Canada balsam, also called Canada turpentine or balsam
of fir, is a turpentine made from
the resin of
the balsam fir tree (Abies balsamea) of boreal North America.
Detailed description
The resin, dissolved in essential oils, is a viscous, sticky, colourless
or yellowish liquid that turns to a transparent yellowish mass when the
essential oils have been allowed to evaporate.
Canada balsam is amorphous when dried.
Since it does not crystallize with age, its optical properties do not
deteriorate.[citation needed] However,
it has poor thermal and solvent resistance.[1]
Uses
Due to its high optical quality and the similarity
of its refractive index to
that of crown glass (n =
1.55), purified and filtered Canada balsam was traditionally used in optics as an invisible-when-dry glue for glass, such as lens elements. Lenses glued
with Canada balsam (or with other similar glues) are called cemented lenses.
Also, other optical elements can be cemented with Canada balsam, such as
two prismsbonded to
form a beam splitter. Balsam was phased
out as an optical adhesive during World War II, in favour of polyester, epoxy, and urethane-based adhesives. In
modern optical manufacturing, UV-cured epoxies are often used to bond lens elements.
Canada balsam was also commonly used for making
permanent microscope slides.
In biology, for example, it
can be used to conserve microscopic samples by
sandwiching the sample between a microscope slide and a glass
coverslip, using Canada balsam to glue the arrangement together and enclose the
sample to conserve it. Xylene balsam, Canada balsam dissolved
in xylene, is also used for preparing slide
mounts.[2] Some workers
prefer terpene resin for slide mounts, as it is
both less acidic and cheaper than balsam. Synthetic resins have largely
replaced organic balsams for such applications.
Another important application of Canada balsam is in
the construction of the Nicol prism. A Nicol
prism consists of a calcite crystal cut
into two halves. Canada balsam is placed between the two layers. Calcite is an
anisotropic crystal and has different refractive indices for rays polarized
along directions parallel and perpendicular to its optic axis. These rays with
differing refractive indices are known as the ordinary and extraordinary rays.
The refractive index for Canada balsam is in between the refractive index for
the ordinary and extraordinary rays. Hence the ordinary ray will be totally
internally reflected. The emergent ray will be linearly polarized, and
traditionally this has been one of the popular ways of producing polarized light.
Some other uses (traditional and current) include:
·
in geology, it is used as a
common thin section cement and glue and for refractive-index studies and
tests, such as the Becke line test;
·
to fix scratches in glass
(car glass, for instance) as invisibly as possible;
·
in oil painting, to
achieve glow and facilitate fusion;
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