Canada balsam

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boreal forest region

Canada balsam, also called Canada turpentine or balsam of fir, is the oleoresin 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. It has poor thermal and solvent resistance.[1]

Uses

Slide of a holotype specimen of a flatworm (Lethacotyle fijiensis) permanently mounted in Canada balsam in 1953

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. Other optical elements can be cemented with Canada balsam, such as two prisms bonded to form a beam splitter.

Canada balsam was also commonly used for making permanent microscope slides. From about 1830 molten Canada balsam was used for microscope slides, then Canada balsam in solution was introduced in 1843, becoming popular in the 1850s.[2] 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.

Canada balsam dissolved in xylene is also used for preparing slide mounts.[3] Some workers prefer terpene resin for slide mounts, as it is both less acidic and cheaper than balsam.

Another important application of Canada balsam is in the construction of the

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;
  • In Buckley's cough syrup.

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. Synthetic resins have largely replaced organic balsams for use in slide mounts.

See also

References

  1. ^ "The Bonding of Optical Elements – Techniques and Troubleshooting". Summers Optical. Archived from the original on February 20, 2007. Retrieved 10 Feb 2009.
  2. S2CID 93268790
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  3. .