What is encaustic painting:

Encaustic painting, the ancient hot wax process, combines molten beeswax damar crystals and dry colored pigments. The painting is done on a birch panel and the paint is applied hot, and manipulated in variety of methods and fused with a propane torch. The painting consist of many layers, each  “burning-in.” Each layer is fused to the layer previously applied. The burning-in process makes encaustic unique from simply adding a waxy ingredient to oil paint. The paint film is a microcrystal in structure that never dries so it cannot darken, yellow, crack or fade. Finished paintings are among the most permanent of all ancient painting media and considered one of the most archival painting medium.  

History 
Encaustic is a Greek word for painting, with the root word “caustic” (to burn) key to its understanding since the paint has to be literally burned in to produce the final paint film. Roman Fayum portraits of the deceased made to adorn ancient sarcophagus were often carried out with encaustic. Examples over 3500 years old can be seen in museums throughout the world. Remarkably they look as though they were painted yesterday, without a crack or diminution of color and vibrancy. Throughout art history, the encaustic method has been investigated and revived most notably in the work of Diego Riviera, Carl Zerbe and most recently Jasper Johns. Currently there is a vibrant incarnation of the process and, with widely available materials and tools, many artists are experimenting with the medium.