billy larson

I work with natural clay in illinois, wisconsin, michigan at several locations in each of these states. Unless it is a grey color fire clay it is very hard to find a silt to clay ratio where the clay is more than 50 percent give or take. Most of the time the silt percentage is higher than the clay and in reality what most people are calling clay is actually silt. So a typical mix is 70 percent sand 15 percent silt and 15 percent clay.
The sand we use is usually not considered coarse or fine but a medium gradation. To fine of sand will lend towards the silty and too coarse more like a concrete.

I will let you know as I progress this winter when I get home, making mix in Texas right now and maybe Oklahoma next....make a pretty red cob

does this make any sense.

I make infield mixes for baseball diamonds and after doing research on making cob the ratios of sand to clay are pretty close.
Cost for a pre mixed infield mix runs from 20 to 50 dollars per ton (one ton is 1.35 yards) plus shipping. A typical load is about
25 tons which would make a wall 1' wide x 8 tall' x 60' long and cost 500 to 1250. Usually they will sell trailer or pickup loads depending on location.
If a person went this way you would need to ask for the tests on the mix for
the actual ratio of sand to clay and also the silt to clay ratio. Most clays in the midwest I work with are a 1 to 1 silt to clay.

Some times the silt to clay ratio is deceiving as silt gets finer toward the coarser clays it performs almost identical, so a sample from
their pile and how it sets up would be wise.

I have brought some straight clay and mix home to experiment this winter with a rocket stove

Mr white cloud,
Thanks, probably what I am going to do is some experimenting. I was just trying to not re invent the wheel. I can imagine the the mohs hardness of the sand vs. the pumice or perlite is a determining factor on
durability and how it holds up under the weight of compression.

I work with sands and clay for baseball diamonds, the mixtures we use are right in line with cob making. 70 percent sand and 30 percent silt and clay with the silt to clay ration of less silt than clay. Too bad its not in NM.

Thanks again

Anyone have any thoughts on using perlite or pumice in place of sand in adobe or cob. I see it is used in rocket stoves for insulation on the riser pipe. How about in between cordwood in cord wood construction in place of the sawdust. I believe the sawdust is mostly to keep the mortar from drying too slow. I think wet perlite or pumice would have the same effect.

I have access to pumice and perlite but no clean sand for 120 miles away.

I am tearing down an old school built in 1927 and taking down the chimney it has fired red brick one layer on the outside, two layer soft brick, and one layer hard fire brick. do you think 2 layers of soft brick be insulating fire brick or common. I am guessing insulating but is there any way to tell?