Water Glass and Glycerin Mountant for Microscope Slides
Richard H. Zander
Res Botanica, Missouri Botanical Garden

September 5, 2013

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WATER GLASS AND GLYCERIN MOUNTANT FOR MICROSCOPE SLIDES

 

Those who cannot stand the tedium of heating slides to melt glycerin jelly may try a water glass and glycerin mountant (WGG). This seems to work okay, but is not suitable for immediate mailing or other disturbance as it does not dry (solidify) as fast as glycerin jelly. It will probably last as long as any glycerin mount, even if the water glass separates out somewhat and crystallizes slightly around the margin of the cover slip. My WGG mounts have lasted three months so far, and show no signs of damaging the specimen, drying up or disintegrating.

 

2 parts water glass solution (sodium silicate solution 40-42 Be)

1 part glycerin (glycerol) mixed with a little water to help it dissolve in the water glass

 

Mix and stir well.

Put in capped squeeze bottle or dropper bottle, but not a glass-stoppered bottle.

 

Soak specimen in water or 2% KOH solution or water and Aerosol solution.

If cells are large and thin-walled and apt to collapse, now add a drop of pure glycerin and heat to force glycerin into the cells, but this is usually unnecessary even for species with moderately large laminal cells.

Then add a few drops of the WGG over the moist specimen, add cover glass. Use one or two drops more than you might think to avoid formation of air pockets because nearly half the solution is water and it will shrink overnight. That half the solution is water is good, since the glycerin then has less osmotic force over time and the cells collapse less freely.

 

The solution will dry fastest around the edges and hold the cover slip on tightly in a couple of days. The solution is basic and moss plants may show interesting, strong, and characteristic color reactions similar to those with KOH solution.

 

The high index of refraction and tinting of leaf cells may allow easy anatomical analysis of Sphagnum species without staining. If a stain is needed, however, only basic stains are useful in the rather basic water glass solution. Safranin O, for instance, works fine. Toluidine blue, orcein, and methyl green precipitate out.

 

If the specimens have collapsed leaf cells after mounting, put the slide on a hot plate until the liquid under the cover slip just boils. I use a coffee cup warmer of the wall plug in type (it’s hotter than those that plug into your computer). Works fine.

 

In time, a brownish deposit may accumulate just within the margins of the cover slip, as water glass migrates to the edges and precipitates. This leaves a perfectly clear large central portion of the cover slip that is mainly glycerin held fast by the peripheral water glass. The user must decide if the very clean-looking glycerin jelly mounts are worth the greater trouble of preparation than the quick and perfectly useable but sometimes less presentable water glass-glycerin mounts.

 

For more discussion see:

 

The Use of Sodium Silicate as a Mounting Medium. Charles W. Creaser and William J. Clench.

Transactions of the American Microscopical Society, Vol. 42, No. 1 (Jan., 1923), pp. 69-71,

available by searching the Web, particularly Google Scholar.

 

Creaser and Clench’s paper advises far more water glass (12:1 water glass to water) than I suggest above, but I have found the medium is then too harsh. The present 2:1 method uses water glass to hold the glycerin to the microscope slide, while Creaser and Clench’s 12:1 method uses glycerin to keep the water glass from crystallizing. Those who find that the WGG formula of 2:1 results in slides that are too syrupy might use 3:1.

 

Note, added November 22, 2013: For those who weary of trying to find a decent mountant that hardens (there apparently are none), try this:  Soak your plant in water or 2% KOH solution. Add 3-4 or more drops of glycerin. Heat to drive off most or all of the water over a coffee warmer (the wall plug-in kind is hottest). Put Elmer’s Washable Clear School Glue around the cover slip margins. Don’t leave anything heavy on the slide since it is easily busted or squashed. You’re done.

 

The Elmer’s clear glue is apparently mostly or all polyvinyl alcohol in water. It is used to hold the cover slip in place, not to prevent the glycerin from evaporating. Heating the glycerin plumps up even the most osmotically sensitive species. The index of refraction is very high and cell details are superb. Few or no bubbles are evident after heating, while glycerin jelly often seems to retain any bubbles from heating. The mounts do not shrink because the water is already evaporated, while water glass-glycerin shrinks greatly as the mount dries and often leaves a residue of crystals around the edges of the cover slip.

 

Because the glycerin mountant and polyvinyl alcohol glue stabilizer are both soluble in water, when the glycerin does begin to dry up, the slide is easily soaked or steamed to remove the cover slip, and the specimen remounted. One can, in fact, dunk the whole slide or that portion with the cover slip into diluted (half water half glue) Elmer’s clear glue, and let it dry. The glue covers the cover slip in a thin flat optically transparent film, and does not restrict casual examination.

 

Note Dec. 6, 2013:

Adventurous persons might try out a new variant:

 

A 1 to 1 mixture of glycerol and Elmer’s Clear School Glue (apparently a thick polyvinyl alcohol solution) works fine. The index of refraction remains high. When water in the glue is evaporated it makes a semi-solid mount. If the cells collapse, heat on a hot plate or cup warmer as with pure glycerol. Make sure the glue and glycerol are well mixed (best to let stand a few days) or the slide will seem to “weep” glycerol after a while.