One of the happiest moments when you are printing an edition is when you realize you have mixed exactly the right amount of ink. The second happiest moment is when you determine that you have mixed enough ink. Why the fuss? Well, it turns out that determining how much ink a plate is going to take for an edition of a certain size is not as straightforward as you might think. For a woodcut, it depends on how much image there is on the plate, the color of the ink, the consistency, how transparent it is, whether it is going to sit on top of another layer of ink or directly on the paper, and so on.
So you make your best guess, mix up a pile of ink and start printing.
Sometimes (most of the time) you get to the end and there is some left. You could discard it (I sometimes do if I have only a small amount and I have mixed drier with it), or save it. Unless there is a lot of it, I usually wrap it in aluminum foil, double sealing the edge and write the name of the print and plate on it with a Sharpie. After a while you end up with a whole pile of these. Most of them are not enough to run an edition from, but they can provide a good starting point.
Before I wrap everything up, I will tap some of each color out on a piece of scrap paper. I make some brief notes on them about what inks I used to create the color in roughly the order of prevalence. I find that if I do that, I can come back to an ink later and hit the color pretty accurately. One of the challenges, however, is that ink (especially light inks) will sometimes change color slightly as they dry (usually darken). Here is where that little packet of ink comes in handy. Unfold that and you have a little bit of wet ink to color match against. If you are trying to match very closely, you can dribble a little thread of your new ink over the old and let it relax into it. Once it does, you will see any subtle color or value variations. Of course, that assumes that your ink is long enough that you can get a thread.
Usually you only have to match that closely if you have made an error and come up short, and have to mix more ink in the middle of the edition. Be prepared to loose a proof or two if this happens as it’s difficult to get a new pile of ink to print exactly like the old one, even if the color matches. I usually have to tweak the characteristics of the ink a little before it settles in.
This all comes to mind because I recently completed an edition where one of the colors (the second one) came out exactly right. I had enough ink and no more. Those last few proofs are always nail biters. There is always the temptation to try and go a little light to stretch what you have so that you don’t have to mix more. That really isn’t a good idea as you are likely to mess up the way they are printing, which is not what you are trying to achieve. On the whole, I would rather mix too much and have some left.