I bought 4 bags of Lincoln 60 fire clay for making cob. This description says its not fire clay. Will it be ok for making the cob mass?
It was first used in the late 1800s by Gladding McBean to make sewer pipe, subsequently for a wide range of architectural purposes. It is also used as a major ingredient in many commercial west coast USA stoneware and middle and high fire pottery clay and sculpture bodies. This material is very smooth and has a unique feel that many potters can recognize with their eyes closed!
Although this material is called a fireclay by many, it is not. It matures around orton 10. It has a unique fired shrinkage vs. porosity relationship: the fired shrinkage increases until cone 6 and then decreases to the point that it is almost zero at cone 10 (thus it actually expands from cone 6 to 11). While this is characteristic of a body nearing it's melting or bloating point, such is not the case with this clay; it's porosity decreases steadily from cone 6 to 11 where it finally reaches zero (without any indication of bloating).
Lincoln clay has several other very unusual properties also:
-It has excellent drying properties (resistance to cracking) even though it has high plasticity.
-It is very plastic like a ball clay yet it feels like a kaolin (it is not sticky as are other clays of the same plasticity).
-27% water is required to make the Lincoln clay plastic enough to work for pottery (whereas a typical plastic pottery clay body is 20-22%). Yet it has a fairly low during shrinkage.
It is thus easy to see why this material was so good for making vitrified sewer pipe.
Hi Allen; I'm sure that it would work ,but I live close to sandpoint and there is plenty of local clay readily available. Lincoln fireclay works great for casting risers and cores, it is also commonly used to space (seal ) firebrick when building a RMH with firebrick. It seems using it for your mass would add unnecessary expense to your build.
Not all who wander are lost... J.R.R. Tolkien
Location: Sandpoint, Idaho
posted 5 years ago
It was 10.50 for a 50 lbs bag at White Block in Spokane Valley. How much sand can I add to it to make cob?
We've used Lincoln 60 in cob and in the high-heat areas of the firebox without problems. It's sticky enough that you don't need much. We often mix in local clays (from the ground, or a clay-bank on a neighbors' property if we can get a tractor-bucket-load or truckload delivered), and then use a dash of the purchased clay to take it up to the right consistency if the local clay isn't quite sticky enough.
I've recently had about 150 pounds of Lincoln 60 go through my hands in recent weeks... My own experience (after a lot of experimentation) was that straight clay pretty much always cracks, regardless of how much water you mix with it. Dry cracks less. Starting with a 5-2 mix of clay/sand I had to work my way up to a 5-7 clay/sand mix before it would dry without cracking. At that point it's more sand than clay (and what I ended up using for mortar). I did notice that it's far less prone to cracking if you dry it quickly...like with a torch. Mine had to air dry inside where it wasn't all that warm..maybe using an IR lamp or leaving it out in the sun on a hot day might help. I don't know...everyone says it's crack resistant, but I was having a hard time with it. Makes a pretty good binder though, and my kids have had fun playing with the extra I have.
Interestingly, the clay I dug out of the yard seems to have a lot of sand already in it, and will air dry crack free every time...though it's not all that plastic.