Natural Resin Mountants

The commonest natural resin used as a mounting medium is Canada balsam. This is an exudate from the balsam fir, Abies balsamea, which is a common tree in a large part of Canada and northern USA. It has a long history of use in optical systems because of its optical characteristics. It was invaluable for cementing lenses together and, of course, acting as a mountant between slide and coverslip. This was because Canada balsam in xylene has a refractive index of 1.524, which is just about the same as glass, ensuring that diffraction of light at the interfaces of the glass is at a minimum. Today it has largely been replaced by synthetic materials.

Canada balsam is originally exuded from the tree bark as a thick syrupy fluid. This is composed of the resin dissolved in a mixture of chemicals, not all of which are suitable for mounting sections. This sticky exudate is collected and the solvents it contains are allowed to evaporate, producing the pale yellow, hard lumps that are needed. If preparing your own mounting media then the purest grade of dried Canada balsam available should be purchased.

Other resins have also been recommended but are not popular. These include gum damar, gum elemi, gum mastic, gum sandarac, and rosin (colophonium). In all cases cleaned, dried gums should be used. If not clean, it may be necessary to dissolve the gums in ethanol first, then to decant the gum solution and evaporate to dryness. These gums all appear to be terpene resins and are related to turpentine.

Formulating Canada Balsam

Dried Canada balsam is dissolved in xylene or toluene, using approximately equal amounts of both. It dissolves somewhat better if the temperature is elevated a little. If so, then the container should be covered to inhibit evaporation. When it is dissolved, the viscosity can be adjusted by adding more solvent to make thinner, or by evaporation to thicken, then filtered through a coarse medium, such as a few layers of gauze. Clean Canada balsam should be used so as to avoid the problem of removing dirt.