What is Medium?
Medium is the binder in a paint, the main ingredient besides the coloured pigment. In acrylic paint the medium is acrylic, a plastic polymer. You can also add a variety of mediums to the paint for thinning and glazing, as well as for thickening, modelling, and textural effects (gels and modelling pastes). They can be used to prepare your canvas, change the texture, adjust the flow, alter the working properties of the colour or protect your painting.
Because they include acrylic resin, mediums maintain or add to the stability of the paint film, and can be used in any amount desired, are mixed with paint for thinning and glazing, and can be used as an adhesive for collage and mixed media work."
Adding mediums instead of water helps to preserve the brilliance of the colour and the natural shine of acrylic paint (unless you use a matte medium). A thinning medium will help the paint maintain its inherent composition while allowing it to flow and be brushed onto a surface easily. There are no rules for combining acrylic mediums with acrylic paint since they are made with the same emulsion base and dry at the same rate.
Different thinning mediums
Gloss medium enhances the natural sheen and brilliance of acrylic paint and dries with a glossy shine. It also makes the paint more transparent and is useful for glazing.
Glazing medium is similar to gloss medium, making the paint thinner and more transparent, useful for glazing. It might also be referred to as glazing liquid. It should be applied in thin layers and dries quickly.
Matte medium is a gloss medium with wax emulsion or silica added. When dry it has a matte, non-reflective surface.
Semi-gloss medium is a mixture of matte medium and gloss medium.
Flow improver thins acrylic paint while maintaining its strength without affecting the character of its finish. It allows the paint to flow evenly and quickly and is useful for creating even washes over a large area.
Retarder is actually not a medium but an additive. It helps the acrylic paint dry more slowly and stay workable longer, allowing you to blend colours more easily and smoothly. Because it is an additive rather than a medium you have to be careful not to add too much retarder or the paint will not dry.
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