[Histonet] Old control blocks

[Elaine allison Hoffman]

 Hi everyone,
Just want to put a question out there. . . . 
How long can we keep control blocks (paraffin embedded tissue) before they go bad?  I just went through our old control blocks in our filing container and some of our control blocks have dates as far back as 10 years ago.  They have not been refrigerated or anything but didn't know if they should be thrown out and start over or how long they are good for?  They are filed in alphabetical order, anything from amyloid controls to yeast control blocks and many others in-between.  Or probably should be tested to see just how positive the staining results turn out to be.  I was just wondering if anyone knew off the top of their head, lol.  
Another brain teaser....
Our cryostat is displaying a temperature of -23 degrees but the thermometer sitting inside the chamber is reading -15 degrees.  The thermometer was just recently purchased and does not need calibrated or anything.  Our supervisor thought we should have a thermometer inside the cryo-chamber just to be certain that the digital display is accurate.  Now we have a dilemma!  Did anyone else experience this problem with the temperature readings with their cryostat?  Not really sure what to do about the different readings.  What should the correct temperature be anyways?
We would really appreciate any feed-back or suggestions.
Thank you,

Elaine Hoffman, HT(ASCP)
Steward Trumbull Memorial HospitalWarren, OH