Black pigment in my lime plaster?

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?

Other random thoughts and ideas are very welcome!

Hi Chris -

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.

Thanks for the quick and experienced based reply, Phil!

Just so I have a data point to to start working from; do you mind sharing the ratios that you were working with?

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.

Here's an article from Ithaka about biochar in interior earth or lime plaster:

https://www.biochar-journal.org/en/ct/3

Thanks again, Phil. And thanks for the tip that the char' will act as a true pozzolan, affecting the lime set.
I'll give that link a close read.

I'll see if I can figure out how my camera talks to my computer and do some before and after shots for your inspection.

-Chris

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).

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!

I dont know if you have access to them, but if you live near an area with Huisache Trees, or know anyone, their seed pods leave giant dark black stains in my cob. The tree resembles a thin Mesquite tree or an Acacia tree since all three are related. I surmise that you could use Huisache beans to make your outer layer as black as you like. Maybe soak done in a lime plaster slurry and you might get black plaster.

I have a question that might be dumb, or might solve your issues.

You know when you take an object and pass it through a candle flame, and that object gets covered in soot, which is hydrophobic and also very black? Well what would happen if you took a weed torch to the outside of the cob beds? You could ostensibly coat the outside of the cob with something designed to produce more soot, which would then stick to the cob, and likely be very black, indeed. Plus, it would be hydrophobic while still remaining breathable.

-CK

I like some version of Chris's idea. I might try a very sap laden wood. Pine would be my idea, but if you have weeds laying around that work then that would be the most sustainable.

Turns out the hot lime really is a controlled substance here. I'm too late this year to get anything on the cob as we're already going into freezing temps at night, so I'll try to get a plasterer on side over the winter whom I can acquire the lime through; and hopefully not too much erosion happens before next June.

Johnny, I'm not near a climate where Huisache grows, but that sounds like a great natural dye. I love it when an observation of a 'problem' results in a useful bit of knowledge to help us work with a natural phenomenon. I'll talk to my local ethnobotanist ( https://beverleygray.com/workshops/ ) and see if she has any suggestions. I wonder if the pine and spruce trees here have enough resin to work for Chris' idea below.

Chris, interesting question. It's the semi burned paraffin wax that causes the soot in 'lamp black', beeswax candles don't do this (to the same extent). The black would work well to absorb the sunlight, but I wonder about the durability in wind snow and rain. As an aside... back in the paddle-wheeler days here on Lake Lebarge, the shipping companies used to mix lamp black with crankcase oil and paint a 70 km line down the ice covering the lake... it opened up a navigable channel, up to a month earlier, to move goods down the river.

Thanks for the ideas!

Better late than never... here's some pics of the raised beds in question.

Chris, when you do get to the lime coat next year I have a couple tips....forgive me if these are obvious.  

1) The finer the grind of the biochar, the better blackening you will get.
2) Be safe....don't breath in any of the biochar dust.  I would grind it wet if possible.  If not I'd wear a mask and do it outside.  Here are a few pictures I took of moist softwood biochar after 20 freeze thaw cycles.  I would think a grinder or ball mill would do a great job.  Maybe a powerful blender.  Heck, pulverizing with a boot even isn't too bad if a grinder isn't an option.  Hardwood biochar is much tougher to crush, but softwood biochar is so easy.  

Anyhow, I just wanted to share the jagged nature of the fragments as a warning against getting any in your lungs.  Always better safe!

Amazing pictures, Greg. Thanks for that!
I will definitely try to break the char down to the smallest particle size that I can. And I'll wear a mask!
I'm pretty low tech around here so I was thinking of pounding it down by hand, or boot, as the case may be... however...
Now you've got me thinking of building a bike powered ball mill. Bunch of old steel nuts inside a plastic canoe barrel or some such...
I'm glad I have the long Winter to plan.

Tell me about your tent or house please

Hi John. It's a canvas wall tent made by this company: Fort McPherson Tent and Canvas

It it about 12x16, up off the ground on a 14x20 deck. It's got a queen sized bed, couch, bookshelves and an air tight wood stove (aka hippy killer).

We don't live in it full time, but we have wwoofers stay with us (right now they are helping split 6 cords of firewood) and they have found it comfortable down to about -10.