Canada balsam

Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine which is made from the resin of the balsam fir tree (Abies balsamea). The resin, dissolved in essential oils, is a viscous, sticky, colourless (sometimes yellowish) liquid, that turns to a transparent yellowish mass when the essential oils have been allowed to evaporate.

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, and also for making permanent microscope slides.

It is amorphous when dried and it does not crystallize with age, so its optical properties do not deteriorate.^{[citation needed]} However, it has poor thermal and solvent resistance.^[1]

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.

Some uses (traditional and current) include:

• in geology, it is used as a common thin section cement and glue, and is used for refractive index studies and tests, such as the Becke line test.
• in biology, to conserve microscopic samples. The sample is sandwiched between a microscope slide and a coverslip and Canada balsam is used to glue the arrangement together and enclose the sample to conserve it. Some workers prefer terpene resin as it is both less acid and cheaper than balsam. Synthetic resins have largely replaced organic balsams for such applications.
• in optical technology, to glue together optical elements such as two prisms to form a beam splitter, or two lenses;
• to fix scratches in glass (car glass for instance) as invisibly as possible.
• in oil painting, to achieve glow and facilitate fusion.
• in Buckley's cough syrup

Canada balsam is soluble in xylene.

See also

• Balm of Gilead, a healing compound made from the resinous gum of the balsam poplar

References

1. ^ The Bonding of Optical Elements – Techniques and Troubleshooting, Summers Optical, http://www.emsdiasum.com/SUMMERS/OPTICAL/cements/manual/manual.html, retrieved 10 Feb 2009