StillSmart

I was in the process of doing a second distilation run. I usually put 2 to 2.5L of spirit about 40% and 1 to 2L of water in the still for the second run.
Then it occured to me I dont actually know why I am watering it down.

I read somewhere thats the best way but cant think why.
Why not just fill the still with 4L of spirit and run that?

Why is it diluted first?

I don\'t know, but I usually \"let it down\" to about 40 or 50 %
I can\'t say for certain, but I\'m thinking that it\'s something about how the molecules of water and alcohol interact and it also stops the still from being over run and the alcohol trying to get out so fast that the small bore of the condenser tube would/could get overwhelmed, possibly causing an over boil

I thought it was so you get a slower run and can make better cuts. Any one else ?

You can see the effects of this by playing around with the calculator, although it goes a bit loopy with higher percentages. It basically makes a cleaner, sharper divide between what you want and what you don\'t want - we know that water boils at 100 degrees and so if we add pure water then we know that stuff is both clean and also staying put in the still (as much as can be expected).
When you do the first run, you\'ve got such a mix of things in the wash that some of it will evaporate at 75 degrees, some at 76, some at 77, 78, 79, etc. You\'re trying to get a chunk out of the middle, so the more you can eliminate the other things and the more accentuated you can make the divides, the easier it is to control.
It\'s not the same principle, but it might help to imagine oil and water mixed together - when you let them settle, you can clearly see the line. If there is only a tiny bit of oil in there, perhaps a couple of drops, you can\'t see them so easily floating on the surface but if it\'s 50% - 50% then you can see it easily.
Also, as umpa says, it means your margin of errors is much bigger. In an extreme (theoretical) example, if you put 99% alcohol through there, then you have to be right on the button with stopping the collection or else you\'re no better off. With 40 or 50%, you can give or take a few ml and it won\'t make too much difference.
Finally, the higher percentage stuff also evaporates more so you\'re going to lose some while you\'re charging the still (very little, but even so) and you\'ll be getting higher percentages coming out into the collection jar, which will again evaporate quicker especially as it is warm.
It\'s not wrong to do it like that, it\'s just many prefer to dilute for these kind of reasons. It works perfectly fine the other way, and indeed that\'s what I did when I first started - mostly for the excitement of seeing how high I could get the percentage if I kept on distilling. You just lose a bit more and it\'s harder to learn good cuts.

I won\'t distill over 50% because I\'ve found before the foreshots start I got strong fumes coming out. Like acetone vapours and it worried me it probably is flammable.

The second reason is with more water it is \"cleaner\" and easier to make good cuts. It is more distinct when it changes. (usually within a single 500ml jar)

Thirdly, you must leave backset, you can\'t let your still run dry.

And last, I\'ve found (today actually) that if you don\'t add enough water and it does get really low you must really crank up the heat to keep the flow going and as soon as you turn off the heat the output stops instantly. (I figured I should of had 9-10L of backset but in fact only had about 3-4L) That means I was cranking the heat to it and inside I only had eight or nine inches in diametre of liquid probably a couple inches deep. Not good.
So yes, add RO water. In a 4L easystill I would think 1L water and 3L 40-50% would be easy to make good cuts.