Any sort of analysis would require access to a lab. I don't know how simple you want to keep things, or what the story time-frame is, but you can possibly amend slightly your idea without much effect on stuff you have already written or ideas you particularly like.
If you want to write an Agatha Christie-style detective quiz with a limited number of people of whom most could have done it, maybe the chemical analysis thing is not entirely feasible without significantly altering your conceived climax of the story. On the other hand, literature is about the plausible, not the actual. Realistic writing, for instance is only a style of writing: the author uses very detailed descriptions, but still, as a result of the nature of the medium, the reader has to use their imagination a lot. Just make sure you do not bring people or events in the story that function as dei ex machina - plot resolutions which come out of the blue without warning. An example of how to avoid this would be to add a scene in the build-up of the story where the detective meets with his researcher friend who faces, e.g. marital issues, but the topic's too complicated and unpleasant so they shift to fountain pen ink behaviour (in layman terms - the expert might throw in some technical terms like viscosity and acidity, but in simple context and with plain everyday meaning).
The chemicals involved in the analysis of something vary wildly. If you want somebody to use a pipette (usually a plastic one-use eyedropper with volume scale on the side) to drop something on the piece of paper and get an unmistakable reaction, I can tell you that's plausible, but most reactions of the sort are not trustworthy because detection with the naked eye is avoided unless you have half a litre of transparent solution which turns really transparent blue (copper does that) or milky (various plant oils do that) if you add -quite a bit- of a suitable reactant (acids usually) in it.
Mass spectroscopy, on the other hand can detect a few molecules of stuff inside other stuff. A sample is burned so hot that everything in it turns into ions (all chemical bonds are broken down and electrically charged atoms float about in a flame. Light passes through the flame, interacts with the ions and gives a measurable signature of light absorption from a sample. This signature is then compared -using software- to other relevant signatures (signature of plain paper of the type we need to analyse, signature of the same paper with ink A, signature of the same paper with ink B and however many inks we need to test). The downside is that for every measurement you need to burn a small piece of the evidence. The paper can be scratched to obtain a sample, though. Another downside is that spectrometres can cost a couple of millions US dollars, so only affluent universities and big corporations have one or two. Researchers often have to pull favours to get some spectrometer time. The preparation and the measurement of a sample can take as low as half a working day.
Now, letting my imagination loose, you can avoid all the above if, for instance, the rare ink of my previous post (call this ink Alpha) happened to turn something easily obtainable and explicable like a pregnancy test stick a deep purple red because, let's say it is an iron gall, and the iron oxidises (with the help of the catalytic proteins embedded in the sponge-like material in the stick) in red-orange rust. it's deep purple because of the gall also present in the ink. Let's now suppose that on the contrary, other (indeed most) inks will not do that, the blue in my previous post (call this ink Beta, just turns the spongy material in the stick a sky blue. The detective gets an (unused) pregnancy test from a pharmacy or a young lady (to be a bit lucky is plausible), gets the pen used to write the note, gets it to spill some ink, adds some water, uses the pregnancy test and - voila- instead of sky blue he gets a beautiful lavender violet (red+blue). The perpetrator needs to have mentioned at some point that the pen is new and that it's never been near this barrel of the rare ink A he managed to secure from the progenitors of Mark Twain; or at least he needs to say something to that effect int the story's build-up section.
I hope I have been of some help.
I do believe this part of the novel is more of the Agatha Christie-Styled close room mystery, but in fact, it is only the very beginning of a much larger, more convoluted plot. The idea is for a witty and logical resolution that ordinary people wouldn't be able to grasp right away until like in Sherlock Holmes the Method is revealed later. And it is for that reason alone, that I continue to read more about it, because the more I know, the more equipped I will be to create a more compelling narrative.
I am trying to move away from chemical analysis, but I won't rule out using chemicals that might be found in say a Photography or Art Studio. The important factor is that there is no time for such in-depth analysis, and results have to be drawn quickly, and they have to be conclusive. I am looking into the idea of chemical analysis of Ink via simple chemicals - the sort that might break ink into various shades. And maybe mass spectroscopy can be utilised at a later date to Confirm the conjecture.
Your final point about the pregnancy test completely Blew My Mind, and it is absolutely incredible how you might have thought of something so brilliant!
In my plot, the ink and the Pen are already confirmed to belong to the owner; what is not confirmed though, is whether the intruder wrote the note, or did the owner write it and is lying about the intruder. It has to be resolved by the detective in a matter of a day while inside the House/Studio.
I will be inspecting, this week, different types of ink behaviour when exposed to water or lemon juice or a sugar solution and such.
Thank you, Ardene.
The main point of all that is that if the note in your story will be seen by anyone who really knows the faker's handwriting well at all, it will be hard for him to make it look as though someone else wrote it and still look as though the person who wrote it is accustomed to writing fluently.
Amberlea's cursive that you showed in post #18 may look to you so different from her print that it looks as though another person wrote it, but I've seen her posts here often enough that I wouldn't need to see the signature on either one to know it was her writing - and if I can tell so easily, think how quickly her family or co-workers would probably know.
I don't know how helpful any of this will be to you - you may have figured this all out for yourself already, or maybe no one who knows the writer will see the note - but I have had fun thinking about it.
That is extremely helpful, thanks for the image. This is what I was looking for, the different forms of the same person's handwriting bring my initial aspect of inquiry.
And you are right, friends and coworkers might find it extremely easy to identify a handwriting. The resemblance of your having written the attached page is evident across nearly all your handwriting styles, specifically, I might point out, because of the general Spikiness of it. The wrong hand is certainly not an option because the note has to be Threatening in a way and that just makes it look silly.
The lucky point for me is that the detective and the person who wrote the note know each other intimately, and hence obviously they are familiar with each other's handwriting - and of course, the plot to resolve the detective has to realise this before too long. Yes, I believe some styles like your Printed Straight (2nd), and extremely Wide and Slanted (2nd last) do a terrific job of throwing one off the scent, even if only briefly.
What I am interested in knowing is, if and only if you have the time to spend on it, whether you used any detectable pressure variations across your handwriting styles - this can be checked by feeling the back of the paper or holding it against the light, but I suspect there won't be any because any experienced writer will use the light hold as a rule.
The other thing I wanted to know is whether the part of the nib that scratched the paper changed significantly (like the left side in one hold vs the right side in another).
It's interesting when I think of it, that maybe this thread might someday end up helping a real detective somewhere, no?
Thank you, Jenny.