[Histonet] unstained paraffin tissue slides storage

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Hello,
I found here a few disscussions regarding the storage of tissue slides but I did 
not find a clear answer to the questions I have. I would really appreciate an 
answer from anybody that has experience with this.

I need to store for long term a bunch of unstained tissue slides for the purpose 
of doing immunostaining even in a few years from now on. Unfortunatelly they 
were stored for about 3 years at room temperature. What it is 
usually recomended: to store them at -20 degrees Celsius? If yes, is it OK to 
store them in the regular 100 slides boxes? And when you need to start an 
immunostaining just take them out of the freezer and let them at room temp for a 
while before starting the stain or what procedure do you use?

I heard some labs keep them in nitrogen gas containers. Do you have any info 
about this?

Any imput is appreciated!
Thanks!