I've seen many questions of people asking about whether or not to dump unfinished ink down the drain or back into the bottle from whence it came. Now, I am personally not able to make a solid decision for you, but I will give some ideas for each side. Personally, I do return unused inks back into the bottle, but that is just my choice. Feel free to make your own choice!
Dumping ink down the drain
Prevents mold and SITB contamination from the pen or from other bottles of ink.
- Sometimes, your pen may become a host of some sort of fungal/mold growth or other SITB (Stuff in the Bottle) contaminations. If you pick up the contamination in pen from somewhere, you will spread it by returning the contaminated ink to the bottle.
Prevents accidental mixings of two different colors
- If you have multiple ink colors that are extremely similar, you may inadvertently dump ink into the wrong bottle! That could cause some unpleasant reactions, that would ruin a complete bottle. It could also alter the color if the colors are not very alike.
- High amounts of wasted ink.
- A typical converter (standard international) holds about 0.5mL of ink and a typical piston and vacuum filler holds about 1.1mL. It may not seem like a lot, but dumping a lot of ink adds up quite quickly!
Returning ink to the bottle
- Saves money and ink
- One of the obvious benefits of returning ink to the bottle is that it ends up saving quite a bit of money and ink. Consider a standard $28 Pilot Iroshizuku bottle of ink that has 50mL. If you dump an entire typical converter (0.5mL) into the drain, you are losing 1% of the bottle and $0.28. Again, this will start to add up if you keep dumping ink.
Contamination of entire bottle of ink
Accidental color mixing
If you realize that you often do not finish a full converter of ink, I recommend not filling the pen fully. Doing about 1/2 of a converter is enough to do about 2-3 pages (depending on nib) and you won't lose too much if you decide to dump the ink.
Also, you may consider doing a "franken-ink" mix (parody of Frankenstein). Dump all your leftover ink into a sample vial no matter the color. You basically have a random mix of colors and sometimes, it turns out quite nicely.
The last option is dumping unused ink into it's own sample vial, separated by colors. If you have a pen with Diamine Hope Pink, assign a vial to it, then dump all unused ink into there. It saves ink and also helps prevent contamination.