Playing with Plasticity (ie varying sand amounts)

You know how you’re trying to solve a problem and finally come to a solution and think wow, that’s so simple, I should have thought of that ages ago??

My clays are sandy. Recently I’ve been playing with my sandstone clay. The first batch had too much sand. Drying shrinkage was only 4% and by the time I add a flux it wasn’t plastic enough. I reprocessed it and removed all the sand. Got 11% drying shrinkage, which is too high. I need to add some sand back.

So, how do I process the clays to get a consistent product from batch to batch? When you’re processing, you settle out different amounts of sand depending on how thick the slurry. It’s hard to be consistent. Sieving is hard work and doesn’t always help when your silt goes through a 100 micron screen. It’s easier to remove all the sand or leave all the sand. I could just process all the sand out and then add some back in but in order to be fairly consistent, I’d either have to completely dry both the sand and clay or use something like Brongniart’s formula to calculate the solids in the slurry and then add the sand. Both methods take significant time and requiring weighing.

I had considered trying to measure the specific gravity of the slurry, but that would require significant testing to figure out the right SG to the right amount of sand still in suspension. Plus, doing it this way, I’m eliminating all the larger sand particles when some of them can be beneficial (variety of particle sizes).

Finally, it dawned on me, I can rough sieve the clay (window screen) then divide it into equal portions by volume. I can then remove all the sand from a portion of the clay and leave all the sand in a portion (measuring the raw material by volume) and then mix them back together after processing.

I will probably be looking at a 3 or 4 to one ratio for my sandstone clay. It won’t be perfect, but it will be easier and still fairly consistent. If I dewater the sandy portion first and add the non-sandy portion into that, I will get faster dewatering. I will still test each batch when I add the flux, but this should cut down on the time I spend mixing.

Pic of my sandstone clay with some sand added back. This is my 4th test batch of this clay. 1) sand 2) no sand 3) some sand 4) a bit more sand