The material commonly used is glycerine (glycerol) or a water–glycerine mixture placed as a drop over the specimen before adding the coverslip.

These are used because:

  • They keep the specimen moist for longer than plain water, slowing evaporation.
  • They act as a simple mounting medium, filling the space under the coverslip and protecting the specimen.

So, to prepare a slide that does not dry quickly, you typically place the specimen on the slide, add a drop of water or glycerine (often glycerine is preferred for slower drying), and then gently lower the coverslip onto it.