This is an additive that is meant to be stirred into the colour to give a thick buttery consistency.
Originally it was formulated to give the perfect paint thickness for screen-printing. However, it is now widely used to thicken colours to achieve an impasto effect in painting.
Add 2 teaspoons of thickener per 250ml of paint. Use sparingly as this a very effective medium (not compatible with fluorescent colours).
Paint that has been thickened with Viscosity Thickener should be used within 48 hours or so.