One of the basic tennants of mounting in resinous media is dehydration. Whatever water a specimen contains must be removed prior to mounting. The dehydration could be acomplished by evaporation in air, via a series of displacements in alcohol, or foregone by virtue of beginning with a dry material such as paper or hair. The process of dehydration itself is straight forward enough that it’s hard to screw up when applied to much of what a novice is apt to be mounting, but it can happen. Let’s take a look at the effect of excess water on a resinous mounting media.

Whole mount without pressure.

In this example a whole Blaptica dubia cockroach (2nd instar) was killed just after moulting and mounted without pressure in natural Canada balsam. The specimen was cleared in turpentine but not dehydrated with an alcohol series or with any other method. It was cured for some few weeks on a low temperture warming table.
Right away one can tell that something is not right with the slide. The balsam has yellowed as is to be expected with a whole mount of this sort but some smudge appears surrounding the specimen. From the photo it may be hard to tell but that smudge is not on the outer surface of the coverslip, it’s on the inside. Visually the defect appears quite minor. Observed under even the slightest magnification it soon becomes clear that the view will never be, clear, that is.

With c-mount camera and 4x objective on BalPlan microscope.

What has happend is obvious. The moisture that remained in the specimen was greater than could be diffused into the mountant. Of all resinous mountants natural Canada balsam is the most forgiving. Balsam can often diffuse a bit of excess water, but in this case there was just too much. All that water was forced from the specimen as it was slowly displaced by the mountant. Because the displacing mountant is significantly more dense than water, the water is forced against the underside of the cover.
A problem like this is at its most extreme in a whole mount without pressure. In a thin section the water will tend to be, well, thinner. They might be thin enough that one could imagine focusing below them to optically section the specimen and still get some use from the mount. Sadly, the significant difference in refractive index will prevent focusing through the water dropletts under any circumstances. Let this example serve as a warning to every mounter, proper and complete (or nearly so) dehydration is a must with resinous media.