I had the good fortune of getting more lessons on coffee perfection recently. I was having a whinge to Kyle at Tartine about how my coffee doesn't trickle out of the portafilter, but rather spurts out in a slightly disturbing fashion. Was it because I wasn't tamping it hard enough I ask?
Kyle, of course, knew what was wrong straightaway. It's not about the tamp pressure - it's about the fineness of the grind. The finer the grind, the slower the flow.
Emanuelle from Ristretto on Howard Lane confirmed this. In his usual chatty fashion, he went on to tell me that hard tamping isn't good for the coffee. In fact, when the water comes out, there is a high risk that it will create a sub-layer within the coffee grounds in the portafilter that results in coffee that hasn't properly percolated through the entire disc of grounds. The result - a failure to achieve that full, perfect, slightly tart flavour.
So there you go.