Canada balsam

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. 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. However, it has poor thermal and solvent resistance.

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 prisms bonded 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. 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.

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