Xavier de Langlais egg matt medium - gives a velvety matte finish, reminiscent of tempera painting. Dilute the medium with water before adding oil paint. Adding the undiluted medium to the oil paint gives the effect of egg tempera.
8-10 drops per amount of paint about the size of a hazelnut, will allow you to get a dull paint regardless of the specific properties of the pigment,
4-5 drops will give the opposite effect - painting will be shiny.