[permaculture] sodium silicate as outdoor paint on earth plaster
raincascadia at yahoo.com
Thu Apr 10 14:03:28 EDT 2008
I've painted the entire earth plaster exterior of my strawbale/cob house with sodium silicate, and for the most part, it seems to be working very well. I used about 2.5 gallons to cover the walls of our small house, which measures about 744 square feet, including the massive walls. Five gallons costs about $50, plus shipping, if necessary.
I did a 1 part to 9 parts water primer, and then a 1:3 coat, with another coat on areas that get rained on frequently. Sometimes, I applied the primer with some soil mixed in to fill in cracks and cover exposed straw. It definitely looks better this way, or with a clay paint applied previously. The paint creates a thin layer of water-permeable glass which binds and protects the exterior from abrasion and erosion from rain. It also seems to decrease water penetration some. One commercial sodium silicate paint brand I found online said the paint lasts up to 15 years. The more layers you apply, the darker the plaster gets, but overall the effect isn't too intense. Sometimes overlapping layers look darker.
The main drawback, so far, is that when applied and dried in full sun, it seems to crack. The hotter the plaster gets, the worse the cracking. The cracking varies from just visible lines to very noticeable cracks with sections raising up and flaking off. However, since it's all silica, it's easy to repair with more clay paint and sodium silicate.
I have only done a little research into the manufacture of sodium silicate, but it seems a huge ecological improvement over lime. The amount of material used is much less than with lime, even if the embedded energy per pound of material were the same. Furthermore, sodium silicate is made from ingredients that are more widely available. Lime involves intense mining and energy use, and probably more CO2 emissions. If anyone knows different, I would really like to know.
The problem is that there seems to be no way to protect earth plaster from the elements that doesn't require a lot of embedded energy, unless you live in a dry climate. Originally, I thought I would just use a combination of strategies (building protected porches, siting water tanks near the walls, planting evergreen bushes next to walls, wide overhangs, etc) to protect my walls, and then replaster when necessary. However, I live in a windy place that gets intense rainfall onto three sides of the house. It seems that even the best made and applied earth plasters (with lots of fine fiber or tons of shaggy-looking straw with clay) only last a few years and after putting three plaster coats on the strawbales and being totally overwhelmed with building, I never want to touch my walls again (which entropy will demand, anyway). I would rather tend to my food forests.
So, at this point, I am recommending the use of sodium silicate. Apparently, it can also be used as a wood preservative. Its alkaline pH protects against rot, and perhaps termites. I haven't done a lot of it, but it seems capable of making a nice, clear varnish.
Another silica-based alternative is called siloxane, but it seems to involve more energy use and it is much more expensive. Also, I don't know what happens if you need to repair or replaster a section, or what kind of soil it creates upon degradation.
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