I'm building an RMH with a steel bell (an old furnace oil tank) which is 5 feet tall, 4 feet long and 2 feet wide. I'm thinking of cobbing it about 6 inches thick on all sides and the top except where the barrel will protrude through the top. See pic of the tank below.
Questions: Can anyone suggest a good recipe for cob? I have sand, can get straw, yellow clay (I think), horse manure. I am looking for a tried and true method as I don't have a lot of time to experiment!
Should I fasten some sort of steel plastering mesh to the tank to help with adhesion?
Will appreciate all the help I can get from experienced cobbers! Thanks in advance!
Cob at 6" thick with straw reinforcement should have enough strength that you don't need to bind it to the tank. Even if the inner layer of cob gets hot enough to char the straw, the outer half should not get hot enough to weaken it. I would suggest to lay up the cob in short enough lifts that it doesn't settle away from the tank as a tall quickly laid cob mass would do as it dries.
Hi Glenn, Thanks for your info above. I was just thinking of messaging you today as I remembered your posts about cobbing your own brick bell!
A few questions: You mention an inner layer and an outer layer. Do you suggest cobbing one complete layer of half the thickness (3 ") and then commence again with a second layer?
Then you mention doing it in "short enough lifts" so it doesn't settle away from the tank. How high would you suggest I go at each stage before allowing it to dry a bit?
You also mentioned that straw reinforcement would likely make it unnecessary to use another means to bind it to the tank. Here I want to ask you about straw and proportions of clay, sand and straw....
I know that proportions will likely vary depending on materials one has available, particularly the kind of clay. But what would rough proportions be? I've read that one might use as much as 3 parts sand to one part clay. Is that "in the ballpark" for a good mix?
Secondly, what proportion of straw is wise and how short should it be chopped? When is it best to add straw to the mix? Do you mix it into your clay/water slurry before the sand, or after the sand is mixed in?
I remember a post last year where someone mentioned that donkey manure with its high content of grass fibres was an excellent substitute for straw which resulted in a cob with no cracks whatsoever. Would horse manure be a good substitute for straw?
Finally, Glenn, is a 6" thickness a good estimate for this project?
It might be good to have clay-sand cob for the inner layer, with no straw to char, but even if you did that, I would tend to want to lay it all up simultaneously, to avoid vertical cold joints. If you do lay it up in separate layers, I would make the inner layer surface very rough, like deep finger jabs or smears, to allow the outer layer to grab securely.
I have found that lifts of about a foot work okay with my materials; your mileage may vary. Test.
Three parts sand to one clay sounds like a standard mix. My particular native clay is a glacial deposit, and comes with lots of sand and gravel mixed in, making an excellent cob all by itself. I guesstimate it has less than 1/4 pure clay after removing the rocks larger than golf ball size. There is a good amount of silt, fine sand, and small to large gravel.
Since my clay already has all its sand, my practice is not relevant, but I would add the straw last. I think you would want less than 6" pieces; 2 to 4" would probably be good, for ease of handling and mixing. The longer the straw, the more it tends to get tangled with the mixing tools and the harder it is to handle. I have found when building kilns that the dried grass I use can allow a wet mix when first applied soon after mixing, and as the grass absorbs water, the clay stiffens up faster than it would without the grass. I'm not positive that this behavior translates to straw.
My understanding is that horse manure works well, but I know as much first-hand as you do.
6" thickness of cob gives a reasonable mass and time delay in heat release. You might be able to use more, but it might start to give too much delay in heating. The colder your climate and the more consistent your heating load, the thicker you want to make your cob. I think 6" gives a structural result while 4" might need some supplemental reinforcing at the dimensions of your bell.
I was able to get a load of clay from a nearby pit that mines it and they say the clay content is about 50%. There seems to be no stones or gravel at all in it. I've just done a test with a jar half-filled with the clay and the other half with water. Shook it well and am letting it settle out to get a better idea of what is in it.
Glenn, you said yours has lots of gravel/pebbles in it and that it is excellent for cob. Would I be wise to mix some gravel into mine?
I don't think you mentioned how much straw you use. What sort of volume of straw would you mix into your clay/sand mix. For example if you have say a 5 gallon pail of clay/sand mixed with water, how much straw would you mix in ( a ball you could gather between your two hands?) less? or more?
Also, you mentioned building in lifts about a foot high each.... how long would you let each lift dry or set before adding the next?
Hope to get started on this as soon as possible after I have some idea of my clay content from the test.
Concrete has crushed stone in it partly to improve the strength, and perhaps partly to stretch the cement (if half the bulk is gravel, there is much less cement needed). I can't say definitively that gravel increases the strength of cob, but it doesn't weaken it, and definitely reduces the amount of clay you have to process.
For proportions, I tend to use a pile of straw that is 1/4 to 1/2 as big as the pile of clay. You will learn the best proportions for your use with practice - you will be mixing a lot of batches You want to be able to take a handful of cob, pull it apart, and have several strands of straw resisting the pull.
The less drying time you give, the better the join between lifts. As far as days, that totally depends on your weather. I would ideally want the previous lift to be stiff enough to not slump if you poke or jiggle it, but no more. That means the lift below that will be very stiff.
Next question that comes to mind: Having never built anything with cob before... what sort of consistency should I be looking for in the cob that is ready to put in place?
I've read of the test of forming a lump into a piece the size of a pencil which should then bend right around your finger without cracking/flaking and if it cracks you need more clay. Is that a good standard to go by before you add your straw?
That's a good test for determining the quality of the clay you have available. For mixing, you want enough water that you can mix the clay thoroughly without leaving dry spots, and it will be much easier on you if it is wet enough to mix without struggling. After the straw is added and mixed in, it should be wet enough to take a handful and slap it into place without dripping, and not have the new cob ooze out of position. It should not be so dry as to not join seamlessly to its neighboring lumps.
Thanks Glenn, One more question.... re proper proportions of sand for the clay you have.... I understand that a properly mixed clump of cob should dry without significant cracking and that cracks would indicate you need more sand. Is that correct?
Looking forward to starting this project very soon!
Correct. I would advise making a series of samples with different proportions, and observing cracking, overall shrinkage (make marks 4"/10cm apart when wet and compare when dry), and relative strength.