Mortar Formula

[larsmith]

OK, the local brick layer who volunteered his time to lay my first 2 courses of my Rocket Heater didn't lay the bricks ( even nearly ) level so I had to tear out his work.

He used a TON of mortar under the 1st layer and the slab and then used too much mortar between each of the firebrick he laid ... so I've just wasted a bunch of store-bought mortar.

Enough of my whining ... but all the above to say that I'm now all-ears to hear from those here who've used a non-commercial mortar to be used between their firebrick ...

What's the ingredience / percentages of materials used to make your home-made mortar ? ( hanging my head, humbled / disappointed / frustrated by this past weekend's experience ).

P.S. added in a whispered tone of voice ... but when I've googled mortar formula, I've seen mention of ( whispered tone ) kitty litter which is mostly bentonite(?).

[canyon]

Find your clay dirt in the area. There are many good reference books to walk you through the testing of subsoils for the properties you want. (Cobber's books or try "The Natural Plaster Book" for example). If you have to buy clay you can get bagged dry powdered kaolin from a potters supply and then add a lot of sand. I make a thick cream slip with my clay dirt and then add 4 times as much sand by volume for my mortar. But yours will be different. You have to just start playing with it and get a feel for it and test various ratios before you do your real thing. In my experience you need to slow down to go faster.

[chronictom]

the clumping litter is bentonite... the non-clumping is not

[Donkey]

Hopefully, you should be able to find clay rich soil at or close to your site. Look at road cuts or some similar place where the sub-soil is already exposed.
Clay needs water to exist, so you could look around and find a permanently damp spot, a place with wet loving plants (rushes, sedges, cattails, wilows, etc.) or a spring. You will find clay under or around these.
My advice?? Dig some out, cover the stuff with water and let it soak for a bit before you mess with it. Some clays take a good long time to fully hydrate and the clumps can add an extra dimension of handling difficulty. So, I'll try to break the stuff up first, shovel it into a bucket or barrel (depending on now much is needed), whip it with a drill mounted paddle mixer and let it sit at least overnight. I go for a consistency like a VERY thick and creamy milk shake, something you can stand yer spoon up in. You will notice that if you leave it that way, no matter how much you whipped it, it will continue to take up water for some time. So, likely as not when you return, you will need to add more water.
When I get back to it, I'll whip it up some more and screen it through a metal fly screen (set in a frame) into another barrel (bucket, what-have-you). Wear clothes that you don't mind messing up, you WILL get fully glopped.. I've found that hydrated clay of the right consistency resists going through the screen, so you've got to work it through with hands (metal tools will f-up the screen wire) and jiggle or bounce it around a bit.
All the above work gets you a smooth clay slip, free of big chunks, roots, most organic matter and other junk that just makes things difficult.
Next, prepare your sand. You need to either buy bagged sand of a known grit, 60 grit works good, 30 is fine, or shake some local, found or rough boughten stuff through the same screen you used for the clay. You want an even grain without random rocks or largish pieces.

Mix together the clay and sand. Your mix will likely be surprisingly sand heavy. You want just enough clay slip to act as glue, every sand grain lightly coated. You should be able to squeeze a handful, let it go and have it just stay together. One part clay to four sand might work, though, depending on how expansive your clay is it may be more. I need one part clay to EIGHT parts sand here on my property, one to six on my friends place closer to the ocean...
Test, fiddle, test again, etc.

Something that has helped me to know is that sand is mostly made of quartz and/or silica, which has a crystalline structure. Crystalline materials tend to deform under heat, clay won't (as much) and this can cause the heat cracking that you see a lot around cob stoves.. It may not matter in a mortar but still, good to know..
You could use grog instead of sand. Grog is clay that has been fired.. Take some clay soil, throw it in a hot fire, after things cool you will notice some changes, notably color changes. It "bisques" and changes properties. It becomes .. Not clay, but pottery (almost).. Perfect for high heat applications as an addendum or even replacement for sand.

WHEW!! That was long winded..
Good luck with that.

[canyon]

Donkey,
My mixing friend and I have been wondering lately about methods of measuring slip density for more consistent batches. Do you know of anyone using some sort of hydrometer or anything else?

[chronictom]

when you are talking about your ratios (4:1,6:1,8:1) do you mean when it is dry or after you have hydrated it into a thick slurry?

In terms of finding out what your clay is like, I took half a bucket of what I though was the best source I had, and am soaking it now. After a couple of days of mixing it up I am going to put half a gallon of it mixed with half of water in a glass jar and shake the shit out of it and then let it settle, so I can get an idea of what the composition of it right from the ground is.

lol, although I'm still not sure what I will do with that info, seems I am just doing it by feel when mixing it anyway...

[Donkey]

It's kinda all happening by feel..

Canyon.. Err... I don't know about all that.
Tom, I think I mean after it's wet...

Guys, I just go for what feels right and try to figure out ratios and whatnot to explain to my helpers and or students later..
Fortunately, natural materials are very fault tolerant! Seeing as how one shovel-full of dirt here can have totally different properties than the shovel-full just 3 feet away....
You gotta go by feel.. A little luck, a lot of experiment, and growing experience seem to be the way.

[chronictom]

I took a 1/3 pail (5 gallon) of clay, which also had some leaves, roots and other dirt amongst it, added 1/3 pail of water and mixed and let sit, mixed and let sit about a dozen times over the space of a day... by the time I was done, most of the leaves, twigs and roots floated to the top, allowing me to scoop them out, and the sand and dirt fell to the bottom, leaving the clay suspended in the water.

It is still sitting in the pail now, when we get back there, (sunday), I expect the clay will have settled into a layer above the dirt. I will mix it up again, and let it sit for a few hours to let the dirt settle again and then pour the slip through a strainer. I figure whats left should be almost pure clay/water. Is this the correct way to get pure clay or is there an easier way? for making bricks for stoves and such its not too bad, but we will be using a lot of clay in the next couple of years and it seems like it may be a lot of work doing it like this.

Also, I tried adding ashes to the slip to bring it to a mixture, it sorta worked for what I was doing with it, but it did crack something horrible, at which point the whole, little bit of clay to act as mortar between sand started to make a whole lot more sense.

If trying to make insulative bricks, using sawdust added, should I mix it just as clay and sand til it feels right then add the sawdust?

[Donkey]

I make bigger batches using garbage cans or 55 gal. drums.
Water and paddle mix in one, screen into the other. I don't worry about getting all of the clay out as the law of diminishing returns catches up at some point. So I'll mix it up no more than three times before i call it good.

This procedure is ONLY for when you need a fine product, plasters and mortars and whatnot. For cob I toss dry clay soil through a half inch screen just to get the big rocks out.. Protects the feet when mixing, dont'cha know.

For the sawdust/clay bricks, I pro'lly wouldn't use sand. Just sawdust and clay.. Perhaps grog, though I doubt it. Less thermal mass is better.. Yes?

[chronictom]

so just take and mix sawdust to the clay slip until it's moldable and start from there?

[canyon]

For insulative sawdust bricks make a clay slip about like heavy cream. Presoak the sawdust overnight and drain. Then, I use a small concrete mixer and toss 6 parts (gallons) sawdust, 1 part chopped straw (for tensile strength if making bricks) and 1 part slip. It only has to mix to just coat the sawdust/straw. This is for low temp insulative bricks. For high temp I would add grogg and more slip although I haven't tried yet. Hope this helps.

[johnjmw]

I understand most of the logic on the mortar/cob mixes but what is Grogg?
John

[Donkey]

Ground (or crushed) fired clay.
Stops acting like clay, becomes an inert filler that won't change shape/volume under intense heat.

[jesse]

A high temperature mortar recipe I've used is 1/2 fireclay 1/2 silica sand-by dry weight, these are available at pottery suppliers. 20-30 bucks for 100 lb batch. Mix to consistency of peanut butter and don't make use more than 1/4" between courses of brick. You could probably sub in whatever clay you have access to, but my application was for a pottery kiln that goes above 2400 so I needed it to be as refractory as possible. Regular lime based mortar doesn't like to go much above 700 degrees F.