Files in the scratch area are never backed up and the disk may be used as a backup disk if some other hard disk gets broken. So never save anything irretrievable in the scratch area. Old files are deleted automatically when the free space on the scratch disk is getting low. The file deletion logic doesn't check file modification times -- it remembers how long a certain filename has existed in the scratch space.
To keep the area clean, the usual mkdir-command cannot be used to create user directories in the scratch area. Scatch-directories are created automagically when your account is created. If for same reason you don't already have a scractch directory, mail guru@nikusla.hut.fi, so that the administrators can create one for you.