Adhesion
Adhesion refers to the degree to which a painting medium adheres to a surface, as well as the way in which this adhesion manifests itself.
Opaque paints adhere to porous surfaces not only on the surface but also deep within the material. Wetter paints penetrate mainly into the surface, while solid media, such as pastels, adhere only to the surface. On non-absorbent substrates, adhesion also occurs only at the surface level, so the binder must meet a different set of requirements.
What kind of surface should we paint on?
- Paints, ink, markers, sprays, gesso, modeling pastes, gel mediums with various effects, gouache, acrylic-based paints for printing and decoration:
The surface must be clean, free of grease, and slightly absorbent. Adhesion occurs both on the surface and deep within the substrate.
- Watercolor paints and ink, Chinese ink drawings, watercolor pencils (with added water), textile dyes, alcohol-based markers and inks:
The surface must be clean, free of grease, and absorbent. Adhesion occurs primarily in depth.
- Oil paints – regular and water-soluble:
Adhesion occurs both on the surface and deep within the substrate. To ensure that the paints adhere well to the substrate, a primer is applied—the oil in the paints penetrates its microscopic pores, and the paint layer bonds to its surface.
Designed for application to non-absorbent surfaces. Adhesion occurs entirely on the substrate, which must be thoroughly degreased and typically requires heat treatment for successful bonding.
Adhesion occurs mainly on the surface. Since it is not very durable, the pastels can be easily scratched off. Almost any porous or rough surface is suitable.
- Dry pastels, chalk and pastel pencils:
Adhesion occurs mainly on the surface and is very weak. Rougher surfaces yield the best results because the dry pigment particles get trapped in their indentations.
Adhesion occurs mainly on the surface. Almost any porous or rough surface is suitable.
Drying time and process depending on the medium
Acrylics, watercolors, gouache, ink , and water-based printing inks dry through a physical process—by the evaporation of the water they contain. The drying time is relatively short and depends on the thickness of the paint layer, the temperature, and the humidity of the air.
Traditional and water-mixable oil paints, on the other hand, dry through a chemical process. The oil absorbs oxygen from the air, causing its molecules to bond with one another and polymerize. Oxidation requires energy, supplied mainly by ultraviolet light, and time, which is why oil-based paints “dry” so slowly. The more light and ventilation there is in the room, the faster the process occurs. There are additives to speed it up. Low temperatures and high humidity hinder polymerization.
Alkyd paints and mediums dry both physically and chemically. The white spirit in which the resins are dissolved evaporates quickly, and the paint dries to the touch much faster than oil paint. After that, the chemical drying process of the dissolved resins begins, which takes about a week.
Dry, oil, and wax pastels, chalk, and colored pencils do not require any drying time, except when they are mixed with water.
Brushstroke
Each coat of solvent-based paint becomes thinner as the solvents in it evaporate. The sharp edge of the brushstroke becomes softer or disappears completely if the paint contains a large amount of thinner or if a thinner is added to it (for example, water to gouache). Only undiluted oil paint preserves the brushstrokes’ texture unchanged even after it dries.
Water Resistance
Some paints are waterproof once dry, while others can be dissolved in water.
The following are waterproof:
Oil, alkyd, and acrylic paints and inks; Chinese shellac-basedinks; traditional oil and wax pastels (excluding water-soluble ones); alcohol-based inks and markers.
They are not waterproof:
Watercolors and ink, gouache, and water-based printing inks.
Dry pastels, pastel pencils, and chalk canbe smudged with water, but they are not completely water-soluble.
Plasticity
Dried layers of paint contract and expand in response to changes in temperature and humidity. The paint’s elasticity determines the extent to which the paint layer can withstand these changes without cracking.
Acrylic paint is the most flexible. Alkyd paint is also relatively flexible. Oil paint becomes increasingly less elastic as the oxidation process progresses. Gum arabic is easily brittle if layered, but watercolors and ink are applied so thinly that it is unlikely to result in a thick, continuous layer of the medium. Furthermore, the flexibility of the paint after drying is enhanced by the addition of glycerin. Gouache is the least flexible due to its high concentration of dry ingredients and the inelasticity of the dextrin. That is why it is recommended to apply it in a thin layer. Oil pastels, on the other hand, remain flexible even when applied in thicker layers.
Translated from Royal Talens’ Artists’ Materials Manual (2020): Martina Petrova
