This is an older thread but I just received this very detailed reply from Richard Frumess, owner of R & F Encaustics:
Here are some thoughts
I don't know if you're familiar with the
painter, Rifka Angel. She painted in encaustic from as early as 1932
until her death in 1988. It is possible to consider her as perhaps the
first encaustic painter in the US to consistently use the molten wax
method (as opposed to saponified wax or wax in oil paint). However, she
used, according to sources, carnauba and montan waxes rather than
beeswax -- two extraordinarily hard, high melting point waxes. Having
organized an exhibit of her work in our gallery in 2005, I found many of
the paintings to be highly fragile. In some cases, large areas flaked
off during transportation. Both the natural brittleness of the waxes and
the difficulty in adequately fusing the paint without blurring the
definitions of her composition would have contributed to its fragility.
Carnauba as a hardener.
You mentioned carnauba as a hardener for beeswax. We've done comparison
tests between added damar resin (no solvent) and added carnauba. Damar
resin, added incrementally increased the hardness of the beeswax
incrementally; whereas carnauba added incrementally increased the
hardness geometrically, going from hardening to cracking the beeswax
with only a slight increase in the amount added. We therefore hesitate
recommending its use as a hardener.
Pigment as a hardener. However,
a large contributor to the hardness of encaustic paint is the pigment.
Some pigments make the paint film fairly soft, others make it quite
hard. We did an 8-year test of several colors and were able to determine
that, while they all go through a curing (hardening) process, that rate
differs among the various colors.
Yellowing effect of damar. The
yellowing effect of damar on encaustic that you mention is noticeable
mostly on the unpigmented medium. We've done tests in which we placed
two sets of panels in different light settings to see the affect that
light had on them. One set of panels was coated with white
filter-bleached beeswax. The other was coated with encaustic medium (9
parts filter-bleached beeswax to 2 parts damar resin by weight) which is
slightly yellow. The films were drawn down to a 10 mil thickness. One
panel from each set was placed in a window sill receiving indirect
sunlight. Two others were placed on a wall receiving ambient fluorescent
light. The remaining two panels were put in a drawer. After several
months they were compared to freshly coated panels.
Window light. The
beeswax panel in the window had bleached even further than its original
whiteness. The encaustic medium, on the other hand, had darkened and
yellowed. This was due to the heat affect of light that will darken
damar, a fact noted by in papers by René de la Rie from the National
Ambient light. Both the beeswax and encaustic medium panels had changed very little.
Absent light. Both
panels in the drawer had darkened but not yellowed. This is a natural
occurrence with beeswax and is easily reversible by exposure to light.
Microcrystalline wax, in contrast, will darken in light due to oxidation, a condition that is not reversible.