How to Make Wax Sealed Microscope Slide Preparations

Sandwiching a transparent microscopical specimen between two pieces of flat glass and then sealing them with red sealing wax was a technique used before the general introduction of mounting in Canada balsam. In the 1830s, William Hill Darker perfected this technique by using two beveled microscope slides, which created a narrow channel between the two slides, allowing the gap between the slides to be filled with red sealing wax. The result is a stronger bond between the two beveled slides as compared to early wax-sealed preparations that used plain microscope slides. This article explores Darker’s wax sealed slide technique, and how to go about making your own version.

Perfecting Ways of Making Sealing Wax Slides

Throughout microscopical literature there are references describing how microscopists, using sheets of glass about the size of today’s standard microscope slide, would sandwich thin-sectioned microscopical specimens between them and join both slides together with a narrow bead of sealing wax to make a single unit.

An early description of this practice can be found in Henry Baker’s 1785 book “Of Microscopes, and the Discoveries Made Thereby, Vol. II, Employment for the Microscope” (1). Baker explains how to preserve delicate crystals of alum precipitated on a glass slide by using a second slide, some thin pieces of cork acting as spacers, and sealing wax:

“Being desirous to preserve some exceeding fine star and Comet-like Figures, I fastened another Glass of its own Size upon Slips where they were formed; having first placed thin Pieces of Cork between, to prevent the Glasses from touching, and after all stopped the Ends and Sides with Sealing Wax, thereby to keep out the Air, which I imagined would spoil them…”

Similar techniques are described in Thomas Gill’s “The Technology Repository” (2), and in Charles Gould’s “The Companion to the Microscope” (3), both published in 1829.

Gill was a patent agent in England, and Editor of his annual repository of information, which he began publishing in 1822. Each volume contains, as the subtitle suggests, “Practical Information on Subjects Connected with Discoveries and Improvements in the Useful Arts”. In Volume I, Gill openly states to the reader that the microscope is a “favorite instrument with the Editor” and in Volume III describes the use of sealing wax in his microscopical preparations:

“The Editor, however, by recurring to the method practiced by himself many years ago, in mounting objects for his microscope between two slips of common window glass, now manages to prepare objects placed between two slips of plate glass very expeditiously and yet sufficiently securely, by merely applying a little melted sealing-wax to the juncture between the ends of the glass slips…he then scrapes off with a knife any excess of sealing-wax upon the surfaces of the slips, and thus renders them as readily manageable as the sliders ordinarily used in the microscope…”

Gould explains a similar approach for viewing “animalcules” using two glass slides and sealing them together with wax, while leaving the fourth (top side) open:

“It consists of two pieces of glass fixed with a small portion of sealing-wax between them, and left open at the top. For viewing animalcules, these pieces should be pressed as close together as possible…If a single drop of water is taken up by the small spoon at the end of the forceps, and spread over the orifice, it will run in between the too [sic] glasses, by which means the surface of the water is rendered perfectly even…”

On the next page, Gould tells the reader about a technique to preserve “curious opaque objects” using sealing wax:

“…they may be fixed on some slips of glass with gum-water, and another glass placed over them, cemented together with sealing-wax.”

These common wax sealed microscope slide preparation techniques would be perfected a few years later by William Hill Darker.

The Bevel is in the Details

William Hill Darker was a highly regarded scientific instrument maker in the mid-1800s and was known for his outside-of-the-box thinking. Darker’s unique thinking is on display with improvements made to the wax sealed microscope slide preparation. He used two beveled microscope slides, rather than two pieces of flat glass, which created a narrow channel for the sealing wax to flow into, establishing a superior bond between the slides.

For a comprehensive historical treatment of William H. Darker and other key microscopists in his orbit, see this article by Brian Stevenson and Howard Lynk.

Mr. Darker’s Method

To make one of Darker’s wax sealed slides, John Quekett provides directions in his book “Quekett’s Practical Treatise on the Use of the Microscope” published in 1852 (4). Quekett refers to the technique as “Mr. Darker’s Method”. I have taken the liberty of converting Quekett’s description into numbered steps:

1. “Two slides of equal size being selected…”
2. “…the edges of each (slide) should be bevelled off on a metal plate…”
3. “The surfaces having been cleaned, the bevelled parts are to be coated with a thin layer of sealing wax…”
4. “…when this is dry, a label, if required, may be gummed to the bottom slide, and then the objects…laid on it; if it be necessary to keep them in place, the smallest possible quantity of gum may be applied to one corner;”
5. “…the top plate is now…laid on the specimens, one of the edges is then…heated in the flame of a spirit lamp, and the groove filled with sealing-wax…”
6. “The excess wax may be cleaned off from the edges by rubbing them upon sand-paper…”

The two key elements of Darker’s wax sealed microscope slide preparations are the beveled edges of the slides and the application of the sealing wax. Quekett’s description left me with couple questions: How does one “bevell off” a glass microscope slide using a metal plate, and how is the sealing wax applied to the beveled edges of the slide? Not finding an explanation in the literature, I turned to YouTube and found videos of stained glass makers using an assortment of tools to create a glass bevel, including polishing wheels, belt sanders, and Dremel® tools.

Quekett’s description of Darker’s method mentions the use of sandpaper in step 6, so I tried beveling a standard glass microscope slide by hand using sandpaper.

I started with 320 grit sandpaper; by holding the slide at roughly a forty-five-degree angle, each of the slide’s edges and corners can be beveled. This process was repeated using 400 grit sandpaper to achieve a smoother beveled surface; this created a nice beveled edge.

Microscope Slide Beveling Made Easy

To get around the hand beveling process you can buy beveled microscope slides made by Propper Manufacturing®.

The Propper beveled microscope slides were invented and patented by Irving A. Speelman in 1968 and are sold today as Bev-L-Edge® slides. In the patent’s Abstract of The Disclosure, Speelman describes the unique use for his beveled microscope slide:

“A microscope slide having peripheral sides and edges or peripheral end edges which are bevelled with a bevelled end edge of the slide operative for smearing of a specimen on the face of another companion slide. The bevelled side edges are structured to provide a wavy configuration to facilitate grasping of the slide for the smearing operation.”

The “smearing operation” is for converting a drop of blood into a thin transparent film. You can read the full patent description (PDF).

Each corner of the Speelman beveled slides is mitered, but not beveled like the rest of the edges of the slide, so you still must hand bevel the corners with sandpaper. Also, the mitered corners are two different sizes (large indicated by the red arrows, and small indicated by the yellow arrows), so if you use these slides for wax sealed preparations, make sure the corners are matched before joining the two slides together with sealing wax.

Faux or the Real Deal?

For this project I purchased both faux and traditional sealing wax. Why buy faux sealing wax rather than the real thing? Faux sealing waxes are used today to seal formal event invitations. They are composed of a flexible polymer substance, which when melted, behaves like a traditional sealing wax. Upon drying, faux sealing wax is less brittle than traditional sealing wax, making it ideal for sending through the postal system.

I also purchased traditional red sealing wax from LetterSeals.com. Pictured below is their Waterstones Scottish Bank of England wickless sealing wax, along with a faux candlestick sealing wax, and a Darker wax sealed slide for scale.

Let’s Do This

With the supplies in place, it was time to make the William Darker-inspired wax sealed microscope slide preparations. Following Quekett’s description, I applied melted sealing wax around the perimeter of each beveled slide edge directly from the LetterSeals traditional wax stick. It was difficult to apply a uniform bead of wax around the perimeter of the slide.

Our Scientific Advisor, John Gustav Delly, recommended dipping the beveled slide edges into the wax to achieve a more uniform wax coating. To dip the beveled slides in wax, I took the bottom of a metal breath mint container and used a standard hotplate to melt some traditional sealing wax into it.

The slide dipping technique created a uniform coating of sealing wax around the perimeter of the beveled microscope slides.

Next, a paper label and hand-sectioned pieces of cork were tacked down with a bit of gum arabic as recommended.

With the bottom microscope slide prepped, the top slide was placed over the bottom slide, and both were run through the flame of an alcohol lamp. After some slight clean-up of excess sealing wax around the perimeter using a razor blade, the Darker-inspired cork slide looked close to my original Darker slide.

I noticed that if you overheat the deposited wax in the finishing step, the wax will begin to migrate into the preparation, as illustrated by the yellow arrows in the above image.

Using additional metal breath mint containers, I melted two colors of faux wax sticks and dipped the beveled slides into each one, creating interesting color combinations:

A Baker-Darker Mash-up

Going back to Henry Baker’s method of preserving crystals on a wax sealed slide, I made a similar preparation using sublimed crystals of Pigment Red 1 grown on a beveled microscope slide. Four small pieces of thin-sectioned cork were added to the corners to create a space between the top slide and the crystals on the bottom slide.

Overall, I was satisfied with my versions of William Darker’s wax sealed microscopical slide preparations. Projects like this make one appreciate the level of craftsmanship required for these historic methods. Making Darker’s wax sealed slide preparations required a few modifications along the way, but I would have been unaware of those challenges if I had only read about them in a book.

References

1. Baker, H. (1785), Of Microscopes, and the Discoveries Made Thereby, Vol. II, Employment for the Microscope, London, J. Dodsley.
2. Gill, T. (1829), The Technology Repository: Containing Practical Information of Subjects Connected with Discoveries and Improvements in Useful Arts, London, T. Cadell Strand.
3. Gould, C. (1829), The Companion to the Microscope, London, G. Davidson.
4. Quekett, J. (1852), Practical Treatise on the Use of the Microscope, London, H. Bailliere.
5. Speelman, I. (1968), Bevelled Edge Microscope Slide, United States Patent Office.