hornbyben if they started uneven but were level after tamping then the density on the side that was higher must be more than the side where it was lower. Once the higher side is fully compressed the tamper is prevented from going deeper, but that doesn’t mean the lower side has reached this point.
The higher density side provides more resistance to flow than the lower, so more water will go through the lower side, giving a shot like tompoland described.
That’s what I thought, except for three things.
First, given enough force, any differential in density should go away, given that coffee is not compressible. This is basic physics.
Second, as I mentioned, I deliberately left the bed uneven, and tamped by pushing down really hard on the ground coffee so that the bed was level after tamping. I poked around with a toothpick, and didn’t find any soft spots.
Finally, I tried this out yesterday. I made a shot like I normally do, leveling the bed, and doing WDT so that things were even before tamping. The espresso was great. I gave that one to my wife, because…
For the next shot I deliberately left the coffee uneven in the basket by quite a lot. I then tamped with enough pressure to make the bed level. No WDT, obviously, because that would have leveled out the bed. I pulled a shot for myself. (That’s why I gave the first shot to my wife, in case things turned out badly.)
The shot wasn’t perfect, but was still very good. I’d put the shot with the uneven bed well within the range of espressos I’m used to getting with my usual technique. The interesting thing is that the extraction time was actually a little longer than what I normally get. This indicates that there’s no soft spot or less dense area that the water could get through more easily. (Point of explanation — I have a 9Barista, so there’s not any manipulation of the extraction once it happens. The length of the extraction depends on the dose and grind of the coffee, and also on how well-packed the puck is after prepping and tamping.)
I’m going to keep leveling the bed and using WDT, but these findings were interesting to me.