Preserving Snow Crystals

It is possible to preserve newly fallen snow crystals, creating one's own snow crystal fossils. These preserved replicas can then be examined by microscope any time, in a comfortable indoor climate, thus obviating the need for doing photomicroscopy in sub-freezing conditions.

Formvar Replicas. A popular snow crystal preservation technique is described by Schaefer and Day for making permanent plastic replicas of crystals. One gram of Formvar (polyvinyl acetal resin) is dissolved in 100 milliliters of ethylene dichloride to make about a 1 percent solution of the plastic. A glass slide is wetted with this liquid after both the slide and solution have been cooled below 0 C. The wet slide is then held in the falling snow. After a sufficient number of crystals have fallen on the slide, it is place in a protected (cold) area until the solvent has evaporated, which takes about five minutes. (The Formvar solution has the interesting property of creeping up over a snow crystal that may rise considerably higher than the depth of the liquid on the slide.) Once the slide is dry, it may be warmed; the water then passes through the plastic shell, leaving behind a replica of the surface features of the original snow crystal. It is sometimes best to let the ice slowly evaporate at cold temperatures; melting can produce enough surface tension to alter some of the delicate features in a crystal. The image at right is from Schaefer and Day.
According to Mason, best results are obtained when the Formvar solution is dehydrated by shaking it up with calcium chloride or phosphorus pentoxide to remove the dissolved water, which otherwise will come out when the solution is chilled and form spurious ice crystals. Also, it is particularly important to use a solution of the right strength. If it is too viscous, small crystals will not become submerged and merely make a crater on the surface; if it is too thin, the solution will run off the slide and not cover a large crystal. Good replicas of natural snow crystals may be obtained with a 1-3 percent solution, but for small crystals less than 0.1 mm in diameter, such as may be produced in laboratory experiments, a 0.1 percent solution can be used.
A slightly different procedure is to catch snow crystals on a piece of black velvet, which can be examined to find particularly interesting specimens. To preserve a particular crystal, one then places a drop of the cold Formvar solution on a cold glass slide with a toothpick, and uses the still-wet stick to gently pick up the chosen crystal. The crystal should adhere to the tip of the stick, and can be placed in the center of the drop. This way one can select and preserve several crystals on a single slide.

Acrylic Replicas. Another effective method uses clear acrylic spray paint, which is readily available in hardware stores. The spray is especially effective for replicating windowpane frost and similar ice structures. The (cold) spray must be applied lightly, since the solvent in the spray can dissolve the ice if too much liquid is present. The best procedure is to precoat the glass slide with the plastic film, place snow crystals on it, and then spray the surface again until the surface is moist. The image at right is from Tape, and was obtained by spraying over a crystal that was placed on a glass slide.