Hi Esther, the tannins and OPCs in oak are what makes it so astringent. Tannins and OPCs are quite polar so they are rather easily water soluble. Hence often extracted simply in hot water – decocting generally from the bark. But a lot of folks use a tincture as you will have read about. 40% alc means 60% water of course so that makes good sense here however one of the sources I use recommends 60% ethanol. That’s most likely to draw out other constituents – the resins mainly. If actually suggest a really low alcohol percentage if you want those tannins – say 20% or 25% so you would water down your vodka up to 50:50. Does that make sense? It really depends on what you want the tincture for… Be careful taking tannins internally over a long time and certainly don’t use them when there’s certain gut problems as you don’t want tannins entering the blood stream. Tannins are super useful though!