Inside the crucible - what IS going on in there?

Seems like a while since we've discussed what we think, speculate, or "know" happens inside the portafilter basket during the various stages of extraction.  A bit of this has come up in another discussion:
however it seems like a more thorough discussion can occur if we break away from that original question.  Some good stuff put up over there, so you should definitely read it if you haven't already.

So, what do you think is going on in there?

(note: I will ask one favor... let's approach this neutrally, scientifically, and skeptically.  I'm pretty sure that we do not actually know exactly what is happening in there... we have developed some really good theories to explain what is happening, have made observations that support them, and have made other observations that may not be fully explained.  So let's approach the subject in that way - submitting what you suspect is happening in there and sharing any supporting evidence you may have.)

Also, if you happen to have any links to other discussions that you feel have covered this well, please share them.  I don't get out as much as I should...

Ok, geek on...

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Me likey this topic.
Me too... a lot! I can't offer anything at all, but sometimes it's great to just get bombarded with deeper thinking stuff... I'm really enjoying reading everybody's thoughts and speculations.


Jason Dominy said:
Me likey this topic.
Marshall Fuss said:
Brian Thayer said:
This may have been asked already, but has there been any move to create a glass basket?
Illy uses a see-through portafilter for its research, though there is speculation it is some kind of polycarbonate, rather than glass. There has been voluminous scientific research, with peer-reviewed papers published, on all the questions raised here. "Espresso Coffee, Second Edition: The Science of Quality," edited by Illy and Viani, is available on and is usually the best starting place to dive into this subject.

Great. I was hoping for something like this. Ordering my copy later tonight.

Thanks Marshall.

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