The difference is far from academic. Paper absorbs the oils, metal does not. Espresso is definitely not filtered, the oils are necessary for a good crema. Ther

The difference is far from academic. Paper absorbs the oils, metal does not. Espresso is definitely not filtered, the oils are necessary for a good crema. There's a fascinating review paper on crema with lots of pictures and microscope images by E. Illy et al. (yes, that Illy) on the subject. https://link.springer.com/article/10.1007/s11483-011-9220-5 https://link.springer.com/article/10.1007/s11483-011-9220-5 [open access]