Neutral buffered formalin
Simple formalin solutions, the same as are used as fixatives, are convenient and effective as preservatives if a few precautions are taken. Chief among these is to change the formalin solution regularly every six months or so. This will allow the tissue to be stored for several years and, as a bonus, it ensures complete and thorough fixation.
Long term storage in simple formalin usually results in formalin pigment and, should the tissue develop this, it will need to be removed before staining. The formation of formalin pigment can be overcome to a large degree by using neutral buffered formalin as the storage fluid, since formalin pigment is produced more rapidly at acid pH, but it is essential that the NBF be regularly changed as formalin pigment may still be formed on long term storage if it is not.
Long term storage in formalin may cause some relatively minor changes to the staining. Basophilia may be reduced, and eosin may not be as avid. These can usually be overcome by increased staining times. These effects may be aggravated by failure to change the formalin regularly.
Although 10% formalin variants are convenient for storage, it is quite feasible to use much lower concentrations of 1% or even less. Once the tissue has been properly fixed by being in a 10% variant for some time, the preservative does not need to be so strong that fixation continues, merely strong enough to inhibit bacterial action.