Ever wonder why your acrylic paint gets frothy sometimes? Curious about why some colors are transparent and glossy while others are opaque and matte?
Knowing what’s in your paint and how these ingredients work can answer some other questions you might have. It can also help your process. So let’s squirt some useful information into your brain.
The four main ingredients of paint
Pigments are insoluble materials ground into fine powder and mixed into a fluid that holds the particles in a suspension. This is what gives the paint its color. Particle size and shape vary between pigments depending on where they come from. Inorganic pigments are typically more irregularly shaped and synthetic organic pigment particles are smaller and more difficult to disperse.
There are two main categories of pigments
Inorganic (mineral) pigments are naturally occurring. Some, like siennas and ochres, are mined while others like Cadmiums are manufactured. These particles are larger and irregularly shaped and paint made from them tends to be more opaque, have lower tint strengths, and a matte finish.
Organic pigments are carbon-based and synthesized in labs from petroleum compounds. The carbon atom is sort of like Mr. Potatohead. Chemists can stick hydrogen, nitrogen and oxygen atoms to it in different configurations to make all these different color families. Thank you, organic chemistry.
Most of these synthetic pigments are relatively new and include quinacridones, phthalocyanines, anthraquinones, azos, pyrroles, perylene, arylamides and dioxazine. They’re transparent with generally high tint strength and a more glossy finish.
I know. This is a lot of information and until you have them memorized, it really helps to have these written down or noted in a color chart. Golden also has a chart on their website that works in a pinch.
A binder is really the glue that holds all the other constituents and dries in such a way that keeps the pigment particles adhered to the substrate. Substrate is the surface of what you’re painting on, whether it’s a canvas or panel or a sheet of Mylar.
In the case of oil paint, the binder is of course oil. In acrylics, it’s an acrylic polymer emulsion (dispersion), which is really tiny little polymer chains suspended in water. So when pigment is added, the pigment particles are suspended in what is already an emulsion.
Emulsion is a general term for any fluid material with pieces of a different substance floating mostly equidistant from each other in it. Like blood, hot cocoa, or the universe.
Because of the properties of oil, oil paints don’t need as many of these as acrylics do, but here are the important additives.
Pigments do not like to be mixed with stuff. The particles want to stick together, a behavior known as flocculation. Wetting agents must be added to both oil and acrylic paints to prevent this.
According to master paintmaker David Coles of Melbourne’s Langridge Artist Colours, a waxy wetting agent called stearate must be added to the oil to allow dispersion of the pigment while mixing. Surfactants alter the surface tension of the solid particles so that they can become wet.
In acrylic paint, the pigment particles are not the only ones that tend to glom onto each other. It also needs an emulsifier to keep the polymer particles in the binder from sticking together. If the polymer particles start touching, they cross-link.
That means they form ionic or covalent bonds and after that happens they are not able to move away from each other anymore. This is how they form a paint film and keep the pigment particles trapped to the surface you’re painting on. This is a desired behavior. It just shouldn’t happen until the paint is on your substrate and the additives evaporate along with the water.
Pigments might be hydrophobic, but you know what loves water?
Preservatives need to be added to acrylic paint to suppress the growth of mildew. If you add much water to a paint mixture to thin it down and then keep it for a long time, though, it will grow mildew. Using mildewy paint will not only smell nasty, but will compromise the integrity of the paint film later.
Thickeners and anti-foaming agents
Acrylic paint has thickeners to help bring the paint to a workable consistency. And they help to keep that consistency uniform between colors within a series.
It just wouldn’t do to open two tubes of heavy body paints and have one be significantly gloopier than the other. That would make it harder to mix them together, especially if you tend to mix colors directly on the canvas.
Because the surfactants that help the pigment particles disperse also make the paint frothy when shaken or worked with a wet brush, an anti-foaming agent is used to reduce this. Otherwise a lather would form as you’re painting and it make it hard to see what you’re doing. And it would look weird when it dries.
4. The vehicle
There’s a lot of disagreement about what this is. I’ve heard people use the words binder and vehicle interchangeably. Although I can understand why, a binder and a vehicle are not exactly the same thing.
The vehicle is not necessarily an ingredient of the paint, but rather a combination of its binder and the solvent used to dilute the paint. The reason why I include it in this list is because the vehicle is created in part by the binder.
Most often, paint out of the tube is too thick to work with efficiently. Some people use it that way for impasto or scumbling techniques, but usually you’d want to thin it out with something. For oil paints, the vehicle would be a combination of oil and a number of other things, like mineral spirits or alkyd medium, or a mixture of things that can be used as a medium.
In acrylics, the vehicle is a combination of the acrylic polymer dispersion and water. Or dispersion and water and some other medium or combination of mediums. The vehicle makes the paint move, how about that?
Just because it’s cool, here’s a fun video demonstrating the manufacturing process of Winsor & Newton oil colors. I want to stick my hand in that, but it would probably get crushed between the rollers.
As promised, here are a couple of really good resources for you if you’d like to get super nerdy and dig into more technical information such as light refraction and particle size in microns, etc.
Deep and dirty reading
Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk (1987). This guy really knows his stuff.
Handprint.com is a great source of information about the physical properties of paint.