I am very interested in this thread.
The surface area/volume point is one I never considered.
As I understand the use of barrels, oaking isn't really the point after the first few batches, it is all about evaporation and concentrating your wine, correct?
I make kit wines now and have thought about a 6 gallon barrel but perhaps barrels make more sense at 12 gallons where you can do two kits or larger batches of wine from grapes?
I will be watching this one closely. I took a wine making class and the instructor said that barrels are a big commitment because you have to keep wine in them pretty much continuously once they are seasoned. And also that the results in terms of rich, deep reds can't be beat due to the concentration effect of evaporation.
There is no right or wrong answer here. It is a matter of experiences and preference.
For me, the difference between the results of a kit without barrel time .vs. a kit with barrel time is really significant.
A person can philosophy about all this, but the proof is in trying it, yourself. Yes, there are pros and cons, but putting those aside, if you buy a six gallon barrel for your typical kits, I will be very, very, very surprised if you don't end up very glad you did. (In other words, don't diss it too much if you have never actually "tried" a kit in a six gallon barrel.
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You do have to be mindful of the fact that a six gallon barrel, especially a brand new one can over oak your wine in only 3 or 4 weeks. The answer here is to be diligent and know how long each kit can stay in the barrel.
That first kit can only stay in for 2 to 3 weeks at the very most, then put it back in the carboy; don't bottle it right away. The next one can pretty much double that time... and so on. (You have to keep track of the cycles!!!) Then when the oaking level goes down to where it takes two or three months to get the right amount of oak, start putting those first kits back in for a month or more, so you can get some more concentration and micro-oxidation. Then you can look forward to the time you can leave a batch in the barrel for a couple of months... that's when the real value of the (neutral) barrel will come through.