Acrylics paints are water-based, and cure through the evaporation of the water from the paint. Extenders, also called "retarders," slow the drying time of acrylic paint from just a few hours to up to a few days, depending on the ratio of extender to paint. Acrylic mediums and extenders are commercially available at any art or hobby supply store, or you can use glycerol in a homemade formula.
Many commercial extenders are glycerin-based. In fact, glycerin is a already an ingredient in most acrylic paints. It gives the paint viscosity in addition to extending the drying time. You can heighten this effect by using glycerol mixed with water. Glycerol is a food-grade additive with medical uses that can be purchased at most health food stores and some pharmacies. It may also be labeled as "glycerin" or "glycerine."
Only a small amount of glycerol is needed, because it must be mixed with water before combining with acrylic paints. One cup of glycerol will last most painters several months, even with constant use. To create the glycerol mixture, pour distilled water into a small bottle, and add glycerol in a ratio of five parts water to every one part glycerol. Close the bottle, and shake vigorously to mix.
Painting with Glycerol
Each artist will develop his own preferred mixture of paint to extender, but this will only come with experimentation. Begin by adding approximately 1/4 tbsp of the glycerol mixture to every tablespoon of paint on your palette. Combine well with a palette knife until the color is uniform and you can no longer see any of the glycerol mixture in the paint. Paint as usual, but be aware that with extended drying times you can mix paints on the canvas as well as move the paints longer for texturing.
You will no doubt have more of the glycerol mixture than you can mix with your paints in one day. It is difficult to gauge and mix the small amounts of the mixture you will need for any one project or paint color. The mixture can be stored for later use. It will keep for several weeks in an air tight container. When you are ready to use more of the mixture, simply shake well and add to your paints.