Our long term volunteer, Livia, 18 from the bay area, USA, has been experimenting with natural paint recipes alongside our eco-construction coordinator, Jack to find the most successful non powdering white paint for the interiors of our traditional Andalucian buildings. 4 different binder types were tested on a gypsum plaster base: flour, casein, lime and linseed oil.
Paints were tested for their colouration, coverage and adhesiveness (non powdering or flaking) on a tester wall, and the best mixes will be tested on whole walls when refurbishing rooms.
The flour paste mixes were deemed the most successful. The paint adhered well and was a good body for fillers, especially clay, which had a lovely matt finish, colour and did not powder at all. A test is planned with marble dust as well when it is sourced, which should be the perfect filler. We used crushed chalk, which worked well but is time consuming and difficult to grind to a sufficiently fine powder, although this can also be bought in a shop. A linseed oil wash can be added over the flour paint to make it water resistant and cleanable.
Caesin was also a good adhesive, especially with Lime, which otherwise would not be compatible with Gypsum. The caesin was extracted from our locally sourced goats milk, which was a very simple process involving the curdling with lemon juice. It can also be bought, but is relatively expensive. Our paints had a yellow tint, which improved with higher proportions of white fillers, and which would not be such a problem if using pigments. Besides the off white colour, there are many vegans on the project, and it seemed a little unfair surrounding them with slightly yogurt smelling paints.
Linseed oil works extremely well as a binder with all fillers, although proportions would have to be honed in order to get rid of the yellow tint, overly reminiscent of smoke stained bar walls. Another unwanted effect of the oil is a loss of the breathability of the walls, so it should be used very sparingly.
Lime works as both a binder and a filler in one, and has been used both in interior and exterior for thousands of years, but unfortunately is chemically incompatible with gypsum, so needs an adhesive additive. Sunseed has been using a mixture with the jelly extracted from macerated prickly pear leaves instead of water for many years, which works well, although has a tendency to flake after 3 years or so. A little linseed oil would very probably extend its working life.
Experiments with natural pigments are also under way. The text in the photos are painted with linseed oil mixed with extracted turmeric pigment, charcoal, the Cochineal insect which lives off the prickly pear, and local purple clay.
A highly recommended book for recipes and ideas: http://www.greenbooks.co.uk/using-natural-finishes
Flour paint (makes roughly 1.5 litres)
1 cup flour
5.5 cups cold water
1.5 cup screened filler (clay, marble dust, chalk etc)
- Mix flour with 2 cups cold water, whisking to remove lumps.
- Bring 1.5 cups water to boil, then add the flour water from Step 1.
- Turn heat to low, stirring until thick paste develops. Remove from heat.
- Dilute the paste with 2 cups water, a little at a time.
- Add filler to diluted flour paste until desired consistency is achieved.