What was encaustic
"In ancient times, beeswax was used in many crafts. It was used as an adhesive, as a protective coating on walls and statues, and as a paint vehicle. For the latter purpose, the beeswax was heated with water and soda or potash. The resulting emulsion is generally called Punic wax". Hermann Kühn. Detection and Identification of Punic Wax by Infra-red Spectrography. Studies in Conservation, 5 (1960), 71.
"The study of the encaustic painting technique of the "Fayum portraits" [...] has set out the problem of the identification of their medium. Literature indicates that it could be pure beeswax or Punic wax. The latter would be the result of a chemical treatment undergone by beeswax according to ancient recipes mentioned by Pliny and Dioscorides. This lead to a more or less saponified wax that could be worked more easily with a brush than beeswax". Sylvie Colinart, Sibille Grappin-Wsevolojsky. La cire punique: Etude critique des recettes antiques et de leur interprétation. Application aux portraits du Fayoum. 12th Triennial meeting, Lyon OM-MCC Preprints 1 p. 213.
"The precise composition of the medium has proved to be a continual source of disagreement. The main theories are that the wax was either applied hot, in the form of pure beeswax, or beeswax with resins added, that had been melted and mixed with pigments prior to application; or that it was applied cold after emulsifying it (by a process known as saponification), which allowed it to be mixed with pigments, egg and oil, and used either while still warm or after cooling". Euphrosyne Doxiadis. The Mysterious Fayum Portraits: Faces From Ancient Egypt. 1995, 95.