There are countless different mounting media available, some of them are quite expensive and for very specific uses (eg. fluorescence microscopy), while others are good for general purpose microscopy and suitable for a wide variety of specimens.
Mounting media for microscopy can roughly be divided into those that are water soluble (aqueous) and those that are not (non-aqueous). Both types can either remain liquid or turn solid. Some aqueous and solidifying mounting media do retain some water. Glycerine gelatin, for example, it is even necessary to seal the sides of the cover glass to prevent complete dehydration of the medium. The mounting medium, therefore remains in a semi-solid state. Non-aqueous mounting media (Eukitt, Euparal, etc) have an organic solvent, which evaporates.
Regardless of the mounting medium used, the specimen must be compatible with the mounting medium. Specimens which contain water (and this includes most of the living things), must first be dehydrated in a sequence of increasing alcohol concentrations before they can be mounted in non-aqueous mounting media. If this is not possible, eg. because this would cause the specimen to shrink too much during dehydration, then the specimen must be mounted in a water-based mounting medium. Algae and other water organisms are quite sensitive to dehydration and water-based mounting media are therefore preferred.
While most specimens can be mounted in aqueous mounting media, there are some exceptions. Bird feathers are hydrophobic and repel water. I found it difficult to mount them in a water-based mounting medium. Bubbles formed and the individual barbules (these are the tiny hair) of the feather wanted to stick together to avoid the water.
Mounting a moist specimen in a non-aqueous mounting medium
This is something which should definitively be avoided for long-term stability of the slide. Some non-aqueous mounting media (Euparal) are able to take a small amount of water, but this is not something to be counted on.
An incompatible mounting medium can result in two outcomes.
- First, it results in clouding of the medium. Water droplets form in the medium. These are subsequently trapped in the solidifying medium around it.
- Second, the specimen (not able to dry completely) might start to decompose due to bacterial activity. The bacteria form decomposition gasses, which result in formation of bubbles. The bubbles grow and eventually are even able to separate the cover glass from the slide. The bubbles expand, break the mounting medium and lift the cover glass off the slide.