Burning Rock

My father has regaled me since childhood with stories of his grandfather splitting rocks, big rocks, boulders, by heating them with fires and then quenching them with water. The thermal shock fractures the rock making it so that one can remove pieces easily. This is safer than dynamite and a bit more accessible.

A benefit that my father had failed to mention is that by doing this sort of fracturing one ends up with many cubic pieces that are great for building walls and cairns. The fractures run along crystal faults much of the time turning round rocks into pieces with nice flat faces.

Over the past five years I’ve been gradually cutting a new road through ledge, in part using this technique. It works amazingly well and has the side benefit of satisfying young pyromaniacs taste for fire. I asked Will, my older son, if I was creating a pyro or curing one with all those fires. His reply was “both!” Realize that for really big rocks and ledge we often burn the fire all day long and even over multiple days as we work the rock. After a while the thrill of the fire dims a bit and it becomes one more chore. I doubt Will will end up “like the mayor’s son.”

I remember Sam, he was the village idiot.
And though it seems a pity, it
Was so.
He loved to burn down houses just to watch the glow,
And nothing could be done,
Because he was the mayor’s son.

Tom Lehre, “My Home Town”

The burning rock bonfires serve other purposes too. It saves on doing dishes, makes for a fun outdoor meal, cleans up wood that is not suitable for household use, provides ash that can used to sweeten our very acidic soil and charcoal which the livestock like to chew on. Apparently eating charcoal helps with digestion, parasites and toxins. In some cultures they regularly eat charcoal to deal with environmental poisons. And, less I forget it and someone else point it out, burning wood produces CO2 which increases the greenhouse gases that cause global warming, something we appreciate here in Vermont. Since I don’t have a SUV or drive much I figure this will have to do for my personal contribution.

Of course, with any bonfire, be careful and don’t burn during dry seasons or let the fire get away from you. Keeping some five gallon pails of water on hand lets you soak down around the fire.

Lastly, if you do split rock with fire and water, beware of the steam which can burn you and that the rock can throw shrapnel. This is one of those – “do it at your own risk” type projects.

At this point I estimate that using this among other techniques I have removed approximately…

(50’x20’x6′ + 14×8’x2’x4′) = 6,896 cu-ft of granite ledge

6,896 cu-ft x 168 lbs/cu-ft = 1,158,528 lbs of granite ledge

Eek! That’s a lot! Over a five year period it comes to 231,705 lbs/year or 115 tons/year or 3.9 tons/week (warm weather work) so I guess that really isn’t so outlandish as it seems at first blush.

75°F/67°F Sunny

About Walter Jeffries

Tinker, Tailor...
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26 Responses to Burning Rock

  1. TalaMuir says:

    That’s fascinating, yet another new thing I’ve learned here!
    That is a fine cairn you have there too.

  2. pV says:

    Walter you blow me away!!!!! I cant amagine that much rock!!!! or moving it! When u say it is over time it makes sense i guess but it still is amasing.

  3. Anonymous says:

    Walter ~ do you spray the fire with a hoset o put the fire out? Would this work so that you could be a distance away from the rock when it splits? Thanks for explaining this process. Your site/blog is a wealth of information.


  4. pablo says:

    I’ve done this very thing before. Of course, it wasn’t intentional, and it resulted in some burn holes in my sweatshirt.

  5. Carol G says:

    I thought I’d stop in and see who the parents are of the two lovely boys who blog that I visited today looking for online friends for my daughter. You have an interesting blog and nice kids.

  6. Carol G says:

    One more thought…do the rocks explode sending rock parts spraying at you when you cool them?

  7. Theoretically the rock could spray shards when it fractures. I’ve not had this happen, yet. I stand back though to try to avoid being in the close line of fire for any potential missiles.

    Faster cooling of the rock seems like it would be better for fracturing it. Ways that I have cooled the rock are:

    1) using the winter – e.g., bonfire on rock in winter and then it simply cools with the cold of winter. Slow.

    2) using the rain – not very fast either.

    3) a garden hose – this lets me stay way back but doesn’t put the water on very fast. Impressive steam. Quick.

    4) Five-gallon pails of water – this is the fastest method. I can throw about 20-gallons of water onto the rock in half a minute or less. Lots of steam. Very fast. I can’t stand back as far as with this method – maybe 8 or 10 feet.

  8. Mama Kelly says:

    Hannibal used vinegar to remove boulders in his path across the Alps. He would heat the boulders, then pour cold vinegar on them, just like what you’re doing!!

    so much work I am so impressed

  9. Leslie says:

    Amazing! I knew that rocks placed into a fire could shatter if you poured water on them, but never considered a bonfire on rock ledge. This is something that I can use to solve a problem here at Palazzo Rospo. We have a place where water runs across our gravel drive and “no way” to dig a ditch… until now :)

  10. Anonymous says:

    Wow, great idea! I will try that. Another way is to drill holes in the rock during the day when it is warm, fill them with water, then insert a wood plug and wait for the night frost to crack the stone. Cheers!

  11. Good idea. I haven’t tried the wood plug. We have done the drill-the-hole-and-fill-with-water-to-freeze method. We do that by drilling the holes in the fall. By spring the rock is all cracked up. I have a new section I hope to do this fall as we gradually terrace our hillsides.

  12. dr dirt says:

    I love your blog… in the upcoming difficult times you guys will do fine… Ha…. maybe those times are here already. Maybe you could make some biochar with all that burning and you would have negative carbon guilt…. Negative guilt, what a concept… keep up the good work.

  13. We've definitely got a negative carbon footprint, deeply negative. Our forests and fields soaks up about 3,057,600 pounds of carbon a year and we generate very little since we travel little, use little fossil fuels and only heat our home with about 3/4 of a cord of wood a year. Our outdoor bonfires, which often double for rock breaking, are probably our biggest contribution. Maybe we should generate more as I would not want another ice age… Hmm…

  14. Louis says:

    Hello Walter, Nice blog. I would like to use this burning technic. With all the experience you have now witch one have the best result? Let say you have a huge rock deep and long, should you do the fire a lot of Time at the same place and move after you are enought deep?

    • First I would dig out the rock as much as I could. Next I would look for existing cracks and veins in the rock. If there were any that would let me reduce the size of the rock then they are where I would focus my energy. If winter were handy I would drill a line of holes in the rock along the grain, veins or potential cracks and then fill the holes with water to freeze. Without deep cold the fire would be my friend. It’s a long process.

      • Louis says:

        Thanks for your input. Let say you burn Wood on the rock, how long you wait until you spread of water ? And what kind of result should I expect when the rock crack how deep and size normaly I could remove rocks ? Just a proportion idea, Are we talking in inch or feet ? I Know it’s random result…

        And did you Try the technique of mixing your water with salt and ice for get a colder water ? If yes did you get better result?

        Thanks again Walter.

        • The greater the temperature differential, the greater the shock to the system and the more cracking. If it is a big rock though you probably aren’t going to succeed in heating the whole thing up so the effect comes more from expanding part of it while other parts stay cold and that causes cracking.

          As to inches vs feet, we cut out many feet deep and about 60′ long of rock this way. It took many bonfires, remove broken rock, bonfires again, etc.

          I don’t think the salt would make enough difference to be worth it and I don’t like salting my soil. Our winters are far colder than the brines I would likely make if I had done that. Salt in the soil isn’t good for the plants though and it’s very bad for rebar, cars, etc.

  15. Louis says:

    Hi thanks for your Quick reply your are right for the salt. I think the same.

    I Will give you news when I Will done.


  16. Farmerbob1 says:


    If you are still doing this, have you considered a trip to the Barrel Man to get a large metal barrel fitted with a harness that allows it to rotate vertically?

    Then suspend the barrel with a chain under a large sawhorse frame made of metal. The entire assembly would look like a child’s swing set, but with a barrel, not a swing. Fill it with water and then move the whole assembly to the fire location with the tractor.

    Then, from a long way away, hiding behind a granite skin or in a ditch, pull the rope and upend the barrel of water. 55 gallons of cold water all at once. Quite the thermal shock!

    • Excellent idea. I’ll remember that. I think it would be very effective.

      • Farmerbob1 says:

        You might already be doing this, but if not:

        If you have a hammer drill, driving a few vertical shafts into the rock around where you plan to start the fire might also help a great deal. When you do the thermal shocking, the temperature changes would then come both at the surface, and from deep within the rock.

        A couple other things:

        Chinese characters in the temperature line.

        The following sentence is incomplete:

        “Keeping some five gallon pails of water on hand lets you soak down around the fire. I have also used the “

        • Yes, we do something along this line. In the fall we drill out lines along the week direction of the ledge and fill them with water. During the winter the water freezes, expanding and cracking. Then the following year we remove the chunks or split them further to make them moveable.

          • Farmerbob1 says:

            Do you, by chance, have any comparable images to show what the land looked like before and after all of your rock-breaking?

            I’d love to see how much you’ve changed the rock layer with simple tools.

          • There are photos from before and I could take some recent photos too as we’ve continued to cut away at the ledge. I’ll put it on my to-do list.

  17. Don Henry says:

    I am planning to do the same thing as you. We have a granite ridge behind us, which hides our view of the St Lawrence. There is a natural “rock cut” in it about 30′ wide and we’ve excavated all the soil, leaving lumps of granite showing. They are bedrock. Do you have good luck building fires on the bedrock, then dumping water, or are you able to build a fire against the side of a rock face. I’m wondering if we can transfer enough heat downwards to bedrock. By the way, we live in eastern Ontario but spent our honeymoon in a cottage in South Hero Vt.

    Don Henry

    • Yes, heat and chill. I also find it very effective to drill holes in the rock in a line, tilt them so water will stay in the holes. Fill them with water and let them freeze over winter. This cracks the rocks. Even easier than the burning rock although much slower.

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