Ultra Matte Medium 16oz by Liquitex
Increases the volume of paint while maintaining the opacity of the color it is mixed into. Opacity of colors mixed with this medium will be higher than if the color had been thinned to the same degree with any other medium except Modeling Paste. The degree to which the color can be extended will vary among the different pigments. When over 50% is added this medium can act as a weak tinting white increasing the value of the original color approximately one level. Light colors will be affected less than dark colors. Colors dry to a matte finish.