Underpainting and/or underdrawing for Encaustic

​It is important to remember that encaustic as practiced today is a modern medium/craft. It’s still not completely understood how the ancients prepared and applied their wax-bound colors. Assuming the best adhesion will be achieved by applying wax directly to the ground, it seems to me that the best approach to transferring the drawing/cartoon to the prepared support would be one that covers the least area, leaving the maximum exposed ground. Also, whatever is applied under the encaustic should be unaffected by heat- egg tempera, it seems to me, could be vulnerable to blistering, and the proteins subject to possible denaturing or cooking. Ink line work should be good as long as coverage is not too broad.   I would also suggest avoiding any broad technique which might emboss or polish the ground. Maybe charcoal powder pounced through a perforated paper cartoon, vine charcoal applied directly, or an alcohol-carried, very light wash of charcoal powder are possibilities as well. 

While Matthew is correct in stating that we are not absolutely sure as to the composition of early encaustic paintings, I am prone to believe that the predominant method was pigments in a salt-water washed and sun-bleached beeswax, applied in a molten state. There may have been some saponified wax mediums, but if one tries to replicate published recipes for such mediums, the results are rather unpleasant in terms of handling. Certainly, ammonia and even sodium carbonate methods of saponification do not yield a satisfactory medium. I have been informed of a wax medium made by melting beeswax into soft soap (really, any natural soap made by using potassium rather than sodium salts). I do not have experience with this medium nor have I seen any historical evidence that it was used in the past.

As to your proposed procedure, Matthew is correct in warning about the possible issues involved in applying a very hot medium over a binder that is readily denatured by heat.
All analysis of Roman occupied Egyptian funerary portraits (ie Fayum Portraits) that I have consulted suggest that if there were preliminary layers, they were most often applied in an animal glue binder (generally termed “distemper”). There is also ample evidence that some Fayum portraits were executed solely in distemper and some possibly in a gum medium. Animal-glue-bound pigments are what I have my students use when they are attempting to reconstruct Fayum portraits that require preliminary layers.
Finally, while it is in no way historically accurate, I can see no technical reason to not underpaint in acrylic dispersion colors as long as the work is executed on panel. Even oil colors could be used as long as the paint is allowed to dry for a sufficient period of time before coving it with wax. I also suggest panels for these works as well. I know that a huge number of artists have used wax over acrylic and/or oil on canvas. That is their choice. It is just a truism that wax becomes very brittle and subject to cracking from even minor vibrations at even moderately lower temperatures. Fabric supports are not optimal for this. If one desired the texture of fabric, it is exceedingly easy to cover a panel with fabric, making a rigid support with the appropriate texture (see our resources section).