I think it’s a great pity the commercial houses do not anymore seal slides in a fine varnish, but that’s just me. -K
Ringing a slide, just as often to be referred to as sealing a slide, was once a ubiquitous practice among not only amateur mounters but nearly all microscopists. The motivation for sealing a slide with any cover was preservation. Of course some permanent mounts were not required to last indefinitely and were not commonly ringed, but for some time seemingly every slide which bore a cover was sealed in some manner. Circular covers were ringed. Rectilinear covers were likely to be sealed as well but the methods were more apt to include waxes and methods that were less well known.
If one has had the opportunity to examine slides of significant age, particularly those that were never sealed, one may have noticed that the mountant is no longer the water-clear of a fresh balsam mount. All balsam will exhibit some tendency to darken over time, some more than others. Balsams that have had their volatile oils driven off and replaced with an alternative solvent may yellow more or less depending on that solvent; Benzene more than, Xylene and Xylene in turn more than turpentine. Other factors are at play as well such as the clearer used or the perfectness of the specimens dehydration. Some more simple factors may be at play as well, for example more acidic balsam tends to oxidize to a greater extent than so called neutral-balsam mounts.
Which brings us to the root of all the yellowing: oxidization. The infiltration of oxygen is the cause of yellowing in many mounts, be they damar, balsam or what have you. This is why then one observes yellowing in a mount it invariably encroaches from the edges of an in-tact cover (or an air bubble) and is more pronounced on the limits of the cover. To prevent the encroach of oxygen and retain the refractive index of the mountant it is necessary to seal the cover. Wax, goldsize, varnish, enamel, even rubber and gold leaf have been employed to seal the borders of coverslips with more or less success.
As a mountant cures the volatile components escape from the edges and the mountant solidifies. Initially the solidification of the mountant at the edges results in a coverslip which seems solidly bound to the slip. If an inattentive microscopists should accidentally rack down an objective too far and crash into and break the coverslip of a slide, the microscopist may be surprised to find that the mountant remains somewhat liquid deep within the mount, even years post preparation. The mountant will not generally begin to yellow as a result of oxygen infiltration until such time as the volatile compounds are exhausted. With many mountants the last of those volatile compounds will not be driven off until long after the mountant is solid.
This is the reason one may have heard that rapidly cured slides are more brittle than slides which are allowed to cure slowly at low temperature. A slide cured rapidly at high temperature is able to exude more completely its volatile components because although it must still escape from the edges of the cover those edges remain more fluid (gas permeable) due to the heat applied. For a quick demonstration one may place a drop of balsam uncovered on one end of a slip and a second covered with a coverslip on the other end of the slide. If this slide is left for several hours on a low temperature hotplate one will notice the uncovered drop (which should flatten nicely) is noticably yellowed and the balsam beneath the coverslip remains quite clear.
When sealed the mountant is prevented from venting it’s last remaining volatile components and retains it’s original appearance for far longer. The substances used to seal a coverslip are effective because they are not covered by a coverslip and so very quickly solidify and create a barrier to oxygen and the escape of the remaining volatile components of the mountant. For this reason sealants which are composed of substances very similar, even thinned with the same solvent as the mountant, may still provide acceptable results.
Some mountants (Euparal for instance) will not oxidize over time and do not need to be sealed to preserve their original appearance. The sealing of mounts may also be done for aesthetic reasons only. Layers of sealant in contrasting colors may be built up around the borders of a cover to great effect and a simple black ring may often prevent the inattentive from placing an unlabled slide upside-down upon a stage. In the end, sealing a slide may be a matter of superficial preference or a means of ensuring ones slides endure in superior condition for future generations. In the next installment we’ll take a practical look at slide ringing.