Pitting On Puck Edge
Debatably the most important factor of espresso making is maintaining the integrity of the extraction chamber volume.
(for an explanation of the extraction chamber volume see the editorial "how to determine your extraction chamber volume”)
How much coffee to put in the coffee basket, and the grind of that coffee, is outside the scope of this article. I have seen this pitting of the puck over the years and thought a quick “throw away” article may be of benefit to those who stumble upon this problem. Assuming we all have the best grooming and dosing techniques in the world, a problem like this pitting can still occur but how?
When the basket is dosed and tamped, the top surface must clear the shower screen so that when the group handle is locked into the group head, the shower screen doesn’t disturb the tamped surface.
As water enters the headspace void and the puck, it begins to swell to inhabit the headspace void, ultimately contacting the shower screen.
When it comes to the relationship between this extraction chamber and the shower screen, the shower screen design is of as equal importance as the process of grooming and tamping. The shower screen function is to form a ceiling on the puck and the junction between it and the basket will be the natural weak point.
The two shower screens above look almost the same, but when examined closely, it can be seen that there is a distinct difference in the angle of the metal rim that surrounds the mesh. The item on the left “angles” or “tapers” to the mesh, where-as the item on the right appears flat with its surface parallel to the mesh. These screens are obviously photographed upside down.
With regards to the mesh on the left, it is not difficult for one to imagine that when the coffee expands into the headspace where this junction resides, the angle creates a “pinched” void which the coffee must penetrate into. As the puck migrates out into this void it becomes “over-extended” and creates a weakness in the puck structure, and if all other parameters are not perfect, can cause puck integrity breakdown, leading to channelling.
After knowing this, look at the puck photos above again. You will see the channel has not only burrowed into the puck but is also around the edge of the puck. The puck erosion in the initial photo with the blue highlighting is quite severe, exaggerated by the contributing factor of blunt burrs. Some will attribute the above pitting to an ill-fitting tamp, and that can be the case. However, the puck in picture 1. was from a customised pullman tamp, machined for a precision fit of those baskets. So…… not this time.
So if you’re having problems with pitting pucks, check your showers before driving yourself mad with tamping technique. The problem may have been out of your control.
Bush and Bush Coffee Systems