When I use my French press (cafetiere) coffee maker, there's these strange marks left around the top of the coffee cup (see picture). Am I doing something wrong, maybe blending incorrectly? Using the wrong coffee type maybe?
I could think of two possibilities. First one is more common, I assume.
In time, the cup calcifies. So, some irregularities causes an unpolished surface. This surface catches color easier than polished ceramic/porcelain. You may try to clean the cup with vinegar. Maybe then with some bleach to get rid of such calcification and organic irregularities. Give it a try.
If the grounds you use includes finer grounds that can pass through the filter, you may end up with a muddier-than-normal French press cup. Those fine grounds may swim on top of the water and leave some trace. If this is the case, make sure about your grind size and consistency.
Your standard French press uses a metal-mesh filter. This allows more oils to pass into the cup (than a paper filter process) and some of them will form a rather small oil slick on the surface. I think you have the oil slick and some fine grounds making up your cup residue. Pouring through a paper filter cone on the way to the cup (after pressing) would reduce or eliminate that (if it bothers you.)
That looks like fines to me. In general, ground coffee has two sizes. The higher quality the grinder, the most consistently the coffee grounds will conform to those two sizes. One size is the actual grind level, dictated in a burr grinder, by the burr design and how closely they are put together. The other particle size are the tiny fragments that break off when you fracture two pieces of bean to make the larger particles. These smaller particles are called 'fines'.
While you will find fines in every grind level, I believe you get more fines the finer you grind. Just thinking logically, if you take a bean and break it one time, then you will get x amount of fines. Break it twice and you get more, etc etc. Effectively grinding finer, will cause more breaks in the coffee bean, releasing more fines.
Fines tend to slip through the metal mesh of a french press very easily, which is part of the reason the coffee has a heavier mouth feel, as well as muddled high notes. All that being said, I can't say I have often found a coating of fines high in the cup, rather they tend to sit in the bottom. This indicates either a huge amount of fines, or as some others have indicated, maybe some crazy combination of oil slick suspending fines? Recommend reviewing your grind level. Do you have heavy grinds in the bottom of your cup? If so, you may need to grind more coarse. French press should be ground at a very coarse setting to help prevent this. By the look of that cup, I would guess that you may be able to swipe your finger around the edge of the cup to capture all of that stain, and feel some grit between your fingers.
Lastly, if you are using pre-ground coffee, almost 100% guarantee the grind level is wrong.