Post by Donkey on Dec 20, 2013 13:43:15 GMT -8
This question comes up with everyone at some point.
Seems right to put this up stickied!
First, take your candidate soil and fill a mason jar 1/3 full with it, add water to just below the top, screw on the lid and shake it up really well. Set it someplace well lit where you can watch what happens next. Rocks and sand will drop to the bottom right away, silt will swirl about for a bit then drop out next, clay will want to stay bound up in the water but will slowly settle on top of the rest and organic matter will tend to float. It's helpful to mark the jar with a felt-tip as each material seems to be done dropping out, no point marking a clay line (below fine, but not above) it can take hours or days to finish depending on the clay. This test will tell you that you have clay in your soil but it won't tell you anything else.
While yer watching the jar, take a goodly handful of the same dirt, wet it and kneed it around in the hands. You should have a sticky clump that doesn't come off the hands (or anything else) easily. Take that clump and roll it into a long worm, a good long worm that can hang off of your palms and then be twisted over on itself without breaking indicates high clay content. When the worm tears, it should separate begrudgingly and leave ragged edges. (Be aware that the perfect cob mix will FAIL this test)
Smash the clump in your hands, you shouldn't be able to wring water out of it and when gently squished between the fingers the surface will not get shiny. If you can wring water out, it's an indication that there is a lot of silt masquerading as clay. Silt and clay feel quite alike as they have similar particle sizes but they are very different materials. Clay holds onto water, silt does not though both feel sticky and slick in the hands. Silt can be easily rubbed off, clay sticks and smears.
It's good to have a lot of clay in your soils, but high clay content (when making cob) is not necessarily ideal. Good cobbers soil has a high amount of sand in it, if the soil is really high in clay (like mine) you'll need to add quite a bit and it's a lot of work, plus money to buy sand. Best to find "readymix" with a lot of sand already in it.
The best test of the quality of your material is to make test pucks or bricks and let them dry out completely. Once they are dry, look them over. High clay content soils will crack, more bigger cracks indicate a lot of clay and/or very expansive clay. Crumbly, easily smashed into dust means high silt or too high a sand content. The ideal is a piece that dries hard with no cracks, something you can't crush with your hands and feels rock-like. Better to have too much clay than too little, a mix can be amended with sand but if you don't have the clay, it's a bust.
If your tests show good for clay, start experimenting with how much sand to add (assuming) make more pucks/bricks, make them the same size and label them well. First puck will be the un-amended soil, second puck add a ratio of sand, add sand and make several mixes changing the ratio as you go. Use easy ratios, 1 part this, 3 parts that, 1/2, 1/1, etc. Carry on till you just know that you've added a stupid amount of sand. Set them all out to dry, then (when they're done) test them to destruction, select your favorite results and make new tests using straw, do the routine again and repeat.
This'll give you an idea of what you've got and what you can do with it. You can read all about it somewhere, but nothing beats fooling with it yourself. The higher sand mixes will have higher thermal mass and tend (depending on the type of sand) not to crack around hot parts of the stove. Higher straw content will tend to have higher tensile strength, higher sand better compressive, and so on.
At some point, you'll want to throw your test into a fire. Let 'em sit in the fire for a good long time, maybe overnight.
The point is to subject each test to more abuse than you think it will ACTUALLY receive..
You want to be able to say "If they survive that, they'll do fine".
Seems right to put this up stickied!
First, take your candidate soil and fill a mason jar 1/3 full with it, add water to just below the top, screw on the lid and shake it up really well. Set it someplace well lit where you can watch what happens next. Rocks and sand will drop to the bottom right away, silt will swirl about for a bit then drop out next, clay will want to stay bound up in the water but will slowly settle on top of the rest and organic matter will tend to float. It's helpful to mark the jar with a felt-tip as each material seems to be done dropping out, no point marking a clay line (below fine, but not above) it can take hours or days to finish depending on the clay. This test will tell you that you have clay in your soil but it won't tell you anything else.
While yer watching the jar, take a goodly handful of the same dirt, wet it and kneed it around in the hands. You should have a sticky clump that doesn't come off the hands (or anything else) easily. Take that clump and roll it into a long worm, a good long worm that can hang off of your palms and then be twisted over on itself without breaking indicates high clay content. When the worm tears, it should separate begrudgingly and leave ragged edges. (Be aware that the perfect cob mix will FAIL this test)
Smash the clump in your hands, you shouldn't be able to wring water out of it and when gently squished between the fingers the surface will not get shiny. If you can wring water out, it's an indication that there is a lot of silt masquerading as clay. Silt and clay feel quite alike as they have similar particle sizes but they are very different materials. Clay holds onto water, silt does not though both feel sticky and slick in the hands. Silt can be easily rubbed off, clay sticks and smears.
It's good to have a lot of clay in your soils, but high clay content (when making cob) is not necessarily ideal. Good cobbers soil has a high amount of sand in it, if the soil is really high in clay (like mine) you'll need to add quite a bit and it's a lot of work, plus money to buy sand. Best to find "readymix" with a lot of sand already in it.
The best test of the quality of your material is to make test pucks or bricks and let them dry out completely. Once they are dry, look them over. High clay content soils will crack, more bigger cracks indicate a lot of clay and/or very expansive clay. Crumbly, easily smashed into dust means high silt or too high a sand content. The ideal is a piece that dries hard with no cracks, something you can't crush with your hands and feels rock-like. Better to have too much clay than too little, a mix can be amended with sand but if you don't have the clay, it's a bust.
If your tests show good for clay, start experimenting with how much sand to add (assuming) make more pucks/bricks, make them the same size and label them well. First puck will be the un-amended soil, second puck add a ratio of sand, add sand and make several mixes changing the ratio as you go. Use easy ratios, 1 part this, 3 parts that, 1/2, 1/1, etc. Carry on till you just know that you've added a stupid amount of sand. Set them all out to dry, then (when they're done) test them to destruction, select your favorite results and make new tests using straw, do the routine again and repeat.
This'll give you an idea of what you've got and what you can do with it. You can read all about it somewhere, but nothing beats fooling with it yourself. The higher sand mixes will have higher thermal mass and tend (depending on the type of sand) not to crack around hot parts of the stove. Higher straw content will tend to have higher tensile strength, higher sand better compressive, and so on.
At some point, you'll want to throw your test into a fire. Let 'em sit in the fire for a good long time, maybe overnight.
The point is to subject each test to more abuse than you think it will ACTUALLY receive..
You want to be able to say "If they survive that, they'll do fine".
