Last year I and a rotating team of wwoofers constructed three raised garden beds from cob.
We dug deep rubble trenches and dry-stacked stone for a good dry foundation- good boots.
However, the cob is, as of yet, unprotected from the elements (rain, snow) and is starting to slowly erode- poor hat. I'd like to give them some protection against erosion by covering them in a lime plaster.
This causes a design problem due to my geography; I live in the Yukon Territory of Canada 62* latitude (not too far from the Arctic Circle)
The ground here stays cold year round. In fact it gets cold the further down you dig.
One key point of building raised beds with cob was to isolate the soil and create thermal mass surrounding the beds- heated by the long sunny summer days.
Problem: Lime plaster is a bright reflective white. Horrible for solar gain.
Are there any chemists/experimenters here that can help me figure out the best way to darken my plaster without destroying it's hardness and water resistance properties?
Since I am already producing bio-char, my current thought is to crush charcoal and add it to the mix.
Can anybody help me reason-out why adding carbon into this lime reaction is a good or bad idea?
Try it and I think you'll find it suitable. I've made some test mixes of lime plaster with biochar as a pozzolan. They worked really well, and the hydraulic quality was similar to mixtures using wood ash. Pieces of cured plaster about one week old went into a bucket of water and were still hard three months later.
The colours I ended up with were shades of grey...I'd recommend doing your own tests to see what ratio works for you.
No worries, Chris! I didn't write them down, but they were probably 5-10% biochar by volume to a stock lime/sand plaster. Overall mix would have been about 70-80% sand, 20-30% lime putty, and the remainder biochar (maybe displacing some of the sand as the biochar went up...I do this as much by feel as anything).
Keep in mind that as the biochar ratio goes up, the lime will set much quicker. Especially if you're using a high ash char with a high pH. The advantage will be a hard and highly waterproof plaster, but I don't know how much freezing will affect it. Will be interested to see your results either way.
I can see nothing wrong with it, but my experience has been the same as Phil's; in adding black colorant, I always end up with a gray and not black.
But for your application, you do not need black in deep, you just need it as a surface colorant to attract the sun. You could do this with paint, but the slight added cost might be worth it compared for all the extra time for something that might not work so well. (lots of mixing and making for gray instead of black).
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I have NO experience with this, but your post got me interested in finding an option for dye (which I think you already have found the best option in creating and using your own bio-char).
Looks like you could probably mix in the char to the plaster when you make it, like you were saying. But then, sounds like it will be grey - which to me is better than gleaming white, anyhow
After, I might try to directly apply the char dust or char-slurry to the surface to blacken it up, and then I saw where sealing with wax is a thing for plaster - that might just be the ticket for "locking in" that dark color and giving a nice protective layer??
I dunno, might be a terrible idea but sounds like it could be fun to experiment with if you have the time and can afford a few buckets of beeswax, good luck!
Travis, gray might be the best I can do, but that's OK. The clay I used in the cob is a light blue-grey and dried even lighter... So though the walls aren't currently gleaming white, they are a light grey. These less than black beds are currently outperforming my old non-raised garden beds so, aside from the erosion issue, the experiment has been a complete success! Mind you, I shouldn't count my potatoes until harvest time.
I don't know much about paint, but, the paint would have to breathe. As the cob is already more than normally 'wet' due to being in direct contact with watered soil (and half filled with hugelculture) I foresee some substantial freeze/thaw spalling if I put a non-breathable layer on the exterior.
That segues perfectly into...
Ty, thank you for an idea! Though I can't afford to buy beeswax (and again the breathability question is in play) I can get linseed oil. I wonder if I could suspend powdered carbon into a hot linseed oil and paint it on? This would give some weather protection as well as darken the surface of the beds.
I think I'll try the lime plaster first as it's proven weather resistant... but If I can't source the lime or I can't get it darkened down sufficiently, at least I'll have a contingency plan!
Thank you all for your ideas and suggestions. Please Keep'em coming!
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