Learned it when I was reading about Moonshine.
It's basic chemistry. Any solid with a stronger affinity for water will steal water from a liquid, or a vapor with a weaker affinity.
They used to use oven dried wheat n oat chaff that was left over from fermentation.
Because alcohol vapor is so hygroscopic, it will take moisture from the air as a vapor, so the limit with distillation is roughly 85%, on a day with low R.H.
Least expensive way to concentrate it was what I was looking for.
CaCl, (Calcium Chloride) will work mo' betta, but I had rock salt on hand.
I'm a brewer so I already had the means to measure alcohol content.
Threw some salt in a cup of 70% ethanol and saw no change.
Hmmm, then the lights came on.
Our RH is high, the Rock salt was almost wet sitting on the shelf.
Weighed some out, popped it in the oven, then weighed it again.
Aha! Now let's see.
Put some dried salt in and brought it to 88%. That's 176 proof.
If you do it less sloppily, 91% - 182 proof is possible.
But kiln dried chaff can get you to 99%.
And keep it there if you seal it tight. :)
Never even leave the cap off for any longer than necessary, it will grab water from the air.
Don't ask how I know that. :(
That all I know about it, so far.
I use the alcohol only for RSO extraction purposes, though.
For cleaning my glass, Acetone can't be beat IMO.
Aloha,
Weeze