A temporary wet mount lasts minutes to hours before it dries out. A permanent slide keeps the specimen in a stable medium under a coverslip, so it can last for years, sometimes for well over a century, as old Victorian slide collections show. Making your own is one of the most satisfying parts of amateur microscopy. It lets you build a reference collection of the things you’ve found.
Permanent mounts fall into three families:
- Dry mounts: the specimen sits in air, sealed under a coverslip.
- Aqueous mounts: the specimen sits in a water-based medium, such as glycerine jelly or a PVA-based medium.
- Resinous mounts: the specimen is dehydrated and embedded in a resin, such as Canada balsam, Euparal or a synthetic mountant.
Choosing a method
| Specimen | Usual approach | Why |
|---|---|---|
| Hair, feathers, insect wings, scales, pollen (dry), sand, fabric | Dry mount | Specimens are already dry and stable; air gives good contrast for fine surface detail |
| Pollen, spores, small plant parts, fungal material | Aqueous medium (glycerine jelly or similar) | No need to dehydrate; good for delicate material |
| Soft-bodied small animals (mites, tardigrades, small insects), some soil fauna | Aqueous gum-based media used by specialists | These media clear the specimen without dehydration |
| Stained tissue sections, stained plant sections, blood smears | Resinous medium | Gives the clearest, longest-lasting result for stained material |
| Diatoms, foraminifera, sponge spicules | Resinous media, sometimes special high-refractive-index ones | Shows the fine structure of silica and lime skeletons |
| Insect whole mounts, larger arthropod parts | Resinous (after clearing) or dry | Depends on whether you want internal or surface detail |
Why refractive index matters
Refractive index (RI) describes how much a material bends light. It’s the main reason different media suit different specimens.
- You see a transparent object because its RI differs from its surroundings. The bigger the difference, the more visible the outline.
- Glass slides and coverslips have an RI of about 1.52. Many resinous mountants, such as Canada balsam and synthetic resins, are close to this, which gives very clear images with few distracting reflections.
- But if a colourless specimen has an RI close to the medium’s, it can almost disappear. That’s why unstained transparent specimens are often mounted in media with a lower RI (water is about 1.33, glycerine about 1.47), or are stained first.
- Diatom shells are made of silica with a fairly low RI. To make their fine markings stand out, diatomists often use special high-RI media, with values well above glass (for example, around 1.7 for some products).
Rule of thumb: stain the specimen, or choose a medium whose refractive index is noticeably different from the specimen’s.
Method 1: Dry mounts
Dry mounts suit things that won’t rot or change: hairs, feathers, insect scales, butterfly wing pieces, pollen, sand, microfossils and fabric fibres.
- Clean the slide and coverslip thoroughly.
- Make a cell if the specimen has any thickness. A “cell” is a shallow ring or well on the slide that holds the coverslip off the specimen. You can buy cell rings, or make them by painting a ring of varnish and letting it harden, or by gluing on a punched ring of thin card or paper. Flat specimens can go straight onto the slide.
- Place the specimen in the centre, arranging it with a fine brush or needle. A tiny touch of dilute gum or glue on the slide can stop it shifting.
- Lay the coverslip on top, resting on the cell walls if you made one.
- Seal the edges with a sealant such as clear nail varnish or a microscope slide sealant (see “Sealing” below), keeping it off the viewing area. Let it dry completely.
Dry mounts are the easiest permanent slides to make. The main risks are moisture getting in (causing mould or fogging) and the coverslip lifting if the seal fails.
Method 2: Aqueous mounting media
Aqueous media are water-based. They accept specimens straight from water, so you don’t need to dehydrate them, which makes them convenient for delicate material.
Common aqueous media:
- Glycerine jelly: a gelatine-and-glycerine mix that’s solid when cool and melts gently with warmth. A traditional choice for pollen, plant material and small specimens. It must be sealed or it will slowly take up or lose moisture.
- PVA-based media: water-soluble media based on polyvinyl alcohol, sometimes with lactic acid and other additives. Used for fungi, small arthropods and other specimens. Some versions contain phenol, which is hazardous.
- Gum chloral media (such as Hoyer’s and Berlese types): used by specialists for mites, tardigrades and small insects because they clear soft tissues. They contain chloral hydrate, which is a controlled or restricted substance in some countries and is toxic. Check local rules before buying.
- Diluted clear PVA glue: many amateurs use clear craft PVA glue, thinned slightly with water, as a simple, non-toxic mountant for algae, spores and small plant fragments. Results vary between brands. Some dry cloudy, shrink, or don’t last well, so test a few slides first.
The basic method (for glycerine jelly):
- Warm a small piece of glycerine jelly until it melts (a warm water bath works). Don’t overheat it, as this causes bubbles.
- Drain excess water from the specimen and place it on a warm slide.
- Add a drop of molten jelly on top.
- Lower a warm coverslip at an angle so air is pushed out (as in our guide to bubble-free wet mounts).
- Let it cool and set. Clean off excess jelly from around the coverslip once it’s firm.
- Seal the coverslip edges. With aqueous media this step is essential for a slide that lasts.
Method 3: Resinous mounting media
Resins give the clearest and most durable results, especially for stained material. They don’t mix with water, so specimens have to be dehydrated first, and often cleared.
Common resinous media:
- Canada balsam: a natural resin from fir trees, used for well over a century. It gives excellent clarity, but it can take a long time to fully harden and may slowly yellow.
- Euparal: a resin-based medium that can take specimens straight from strong alcohol without a separate clearing step. Popular with amateurs for insects and plant material.
- Synthetic mountants (such as DPX-type and other commercial resins): used in histology labs; quick-drying and generally stable.
- UV-curing resins and optical adhesives: some amateurs use these because they harden in minutes under UV light. Quality and long-term stability vary between products.
The general workflow:
- Fix the specimen (preserve it), for example in alcohol or formalin, depending on the material. Many amateur specimens are simply preserved in alcohol.
- Stain if needed. For example, stained sections for beginner histology, or methylene blue for some cells. Blood smears are stained and dried; see how to make a blood smear slide.
- Dehydrate through a series of increasing alcohol concentrations, for example 50%, 70%, 90% and then absolute (100%) ethanol or isopropanol, a few minutes in each for small specimens. Moving gradually avoids distortion. Every trace of water must be removed, or the resin will go cloudy.
- Clear (for balsam and most synthetic resins). Transfer to a clearing agent that mixes with both alcohol and resin. Traditionally that’s xylene or toluene, but there are less hazardous substitutes, and clove oil is a traditional option for some specimens. The specimen usually looks transparent when properly cleared. Euparal skips this step.
- Mount. Place a drop of resin on the slide, transfer the specimen into it, and lower a coverslip at an angle.
- Cure. Let the slide dry flat (see below).
Lowering the coverslip and curing
- Use the right amount of medium. Enough to fill the space under the coverslip, but not so much that it floods out. It takes practice. Small amounts of excess can be cleaned off later.
- Lower at an angle, touching one edge first, to push air out ahead of the medium.
- Small bubbles in fresh resin often rise and escape on their own, or can be coaxed to the edge with gentle warmth.
- Thick specimens may need coverslip supports, such as fragments of coverslip glass or a cell ring, so they aren’t crushed.
- Weight the coverslip if needed. A small clip or a light weight keeps it flat while the medium sets.
- Cure flat. Store new slides flat, in a warm, dust-free place. Synthetic resins may be firm within a day or so. Canada balsam can take weeks or months to fully harden at room temperature; gentle warming speeds it up.
- Clean up excess medium once it has firmed. Scrape gently with a blade, then wipe with a little of the appropriate solvent.
Sealing (“ringing”) the coverslip
Sealing, called ringing in traditional microscopy, protects the mount from air, moisture and evaporation. It’s essential for dry and aqueous mounts. Resin mounts usually don’t need it, though it can tidy the edge.
How to do it:
- Make sure the slide is clean and the mounting medium has set.
- Apply a thin, continuous band of sealant all round the edge of the coverslip, overlapping slightly onto both the coverslip and the slide. A fine brush works well. A slide-ringing turntable, where you spin the slide and hold the brush still, gives neat round rings on round coverslips.
- Let the first coat dry, then apply a second for a more reliable seal.
Common sealants include clear nail varnish (convenient, though some brands crack or yellow over time), commercial slide sealants, and traditional ringing cements. Whatever you use, check it doesn’t dissolve or react with your mounting medium.
Labelling and storing slides
- Label every slide with what it is, the date, the location if collected, the stain and the mounting medium. Many people use small self-adhesive slide labels at one or both ends. Pencil on a frosted end also works.
- Keep a notebook or spreadsheet with a number for each slide and more details.
- Store slides flat, especially new ones and any in balsam or glycerine jelly, which can creep slowly if stored on edge. Flat slide trays or cabinets are ideal.
- Keep them cool, dry and dark. Light can fade stains, and humidity encourages mould on dry mounts.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Resin mount looks milky or cloudy | Water left in the specimen or medium | Dehydrate more thoroughly; use fresh absolute alcohol; work in dry conditions |
| Bubbles under the coverslip | Coverslip dropped flat; too little medium; medium too thick | Lower at an angle; use a little more medium; warm the medium slightly |
| Medium retreating from the coverslip edges | Too little medium; solvent evaporating as it cures | Run a little more medium in at the edge; use enough medium at the start |
| Specimen invisible | Its refractive index is too close to the medium’s | Stain it, or use a medium with a different refractive index |
| Specimen shrunken or distorted | Dehydration was too fast | Use more gradual alcohol steps |
| Crystals forming in the mount | Medium unsuitable for the specimen, or contamination | Try a different medium; keep everything clean |
| Glycerine mount drying out or moving | Edges not sealed | Clean the edges and ring with sealant, two coats |
| Stain fading | Light exposure, or a stain not suited to the medium | Store in the dark; check stain-medium compatibility |
| Mould on a dry mount | Moisture got in | Dry the specimen thoroughly before mounting; seal well |
Some flaws in finished slides come from the preparation itself, not the specimen: bubbles, stain precipitate, folds and debris. Learn to spot them with our guide to microscopy artifacts.
Safety
Several chemicals used in slide-making are hazardous. Read the safety data sheet for each product before you start.
- Xylene and toluene are flammable and harmful if breathed in or absorbed through the skin. Use them only with good ventilation, away from flames, wearing suitable gloves. Many amateurs choose a less hazardous clearing agent or a medium like Euparal that avoids clearing.
- Alcohols (ethanol, isopropanol, methanol) are flammable. Methanol is also toxic.
- Chloral hydrate and phenol, found in some aqueous media, are toxic and in some places legally restricted.
- Formalin, if you use it as a fixative, is toxic and a known health hazard. Many amateurs use alcohol instead.
- Work on a protected surface, keep chemicals clearly labelled and away from children and food, and dispose of solvents according to local rules, not down the drain.
Where to start
If you’re new to permanent mounts, start with the easiest wins:
- A dry mount of a feather barb, a butterfly wing scale or some pollen, sealed with varnish.
- A glycerine jelly mount of pollen or a moss leaf.
- A stained onion skin, dehydrated and mounted in Euparal or a synthetic resin.
From there you can move on to more ambitious projects, such as sponge spicules mounted in resin or a collection of tardigrades from moss.