Vac pots are special in their combination of (1) total-immersion brewing (2) stable hot-but-not-too-hot temperature (3) cloth filtration. (2) and (3) are especially important I think. Cloth isn't necessary but in my opinion it's the best way to go. The agitation is more controlled than that through metal filters (I've tried them all...all), mouthfeel is denser than paper-filtered coffee but it lacks the sediment of metal-filtration. I did a post on maintenance some time ago.
My technique is dialed into my particular grinder and slightly unusual vac pot set-up. In other words, I could tell you what I do and how I do it but there's a good chance that it'll be completely irrevelant to your vac-pot purposes. So I'm just going to say some things that are important to keep in mind.
* Centering your cloth filter. If you filter isn't properly tied and centered, you'll get big-time bubbles leaking up from the side. Use a non-metal stirrer (non-metal because the glass gets even more delicate when hot...just a precaution) to move the filter around until the major bubbling stops. You're looking for gentle bubbles through the center; a gentle stream from the side of the filter isn't ideal but it's workable. If you have major turbulence, it's best to just abort...you're not likely to brew anything tasty. Those big bubbles will most likely over-agitate/over-heat certain parts of the coffee bed, leading to unpleasant bitterness.
This is the main reason why I don't like those vac pot methods that suggest putting the coffee into the upper chamber before the vac pot is attached or before all the water has siphoned up. How are you supposed to adjust the filter?
* Crust. Vacuum pot is technically total immersion brewing...all the coffee gets wet. For a while at least. Without periodic mixing, a crust forms and the upper level of coffee won't be brewing the same rate at the lower level. Not the worst thing, but I like to fold in the crust mid-way during the brew. Gently.
* Draw-down. Once you take the pot off heat the vapor will condense and the filtration will begin. The speed of this has a lot to do with ambient temperature, I've found. This opens the door to inconsistentcy. If you have a vac-pot with strong glass (this isn't recommended with a Yama), you can use a cool towel on the bottom globe to both speed up the draw-down and make it more consistent. Not necessary but helpful.
* Cool-down. Vac pot coffee emerges crazy hot. I like to cool it down fast for the same reason that I don't like using air pots or electric plates for coffee. The residual heat is causing chemical changes that generally aren't great for the coffee taste-wise. I'll usually decant into a Chemex before pouring into a cup to cool things down. This step isn't just about impatience; it's about making things tastier.
* Dose. I think a slight updose is nice. Some people double-dose, which I think is crazy, but whatever suits your taste.
* Just in case you've got the same instruments. I grind with a Virtuoso and steep with a two-cup Hario. Grind is medium, stirs are qucik and gentle and at the beg/mid/end. On heat for 90 seconds, draw-down is 30 seconds. I use a cool towel. Generally 19-22g of coffee per ~8 ounces of water, temperature 196-200 F, coffees I use are very dense. On this point your mileage *will* vary.
Special thanks to Barismo and Simon Hsieh, from whom I've learned a lot.
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