I'm going to break this topic out of my oil fired furnace thread, since that one is really supposed to be about my furnace.
A few days ago I decided to gradually test my way up through some green sand recipes by starting out with US Silica F60 silica sand, and Whittaker bentonite 149 (probably sodium bentonite, but not definite).
The procedure would be to start at about 4% clay and 4% water, and do a trial, then increase the clay percentage and repeat, until I felt the greensand produced good results with the minimum amount of clay, specifically for my sand and equipment..
For the first trial, the ingerdients were mixed in the proportion of 50 lbs sand to 2 lbs, clay, or a little less than 4% bentonite, dry weight. 2 lbs of water was also mixed in.
I waited a day for the clay to full absorb the moisture, and have checked it for a few days since.
Conclusions:
It is way under-bonded.
It is also way too wet.
In fact, I rate it as unusable as-is, just by feel.
Therefore I've added 1/2 lb of additional bentonite, to bring the clay percentage to a little under 5%. It was thoroughly mixed in and returned to a plastic tub for aging.
No additional water was added. Water reduces through slow evaporation over time in this setup. When the sand reaches a state were the clay bonding feels sufficient, I will correct the moisture to 3%, as measured by drying and weighing a sample.