A few threads on this topic already.
As you are doing, you need to test each box to ensure they are unaffacted by base fog.....it 100% depends on their age and how they have been stored, plenty of good advice in the thread, I would have to challenge the opinion that RC paper does not last as long as FB, in the early days of RC papers 70's early 80's or when 'developer incorporated' that was the case, but certainly since the
90's I would expect all our RC products to be as stable ( unexposed ) and fog resistant as the FB products.
Longevity of unexposed papers is really down to storage, yes you can freeze the product and that undoubtedly is the way to ensure they last the longest, but probably impracticle for most users, but normal storage, in a cool, dark and dry environment ( less than 20c ) should ensure many years of life, minimum 5 years and many examples on APUG of 10+ years.
As per the thread contibutors say, ageing papers exhibit lower top end contrast ( and D.Max ) and increased level of base fog.
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