificial chemical compounds that may make paint harmful for us and for the environment. So to start we should pay attention to the components of the paint we are using and be aware of the harmful substances it may contain.

In the past, paints containing lead as an additive were extremely common and used in many houses. Approximately 87% of homes built before 1940 in the US contain lead-based paint and many properties and rental units still contain traces of it. Lead-based paint is most dangerous when it is deteriorating—peeling, chipping, chalking, cracking, etc. [1]

Nowadays we know that lead is toxic, especially for children, as they can absorb up to 50% of lead from food, dust or contaminated water, while adults only absorb 5-10%. It interferes with numerous enzymes inside human organ cells. It can be stored in our nails, bones or teeth lasting up to 30 years. Moreover lead in the environment can result in decreased growth and reproduction in plants and animals, and neurological defects in vertebrates. [2]

Solvents are various low viscosity, volatile liquids including petroleum mineral spirits and aromatic solvents such as benzol, alcohol, esters, ketones, and acetone. Organic solvents react in the atmosphere in sunlight, producing an air pollutant known as ground-level ozone. High concentrations of ground-level ozone seriously affect human, animal and plant health. It is important for health and safety to store organic solvents safely.

The most popular of synthetic resins are alkyds, acrylics, epoxies, and polyurethanes, while coconut, soybean oil, and the one we will be using – linseed oil, are among some natural resins examples. [3]

The natural paint we have been looking up and testing is flour paint, and in this article we want to share with you our findings!

Made with a base of flour (who would have guessed) water, soap, linseed oil, natural pigments and iron sulphate (a common agriculture additive).This ecological paint can be easily made at home and is a cheap and safe alternative to prevailing industrial paints, without emitting any Volatile Organic Compounds (VOCs) [4]

If you want to learn more about this traditional wood protection and painting technique and our experience with it or want to be able to make it yourself, keep on reading

About Flour Paint

Flour paint might be known by plenty of names, such as “Swedish paint” or “ocher paint”, but the former owes to its base ingredient. This recipe has its origins in Sweden and it’s traditionally used in Swedish Cottages as lime wash in European homes. [5]
This recipe became more and more popular throughout the Western World, and nowadays is a well liked exterior paint for wood protection. Its popularity is not a matter of faith, but a combination of good performance, a characteristic finish, cost effectiveness and respect for the environment. The recipe is simple to make (even from home) using natural ingredients and with an easy and safe application. Other great features include being inexpensive and eco-friendly while being incredibly durable. It has been successfully tested throughout history in harsh and wet Scandinavian climates. This formula protects wood sustainably and naturally for up to 10 or even 15 years for darker colours, as well as providing an eye-catching colour palette with a matte finish with visible wood veneers.
Wood is a breathing and alive material, this formula allows it to breathe and to release moisture quickly making it perfect to keep wooden constructions in good shape in an ecological way.

A little bit of History

Flour Paint is an evolution of the traditional recipe of the so-called Swedish paint. You might recall the characteristic reddish-brown colour that covers its countryside. This natural paint recipe has its first notices in the Swedish city of Falun, where the largest copper mine in Europe at that time was settled.

On its origins the pigment used in this wood painting method was a by-product of the copper industry. The mineralization of the mine’s tailings and slag added by smelters began to produce a distinguishing red-coloured pigment known as “Falu-red” which was basically the ground of these mine areas.[6]

At first this slag was treated as rubbish but its composition, rich in iron, copper, zinc and lead, was found to be a good conservator for wood due to the antifungal and bactericidal properties of these elements.
A formulation of this pigment heated together with wheat or rye flour, water and linseed oil ended up being an excellent anti-weathering and permeable paint. Its accessibility, unique effectiveness and cheap production soon made Swedish paint a staple, first for cottages and barns in the countryside and later in other wooden buildings in the ongrowing cities.

Sweden consists of almost 70% forest grounds, and 83% of it is coniferous forest so gathering wood as a construction material is very accessible and that’s why there is still a significant amount of wooden buildings in Sweden. This paint and wood protection was also an affordable way to paint houses and its reddish colour was a great alternative to get wood to mimic brick walls which were a symbol of richness, hence the expensive price of the material. [7]

“Swedish standing volume by tree species in percent” Riksskogstaxeringen, SLU, 2017.


This traditional special ingredient based on iron oxide gives strong resistance to the harsh climate, yet makes the paint more toxic for humans. The challenge was finding a pigment for this paint recipe which was safer while conserving the characteristic colour palette of Swedish paint.
And, thanks again to nature, we found an alternative: natural earth pigments. By changing the original pigment, we move into what we now call “flour paint” as a more generic term. The pigment that is usually being used instead of copper is ochre.
Ochre is rich in iron compounds, which is also a good wood protector and changes colour when heated providing an amiable palette of pigments. Ochre has a yellow colour which after rising to 200°C becomes orange/red and with more heat comes to darker tones. It can also be made using other types of pigments if we look to have a wider colour choice but keep in mind natural colour pigments offer much better resistance to UV and time.

“Flour Paint with Ocher Pigment Colour Palette” blog

Application and Performance

How and where to use it

Flour Paint is particularly suited to paint wooden vertical exterior surfaces. It can be widely applied on old and new timbers but for a proper result the surface must be dry and clean from any previous paint or varnish because residues could disturb bonding of paint with wood, to avoid that the surfaces must be wiped out or brushed off from dirt and dust.
If we are painting old wood and any mildew is present it is recommended to apply black soap directly to the surface and wipe it off as a deeper cleanser.

A good quality of Swedish style paint is that even if it fades away with time it does not peel or crack. This means that it won’t need to be scraped off before repainting, saving an extra step and extra residues. For repainting over flour paint you only have to brush off the loose pigment and dust and then reapply, making its maintenance over time very convenient.

Softwoods are the perfect match for Flour Paint, as the majority of Swedish forests are coniferous, paint was made to work on these types of trees rather than in deciduous or harder tropical woods, which may start darkening and some brown stains could show through the paint.
Flour paint is not recommended on horizontal surfaces because it is not very resistant to physical disturbance such as sitting or walking. It is commonly used for exterior but it can also be used in wooden interiors, in this there’s no need to use additives against humidity as iron sulphate. [8]

Our experience: Prototype Diary

This natural paint is a perfect alternative to those who are looking for low-tech alternatives, like to cook, and get hands-on work. As this pretty much defines the character of Critical Concrete, we have been making and testing our own flour paint and now we want to tell you how it turned out!

What are we testing?

Our first try was in the wooden structure we built for the social intervention in Apulia. Where, as a first approximation, we decided to cover parts of vertical surfaces with flour paint and observe what happens. We learned that if we want to use it properly we will have to test how it will work in various circumstances, as it went differently from our expectations.
In order to check the quality of the flour paint we tested on wooden boards how the paint reacts in different ways of preparation, with wood protection coating or undercoating and changing weather conditions to understand how it will perform in Portuguese climate, exposing the paint to sun and rain.
Additionally we wanted to experiment how the paint behaves together with tricoil oiling, the coating we usually use for extra protection. After preparing five samples, we applied paint either with and without the previous tricoil oiling as an undercoat and waited different periods of time between oiling and painting.

Procuring the correct recipe and preparation method for this experiment was key. After conducting research we chose the following recipe [9]:

Proportions for 10l of paint.

Sample Testing and Conclusions.

“Flour Paint Sample Testing Diary”

Samples 1,2,3,5 look good after months of rain and external exposure. With all tested samples and as the sample 1 looked equally protected without any pre-coating and flour paint is meant to be used bare wood we can assume and confirm that oil is not necessary before painting, moreover oil can give difficulties to get passable results if we don’t wait a certain amount of time so the wood absorbs the oil completely before painting.
Since we didn’t experience any variations between the samples concerning the different amount of time waited until being exposed to external factors we can assume that once the paint is well dried, after a couple hours in dry conditions and moderate temperature, the wood is ready to be exposed to the various climatic conditions.

Recipe Testing and Recommendations

After this experiment we took hand of the flour paint again for our solar water heater window frames. We made the paint two times, so we were able to compare and improve the recipe and technique for the future.

This is what we did and what we learned:

We adjusted the original proportions of the recipe to make 2 litres:

Tools we used
Cooking process


The first time making the paint we had to reduce the mixture with 0,5l of water to get the right consistency and some paint stuck to the bottom, we deduced that this was due to too much heat so too much water evaporated. Second time we did it we completely turned off the heat while mixing instead of reducing the intensity to stop the boiling as we did during the first attempt. By doing this we almost get the right consistency, having to add just a bit more water to make it perfectly ready to be used.
Another annotation is that the paint had some lumps, from mixing the flour with the water. This didn’t seem to be too much of a problem while applying the paint, but is something that could be improved by using a mechanical mixer instead of mixing it by hand in order to obtain a more uniform body.

While sourcing for the information needed to make our own flour paint we gathered and took notes of other considerations to take into account while applying the paint.

General Conclusions.

Research is explaining how paint transformed and improved along with time. Respecting our environment and people sharing the planet with us, flour paint is a choice which brings many benefits, is perfect for exterior walls, as it is kept wood influenced by different weather in great condition.
Our sample testing proved that flour paint fulfils its role in portuguese weather conditions which are full of rain in autumn/winter season and is giving plenty of sun in spring/summer. This encouraged us to try it in the solar water heater we just built in order to protect our brand new window frames and give a nice and blended finish to our house façade.

If you want to know more about our solar water heater façade project make sure to stay tuned …
















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