We stain peripheral blood smears with May-Grunwald-Geimsa. But some smears do not properly maintain staining properties during long time maintaining. Especially after about two years, they loss basophilia of nucleus and we only see a hollow space in location of nucleus; however, cytoplasm maintain its staining characteristics for longer. Indeed, this occurrence does not happen for all slides. We also have slides stained more than ten years ago but despite it they have good staining characteristics

We coverslip all of our slides that we intend on keeping long term. 

Posted August 28, 2016
The easy solution at the question, reported by colleenbennett, on the usefulness of cover the blood films with glass coverslips, not is ultimate because blood smears are preserve better without using coverslip. For storage stain peripheral blood smear slide a non-aqueous/permanent mounting medium (Entellan, DPX, Canada Balsam, and Neo-Mount) is recommended. As soon as the slide is completely dried, a few drops of a non-aqueous/permanent mounting medium are brought up on the preparation. Avoid air bubbles under the cover slip when the slide is covered. After drying time of 20 - 30 min, the smear can be examined under the microscope and stored in an archive. The so preserved preparation remains color stable for a minimum of 10 years. I have realized a permanent slides adding a drop of Canada balsam on the slide and adding a coverslip. Warm the slide to help the Canada balsam spread. Store flat and allow to harden over several days. We used for stain thin smear peripheral blood a quick procedure in which after fixing step, 1 min. in fixative solution (dissolve 0.002 g di Fast green in 1000 ml of absolute methanol or in denatured alcohol), the slide is rinse in tap water and dipped for 45 sec. in eosin G buffered (dissolve 0.95 g Yellow Eosin in phosphate buffer 0.2 M at pH 6.6 [dissolve in 300 ml warmed dH2O, 12.52 g Na2HPO4·2H2O, and 12.5 g di KH2PO4, check pH at 6.6, and dissolve 1 g NaN3 and make to 1 liter with dH2O]), then rinse the slide in tap water and dipped for 15 sec. in methylene blue-azure A buffered (dissolve 1.6 g methylene blue and 1.0 g azure A in phosphate buffer 0.2 M at pH 6.6 [dissolve in 300 ml warmed dH2O, 12.52 g Na2HPO4o2H2O, and 12.5 g di KH2PO4, check pH at 6.6, and make to 1 liter with dH2O]). Rinse the slide in tap water and allows to stay in vertical position for air dry.