Correcting the Damage
After determining that the slide is a good candidate for rehabilitation one must remove the the degraded mountant and the cover glass. Doing so is not a difficult or tedious task but it will require a degree of patience.
Removing the Cover
The first step is to determine a solvent that will dissolve the mountant without unduly harming the specimen. Working with the knowledge that most mountants which discolor to yellow in areas where it has been exposed to the air contain a natural resin, one finds a starting point to identify a suitable solvent. In many cases a comparatively gentle solvent such as the pure gum spirit of turpentine will prove satisfactory. Whatever the mountant and solvent, placing a drop of solvent on an area where the mountant extends beyond the cover slip is the simplest means of determining its suitability. After a few minutes the heretofore brittle mountant will take on a sticky, thick syrup consistency if the solvent is able to dissolve the mountant.
For some synthetic mountants one may find alcohol works well, however, the frequent use of alcohol as a destain in the initial slide preparation may lead to the loss of vibrancy in the specimen. Other options of more rapid action (but more pronounced hazard) may be benzine, xylene, dichloroethane, or chloroform. On the opposite end of the spectrum in cases where more gentle action or greater safety is desired one may employ various essential oils such as the classic, clove oil.
Once a solvent is identified one need only place the damaged slide into a suitable vessel filled with an ample quantity of solvent. If many slides are to be rehabilitated one may choose to employ a large staining vessel and rack. If only a few must be processed a small Coplin staining jar or in a pinch a slide mailer may be used. One should keep in mind that as the solvent acts upon the mountant its strength will be depleted and the time required will be greater.
Above may be seen a few suitable vessels. The plastic slide mailer is useful only for the less volatile solvents and is recommended for no more than one or two slides. The vertical Coplin jar has a volume more than double that of the slide mailer and is well suited for from two to four slides. The horizontal Coplin jar has slots for ten slides but the most that can be recommended is eight. One may of course load each vessel to capacity if one is willing to permit the dilution of the solvent to such an extent that the process will take a week or more.
After a period of from 24 to 72 hours one may expect the mountant to be largely dissolved. At this time one may remove the cover glasses. Occasionally the condition of the mountant upon removal from the solvent will be such that the cover will have come off without the need for additional effort. If a cover glass should remain in place one will need to raise it by carefully inserting a needle or blade under one edge. Once accomplished, surface tension should be overcome and the cover easily removed. If the cover is seen to flex significantly one must forgo removal for the time being. A cover glass support (or more economically a portion of a broken cover glass) should be inserted just enough to maintain a slight lift on one side of the cover glass and the whole then returned to to the solvent.
Provided the slides were well prepared initially and the solvent selected was not undually harsh there should be no risk of sections coming away from the slide or adhering to the cover∗.
Removing the Mountant
With the cover removed one should take care not to permit the surface of the sections to dry. There is liable to remain a significant quantity of partially dissolved mountant on the slip. To remove this, one will require additional solvent. Resist the urge to employ the same solvent that was used previously; it has been rendered too weak and will now contain all manner or particulate contaminants. Depending on the time one has available one may proceed with one of two methods described below.
The fast method requires a slide clamp, a circle of filter paper, a funnel, a ring stand, a beaker, and a pipette. Pour a quantity of solvent into the beaker and placing the filter paper in the funnel position it in the ring stand over the beaker. Grasp the slide with the clamp and holding it in one hand over the mouth of the funnel use the pipette to send a gentle stream of solvent over the slip so that any particulate matter that washes off is caught on the filter paper. Refill the pipette from the beaker and repeat until the slip appears free of undissolved mountant. This method is particularly well suited for use with more profoundly damaged slides or particularly delicate specimens as it affords a degree of fine control and permits the technician to immediately notice any detriment to the specimen.
The slow method is simply a continuation of the previous process. One should use a new vessel and again additional solvent. If one is forced to use the same vessel one must be sure it has been scrupulously cleaned. Take care that all slips are arranged so that any particulate mater dislodged from one slip will not fall upon another. In practice this means that wherever possible slips should be positioned back to back. After a period of three to twelve hours all the solvent should be removed. Rinse the slip then with a pipette of fresh solvent to ensure no particulate contaminants are present on the surface. This method should not be used with powerful or excessively volatile solvents. It is also unsuitable for deeply or contrast stained† specimens.
The next post will treat with re-remounting the slip. If you’re not used to working with large cover slips you’re in for a treat! -K
∗For some subjects it was accepted practice to adhere sections to the cover glass rather than the slide. This is exceedingly uncommon in preparations younger than 80 years or so. Thicker sections are sometimes attached to the cover but such preparations were likely made by students in an effort to salvage an otherwise unusable section, so be cautious with slides that have the signature or a novice. There are methods for handling such slides but that would make a long post longer.
†Of course one could go right ahead and completely destain the specimen. Followed to the extreme this method would provide one with what is in essence a newly prepared slip ready for staining. Again though, I am trying (and failing) to keep this short.