I am curious to know why you chose to test how the ink behaves with the same pen held at various angles.
I did that to emulate writing in that ink with a number of pens that have different (round-tipped) nib widths and different degrees of "wetness", at least with regard to outcomes on the page.
I discovered that this Jinhao X450 nib, which I essentially salvaged from scraps and then reground by myself artlessly, now works almost like a Sailor Zoom nib but at a smaller scale, and allows me to put down significantly different line widths by varying the angle., just like the nib on my Sailor kabazaiku pen does as well. I also figured long ago that there is no single level of "wetness" for even just one pen.
Doing it this way is more economical on time and consumption of ink, instead of going through my stash of desk pens originally bought for testing ink, because I'm not filling multiple mutually incompatible (Pilot, Sailor, Platinum) converters. Even if I were to limit myself to five different Pilot pens -- which was tempting, given that I have to (at least partially) fill a Pilot converter for the Parallel pen -- I'd still be "wasting" ink because some will get trapped in the feed of each pen, by the time I'm done with it.
I'd considered fitting different Pilot MR-style steel nibs into a single pen serially, while keeping the same ink reservoir (most likely a CON-B converter) connected to the same feed, but then it'd definitely get messy in terms of inky fingers. And then, I don't know that I would get the same fit between a particular nib and the feed every time, so a particular F nib on that feed and pen may be "wetter" or "drier" each time I install it to test an ink.
Between "wetness" and nib width grades (or line widths), as variables, how an ink would look at different degrees of "wetness" is more relevant to me as a user, since I almost always prefer to write in the Extra Fine to Fine range of nib widths (except when I using a Stub or Italic nib to put down section headings, or write place cards, etc.), but I can't control the variability in wetness of all my EF- or F-nibbed pens. Fitting EF, F, M and CM nibs onto a single Pilot pen, but sacrificing any point of reference for "wetness", would therefore be counter-productive to the purposes for which I'd be testing and reviewing an ink.
Crud on the nib can also tell us something about the degree of saturation, and again, solubility.
An almost saturated solution would be prone to precipitate out if the solvent evaporates a little bit. Perhaps some inks have an ingredient that is close to a saturated solution, so that as water in the ink evaporates the crud precipitates onto the nib. A pen that seals very well ought to be more resistant to this crud.
WalterC, I agree with you. When crud forms on a nib while the pen is capped and unused, that always tells me the cap is not sealing the nib and feed effectively. Some pens never do; others may fail because of user (i.e. my) error. I now know the caps on Nemosine Singularity pens don't seal well; I have a number of them, and I recently tested the issue by filling their caps with water and then leaving them to sit with the opening facing the ceiling; they all leaked behind the bend of the clip. Put Monteverde Fireopal, and I suspect Diamine Oxblood (but I never tried that combination), in a Nemosine Singularity and nib crud is guaranteed. On the other hand, even after a full converter of PenBBS #274 Obsidian (dark blue-black) ink had evaporated from a Nemosine Singularity while capped, there was not even the slightest hint of crud. I could try putting KWZ Ink Walk Over Vistula, which is known to be very saturated with dye, in one and let it dry from evaporation while capped over a number of weeks, and I suspect there would be no crud.
I thought my bottle of Diamine Oxblood would never produce crud, at least not in my Pilot Elite 95S which has been filled with the ink for the past twelve months or so. A few weeks ago, it did -- because I failed to cap it properly and the cap was ever-so-slightly loose. After a fresh fill and proper capping, it hasn't happened again even though I only use or check that pen sporadically.
Maybe the very best way to scale saturation would be to weigh in a certain volume of ink, let it evaporate until dry (or use a desiccator), and then weigh out the solute(s)....
The concept of saturated ink may relate more to our perception of color than to the chemistry definition. Lapis' definition probably works for a single dye, but may not work for comparing different dyes (some dyes are more intense than others). Still Lapis' definition tells us something that might be useful -- i.e., more mass of stuff may relate to how much pen cleaning is needed for that ink.
My understanding of saturation, when it comes to ink, comes from skim-reading patents held by the likes of HP for its inkjet inks, and the dye load idea corresponds well with lapis' suggestion of removing the solvent(s) from a known volume of ink and weighing what remains; but then, perhaps not all of that may be dye.
To be honest, I'm not even sure why a fountain pen user would want to be informed whether an ink is saturated. A view to use it diluted, perhaps? It's not something I'd personally do, though. Worried about staining and/or ease of cleaning? I think the proof is in the pudding for that one; if a component of the ink reacts with the material of the converter or eyedroppered pen barrel somehow and caused permanent discolouration, then it doesn't really matter how much of it is there in the ink; the clear or perfectly white plastic will never look the same again.