By THOMAS T. P. BRUCE WARREN.
This oil, which I obtained from the fully ripened nut of the _Jugluns
regia_, has so many excellent properties, especially for mixing with
artists' colors for fine art work, that I am surprised at the small
amount of information available on this interesting oil.
Walnut oil is largely used for adulterating olive oil, and to
compensate for its high iodine absorption it is mixed with pure lard
oil olein, which also retards the thickening effect due to oxidation.
The marc left on expression of the oil is said to be largely used in
the manufacture of chocolate. Many people, I am told, prefer walnut
oil to olive oil for cooking purposes.
The value of this oil for out-door work has been given me by a friend
who used it for painting the verandas and jalousies of his house (near
Como, Italy) some twenty years ago, and which have not required
painting since. In this country, at least, walnut oil is beyond the
reach of the general painter, and I do not know that the pure oil is
to be obtained as a commercial article, even on a small scale.
It was in examining the properties of this and other oils, used as
adulterants of olive oil, that I was obliged to prepare them so as to
be sure of getting them in a reliable condition as regards purity. The
walnuts were harvested in the autumn of 1887, and kept in a dry airy
room until the following March. The kernels had shrunk up and
contracted a disagreeable acrid taste, so familiar with old olive oil
in which this has been used as an adulterant. Most oxidized oils,
especially cotton seed oil, reveal a similar acrid taste, but walnut
oil has, in addition, an unmistakable increase in viscosity. The nuts
were opened and the kernels thrown into warm water, so as to loosen
the epidermis; they were then rubbed in a coarse towel, so as to
blanch them. The decorticated nuts were wiped dry and rubbed to a
smooth paste in a marble mortar. The paste was first digested in CS2,
then placed in a percolator and exhausted with the same solvent, which
was evaporated off. The yield of oil was small, but probably, if the
nuts had been left to fully ripen on the trees without knocking them
off, the yield might have been greater. It is by no means improbable
that oxidation may have rendered a portion of the oil insoluble. The
decorticated kernels gave a perfectly sweet, inodorous, and almost
colorless oil, which rapidly thickens to an almost colorless,
transparent, and perfectly elastic skin or film, which does not darken
or crack easily by age. These are properties which, for fine art
painting, might be of great value in preserving the tinctorial purity
and freshness of pigments.
Sulphur chloride gives a perfectly white product with the fresh oil,
but, when oxidized, the product is very dark, almost black. The iodine
absorption of the fresh oil thus obtained is very high, but falls
rapidly by oxidation or blowing. A curious fact has been disclosed
with reference to the oxidation of this and similar oils. If such an
oil be mixed with lard oil, olive oil, or sperm oil, it thickens by
oxidation, but is perfectly soluble. Such a mixture is largely used in
weaving or spinning. Commercial samples of linseed oil, when
cold-drawn, have a much higher iodine absorption, probably due to the
same cause. Oils extracted by CS2 are very much higher than the same
oils, especially if hot-pressed.--_Chem. News._
* * * * *