Sheesh, I'm nearly 70 and I barely remember Canada balsam! It's a
wonderfully sweet-smelling resin. Lillie described it as almost pure
abietic acid. It would destroy most stains within a few years -
neutralization of the balsam with potassium carbonate was said to
prevent this, though I don't see how.
Canada balsam took at least a month to set solidly enough that you
could file the slides. This "curing" could be speeded up by putting
the slides in flat trays in a 37 degree C incubator for a week. At
Johns Hopkins an ancient walk-in bacteriology incubator was still in
use - I loved to go in there because it smelled so nice. The synthetic
mounting medium in use around 1965 also benefited from a few days'
curing, so the incubator remained in use.
Around 1970 I took the coverslip off a 1932 Levaditi silver stain of
syphilitic infant liver, so I could replace it with a coverslip thin
enough for me to photograph the little black spirochetes under an oil
immersion objective. Took a week in warm xylene to get the thing to
Canada balsam is still around - look at a can of orange soda and
you'll see it contains "glycerol ester of wood rosin". Yummers!
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