Anyone who's ever painted (miniatures or otherwise) has probably experienced the frustration of trying to mix a color that has separated. Certain paints are especially famous for this (yes, GW Foundation paints and P3 metallics, I'm looking squarely at you), but every color has a tendency to settle over time.
Without boring anyone to tears, hopefully, I'm going to explain a bit about why it happens and then offer a few suggestions on how to, if not prevent paint separation outright, then at least make mixing a little bit easier. Paint, for simplification's sake, is generally made up of three different elements: binder, carrier and pigment. The carrier is the primary liquid in most acrylic paints, and is responsible for the overall "flow" of your paint when wet - with most acrylics, the carrier is deionized water, whereas with solvent-based acrylics (such as Tamiya Acrylics) it's a general-purpose thinner (a variation of rubbing alcohol, perhaps) that serves essentially the same purpose. The remaining elements are the "solids" of the paint that are in suspension in the carrier; pigment being the coloration, and the binder being what "fixes" the pigment in place once dry and after the carrier and whatever other liquid elements in the paint have evaporated away. As you might guess, the pigment and binders are usually heavier than the carrier medium and over time tend to separate out, if only due to gravity and particle size. Another partial reason for "settling" might be due to evaporation of the carrier in paint pots that don't seal appropriately - that is, without enough carrier liquid to keep the other ingredients "afloat" in suspension, all the more solid elements of the paint will start to clump together and sink as separated-out "sediment."
With all that out of the way, here's a few ideas as to how to limit the amount of settling in your paints:
- Keep your pots stored upside down. Not only does this make the colors themselves more visible in the rack with just about every bottle design I've seen so far, but it causes the paint itself inside the jar to act as a "barrier" of sorts to limit the amount of air able to pass into the paint pot. Less air equals less evaporation of the carrier liquid inside the pot, and thus "thinner" (and easier to mix) paint. Any separation of paint elements will have the pigments and binders settling towards the lid of the pot, as well, which generally makes for easier re-mixing if you shake all your paints periodically to maintain "freshness."
- After every use (or thereabouts) add a little bit of liquid back into your paint pot - in most acrylic paints, this would be deionized water. For the sake of limiting impurities in your paints, you want to use water as physically pure as possible, especially if you live/paint in an area where there is hard water. Deionized would have the fewest impurities but be the most expensive, then on to distilled water, then purified water, and finally to tap water. The reason for adding in liquid? To replace whatever amount of carrier has evaporated or been otherwise lost, which, while not technically remixing your paint for you, essentially allows it to maintain a closer-to-new proportion of carrier (or a close enough substitute) in relation to the other paint elements and thereby making the paint less thick overall and easier to reconstitute.
- Last and maybe best, use an agitator of some sort inside your paint pot. Ideally you want something evenly shaped and small enough to move around freely inside the pot when shaken, made of an inert material so as to not chemically interact with the paint in any way, but still heavy enough to actually shift settled contents and stir things together again without having to break a wrist with the effort. I've seen mention on several forums of folks using various things as in-pot agitators, primarily leftover trimmed-down sprue but also items such as white metal clippings, BB's, or even stainless steel ball bearings. Personally, I've tried most of these and found them less than satisfying: the sprue wasn't heavy enough, the white metal awkwardly shaped and reacting (slightly) with the contents of the pot, BB's outright rusting from the core once their inert nickel plating wore down enough, and ball bearings being expensive unless bought in bulk - also a little hard to find. My solution? A trip to the local arts and crafts chain store (Michael's, in my case) to buy glass beads - I've found 6mm to be a good and fairly inexpensive size. If you go this route, I'd suggest that instead of buying loose glass beads from the beading/jewelery-making section where they are a little pricey for the amount you get, I simply bought a few on-sale faux pearl glass bead necklaces - twice the amount for about half the cost. (Thus far I've had the luck on several trips to pick up mine on sale - 65-count necklaces of 6mm round and durable glass beads for $1.50 US!) They're even reusable once you use up all the paint in a pot, responding just as well to paint stripping as your average white metal figure. I use glass beads as agitators for paints from Games Workshop (both the newer short style pot as well as the old canister-type), Vallejo, P3, and Tamiya, and also to keep my several baby food jars of different self-mixed thinners from thickening up.