Interference acrylic is the paint that looks like nothing in the pot and startles people on the canvas. Squeeze some out and you get a pale gray sludge with a faint tint. Brush it over a black ground and it flares into a color that shifts as you move.
It is also the paint most often bought and abandoned, because almost everything about using it is counterintuitive. Here is how it actually works.
What it is
Not pigment. That is the key fact and everything else follows from it.
Interference paint contains flakes of mica coated with an extremely thin, precisely controlled layer of titanium dioxide. Light hitting a flake partly reflects off the top of that coating and partly passes through and reflects off the bottom. The two reflected waves are slightly out of step with each other, and depending on how far out of step they are, some wavelengths reinforce and others cancel.
The color you see is produced by that interference, not by a colored substance. It is the same physics that puts color in a soap bubble or a film of oil on a puddle. Change the thickness of the coating and you change the color — which is why interference paints come in a range of hues made from what is, chemically, the same two colorless materials.
The flip
The property that gives these paints their character, and the one worth understanding before you buy any.
Because the color comes from wavelengths canceling, whatever is canceled in reflection is still there in transmission. So an interference paint shows one color when light bounces back at you and its complementary color when light passes through. Interference green flips to red-violet; interference blue flips to orange. Tilt the canvas and the color changes — that shifting is the effect people buy it for.
It also explains the disappointment. If the light is passing straight through the paint and hitting a white surface underneath, the two effects wash each other out and you get gray.
Which is why it needs a dark ground
This is the single most important practical point. Interference paint over white reads as a faint milky haze. Over black or a deep dark, it reads as a luminous pastel that shifts as you move.
If you take away one thing: paint the area black first, let it dry properly, then glaze the interference color over the top.
Over a mid-tone it does something in between, and over a strong color it produces the most interesting results of all, because you are seeing the interference color layered against a hue rather than against neutral darkness. Interference blue over a deep red is worth trying just to see what happens.
Thin coats, not thick ones
The second counterintuitive thing. Piling it on does not strengthen the effect — it dulls it, because the flakes stack up at random angles and cancel each other out.
What you want is a thin layer where the flakes can lie flat and parallel, all catching the light the same way. Several very thin coats, each fully dry before the next, produce far more shimmer than one heavy application.
Brush in one direction rather than scrubbing back and forth, for the same reason. The flakes align with the stroke, and aligned flakes are what makes the surface flare rather than glimmer weakly.
Mixing it
Interference colors are almost never used straight. Three ways to use them:
With a clear medium. Thin with an acrylic glazing medium rather than with water, and you get a transparent shimmer you can lay over a finished area without changing its color. This is the most useful thing you can do with it and the one people skip.
Into another color. A small amount stirred into a regular acrylic gives that color a pearl quality. Add it to a color already close to the interference hue and the effect is subtle; add it to a contrasting one and you get something that shifts between the two.
With a touch of black. A very small amount of black added to an interference color intensifies it, because you are supplying the dark background from inside the paint rather than underneath it. A little more gives you an iridescent gray. This is worth experimenting with on a scrap, because the difference between intensifying and killing it is a very small quantity.
Whatever you do, test on a scrap and let it dry. Interference paints look completely different wet, and no other paint misleads you as badly at that stage.
Interference and iridescent are not the same
Worth separating, because the labels get used interchangeably in shops.
Iridescent or metallic paints contain mica coated to look like gold, silver, bronze or copper, or actual metal flakes. They are largely opaque and they look metallic over any ground.
Interference paints are transparent and rely on what is underneath. Over white they nearly disappear.
If you want a metallic sheen, buy iridescent. If you want a color that shifts as the viewer moves, buy interference. Buying the wrong one is the most common reason people conclude the paint is faulty.
Durability
One genuine advantage over metallic paints made with actual metal flakes: mica is chemically stable. It does not oxidize, so it does not tarnish, corrode or darken with age the way a copper or bronze flake will. Titanium dioxide is likewise stable and does not fade in light.
So the effect you get on the day is broadly the effect you will have in twenty years, which is not true of every specialty paint.
Where it fits with a numbered canvas
To be clear: interference paint is not part of a paint by numbers kit, ours or anyone's. The colors in a kit are matched to the design and mixed to cover flatly, which is the opposite of what this paint does.
Where it can earn its place is as a final touch on a finished canvas, over paint that is completely dry, used sparingly:
- Water. A thin glaze of interference blue or green over the darker areas of a lake or sea gives the surface a shift as you walk past it that flat paint cannot.
- Snow and ice. Over the shadow blues rather than the whites, where the ground is dark enough for it to register.
- Night skies. The natural home for it, since the ground is already dark. A shimmer through an aurora or across a moonlit sky.
- Wings and feathers. On a hummingbird's throat or a dark bird's back, where real iridescence exists in the subject.
Two cautions if you try it. Work on a dry canvas, and test the effect on a scrap of the same dark color first, because the ground changes the result completely. And use it in one or two places rather than across the whole picture — an entire canvas of shimmer reads as a novelty finish rather than as light.
Our note on layering acrylic paint covers building up transparent coats properly, glazing covers the technique this paint depends on, and the night designs in the kit collection are the ones where it would actually do something.
