Mortars are used to create effects of materials and reliefs, especially to create backgrounds. They are best applied with a painting knife. Artist Acrylics Pumice Stone Mortar is a mortar made from acrylic polymer and finely crushed pumice stone. It lends a look of a thin material consisting of small black particles to a fine-grained material effect. It can be used pure or mixed with acrylic colours adn has a semi-matt finish.