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Making a Permanent Mounted Slide
A drop of pond water under a coverslip is a wet mount, and it has about twenty minutes to live. The water creeps out, the coverslip drifts, air pushes in from the edges, and whatever you found is gone before you can show it to anyone.
A permanent mount replaces the water with a resin that sets clear and hard, sealing the specimen between glass forever. Victorian slide-makers turned this into an industry — there are mounted diatom arrangements from the 1870s that are still perfect — and the chemistry has barely changed.
The reason it works is not just preservation. The mounting medium is chosen to have a refractive index close to glass, so the specimen stops scattering light at every boundary and detail that was lost in glare simply appears. A good mount makes a specimen LOOK better, not only last longer.
Beginner
3 hours
Instructions
1
1
Clean the glass properly
Clean the glass properly
Wash slides and coverslips in detergent, rinse, dip in alcohol and dry with a lint-free cloth. Handle them by the edges only.
Grease makes the mountant pull away into islands as it dries, leaving a permanently ruined slide that looks fine while it is wet. A fingerprint at 400× is a landscape. This is the same lesson as cleaning a TLC plate, and it fails the same way: silently, later, and only after you have done all the other work.
Materials for this step:
Microscope Slides with Coverslips20 pieces
Ethanol - Lab Grade, 95%100 mlTools needed:
Nitrile Gloves2
2
Fix the specimen so it stops changing
Fix the specimen so it stops changing
Soak the specimen in a fixative — formalin, or a simple alcohol-acetic acid mixture — for long enough to reach the middle of it. Thin material needs minutes, thick material needs hours.
Fixing cross-links the proteins so the tissue stops decaying and holds its shape through everything that follows. Skip it and your beautiful section turns to mush in the alcohol. Work with formalin only in good ventilation and gloves — the vapour is an irritant and a sensitiser, and an alcohol-acetic fixative is perfectly adequate for plant material.
Materials for this step:
Formaldehyde Solution (37% Formalin)50 mlEthanol - Lab Grade, 95%200 ml
Acetic Acid (Glacial)20 mlTools needed:
Glass JarNitrile Gloves
Safety Goggles3
3
Dehydrate in steps, never in one jump
Dehydrate in steps, never in one jump
Move the specimen up a series of alcohol baths — 30, 50, 70, 90 and finally 100 per cent — giving it time in each.
Water and resin do not mix, so every trace of water must go before mounting. But dropping wet tissue straight into absolute alcohol pulls water out so fast that cells shrink and distort, and delicate structures collapse. The graded series is slow and it is the difference between a section that looks like tissue and one that looks like a raisin.
Materials for this step:
Ethanol - Lab Grade, 95%500 mlTools needed:
Glass Jar
Stopwatch4
4
Mount without trapping air
Mount without trapping air
Put a single small drop of mountant on the slide, lower the coverslip from one edge at a shallow angle, and let it fall the rest of the way under its own weight.
Dropping it flat traps a sheet of bubbles that you will never get out. Lowering from one edge pushes the air ahead of the advancing liquid. Use less mountant than feels right: too much oozes out and takes weeks to harden, too little pulls air in from the edges as it shrinks. One small drop for a standard coverslip.
Materials for this step:
Microscope Mounting Medium10 mlTools needed:
Graduated Pipette (Mohr)5
5
Dry it flat, and label it immediately
Dry it flat, and label it immediately
Lay the slide flat, somewhere warm and dust-free, for days to weeks depending on the mountant. Write the label as soon as the coverslip is down.
Tilted slides let the coverslip creep and the specimen ends up off-centre or half out. And an unlabelled slide is worthless within a month — you will not remember what it was, where it came from or how it was stained. Species or material, source, date, fixative, stain: five fields, thirty seconds, and the slide stays useful for a century.
6
6
Why the mountant makes things visible
Why the mountant makes things visible
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Materials for this step:
Glycerin (Vegetable)50 ml7
7
Ring the semi-permanent mounts
Ring the semi-permanent mounts
For a glycerol or water mount, paint a ring of clear nail varnish around the coverslip edge in two thin coats, letting the first dry.
The ring does two jobs: it stops evaporation and it glues the coverslip down so an objective cannot drag it. Two thin coats beat one thick one, which shrinks as it dries and pulls away from the glass. Victorian mounters made this decorative — coloured rings and gilt paper — and the practical version is exactly the same thing without the flourish.
Materials
6- 20 piecesPlaceholder
- 800 mlPlaceholder
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- 50 mlPlaceholder
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