This clouding is due to an incompatibility between the specimen and the mounting medium. This happens when a non-water based mounting medium (such as Canada Balsam, Euparal or other resin based media) is used with an improperly dehydrated specimen. The water from the specimen is driven out by the solvents of the mounting medium but are not able to mix with the mounting medium, forming bubbles.
Mounting a moist specimen with a non-water based mounting medium is also problematic because the specimen is not properly preserved. The mounting medium prevents a drying of the specimen. Bacterial decomposition can then start to take place.
A clouding can be prevented by first dehydrating the specimen. This can either be done in air (for those that do not shrink) or by placing it into subsequently higher concentrations of alcohol. The step-wise increase in alcohol concentration slowly withdraws water, reducing the possibility of specimen shrinkage. various concentrations of ethyl alcohol (ethanol) can be used for this. As ethanol itself also contains small amounts of water (from air moisture), it is sometimes recommended to use isopropyl alcohol (isopropanol) for the last dehydration step. This alcohol is “dryer” than ethanol as it has a lower tendency to attract air moisture.
If Euparal is used as a mounting medium, then it is possible to transfer the specimen directly into the mounting medium. If other mounting media are used (Canada Balsam, etc), then it is necessary to infiltrate the specimen first in xylene before mounting. Be aware of health risks when inhaling these solvents!