what make a dry puck ? does that means a good shot of an espresso?

A good cup of espresso is defined by taste.
If the puck is dry or soupy, who cares? Are you drinking the puck?

There are a couple of things that can be diagnosed from a puck -

Is it cracked? Are there pinholes? These indicate channeling.
Is the coffee smashed against the shower screen? Dose too high?

Not much else, I think.

matt1203 wrote:what make a dry puck ?

If the puck doesn't make contact with the dispersion screen when the coffee expands, the puck's surface will be wet with a sandy/lunar-esque texture. As long as the puck is consistent, wet or not, it's fine. For reference, see prior discussions 1, 2, 3, 4, 5, 6, 7, 8, 9 ... or just puckology in general. If you want a dry puck, dose a few grams more and/or wait longer before removing the portafilter.

matt1203 wrote:does that means a good shot of an espresso?

No.

allon wrote:There are a couple of things that can be diagnosed from a puck...

I've not discovered a correlation between the appearance of the puck post-extraction and the taste of the espresso. Moreover, there's nothing inherently wrong with small puddles of water on the puck's surface, though it should be consistent from shot-to-shot. That is, if you see big puddles one time, dry as sand the next, that's a problem. But if the puck's surface looks and feels basically the same each time, I believe you've exhausted the value of puckology.

That's why I am wary of claims that one can see evidence of channeling on the puck's surface. Afterall, most espresso machines have 3-way valves and they depressurize from 130 PSI to 0 in an instant. I think that any fissures are as likely caused by rapid depressurization as channeling during the extraction.

HB wrote:That's why I am wary of claims that one can see evidence of channeling on the puck's surface. Afterall, most espresso machines have 3-way valves and they depressurize from 130 PSI to 0 in an instant. I think that any fissures are as likely caused by rapid depressurization as channeling during the extraction.

I've had obviously channeling on my lever machine (no 3-way) and had a fissure there - that was before I figured out the dosing and forced a too-high-dose shot even though I should have known better.

And on my Faema C85, the bottomless PF makes a much better diagnosis as to channeling, but when I have screwed up, I can see the evidence on both sides of the portafilter.

You won't see evidence of side-channeling in the puck, but I believe pinholes are valid evidence of channeling.

I have to admit that I never look at my puck any more. I did alot when I started out but realized it didn't add much value.

allon wrote:A good cup of espresso is defined by taste.
If the puck is dry or soupy, who cares? Are you drinking the puck?

I agree. I think folks pay far too much attention to their puck.

I would use the puck as an indication of where you are when your espresso is good.
Puckology is a usable tool, but not a divination. You can't find your way to great espresso using puckology, but it can' be the bread-crumbs on the rail back, if you stray.
Knowing what your machine with your technique in your house with your coffee does to the end result is useful knowledge, on in that a remarkable change in the appearance of the puck that is accompanied by a remarkable change in the quality of your espresso might help you diagnose what went wrong.
If your pucks are dry and your espresso is great, and suddenly your pucks are soggy and your espresso is horrible, the two may be related. If you're getting early blonding and a ton of spritzes in your espresso, and you notice that there are funnel craters on your puck, that could indicate something.
Knowing what the puck looks like when you're trying to adjust a few parameters, and noticing how those changes in parameters change in the visual result, can be a handy guide for later. If you can track what changes make what improvements in the cup, and correlate those changes to the changes in the pucks appearance, that may be handy when you get a new bean, temps change, or you suddenly notice problems in the pull/cup.
But I agree with the rest of the folk here. Changing technique just to satisfy what you think that puck should look like is folly. Make any changes only to improve the result int the cup. If you note a simultaneous improvement in the cup and a change in the puck, it's worth noting, and storing that info away to use as a possible correction in the future if it's needed, but other than that, puckology and astrology have the same impact on espresso.