There are a couple of separate issues here. First, there is
no evidence to suggest that oil of spike is less toxic than Gamsol, which is a
very pure odorless mineral spirits (meaning that aromatics and other more toxic
fractions have been removed). These highly refined aliphatic solvents are
probably the least toxic organic solvents that are miscible with oil paint. It
is important to realize that there is a very different “feel” under the brush
between “regular” mineral spirits, OMS, gum turpentine, oil of spike, etc. This
may be mitigated, obliterated, or augmented depending on what other components
are added to the diluent.
Spike oil is probably most useful due to its very slow
evaporation rate, this can be exploited for certain technical effects. It is not
without its dangers, though. It’s very slow evaporation rate also means that
using this solvent in subsequent and upper paint layers can allow for the
biting into and disruption of lower layers. Unlike paints like gouache, which
can be rewet many times, biting into and dissolving semi-dried oil paints irreversibly
diminishes film strength. This is not to say that one cannot use oil of Spike
responsibly, but I would restrict its use to ala prima or to paintings with
relatively few and well-dried layers.
As to over use of diluents contributing to a weakened oil
film, think of it like this, while in theory a diluent is not changing the percentage
relationship between pigment and binder, there is a point where pigment
particles are so separated and the binder so attenuated that the resulting
paint film is friable and underbound. This situation is not uniform for all
media. It is important to realize that drying oils are not really great
adhesives and their ability to bind pigments is rather limited. Acrylic
dispersion binders, on the other hand, can be thinned to a great degree and
still adequately bind the paint film (within reason).
As to walnut oil, it produces a slightly less resilient film
than linseed oil. Linseed oil certainly creates the strongest paint film, but walnut
oil may skin less readily, resulting in less wrinkling of thick paint films.
This is not universally agreed on and some blame the paint defects in early 16th
century Italian oil paintings on their use of walnut oil. We at MITRA feel that,
until there is conclusive evidence, walnut oil it is a perfectly reasonable
binder for oil paint.
As far as “less yellowing” much of the current thinking
about this phenomena is that the three major (highest grade and selected for color
stability) drying oils do yellow at different rates, linseed the soonest and
poppy the last, but that, over time, they all yellow to somewhat the same
degree. With this in mind, we feel that it is probably more important to choose
drying oils based on their handling and film forming qualities.
I hope that was of some help.