Given the exact same artwork, hanging in the exact same lighting conditions , matted/mounted in the exact same way, bar the glass, what difference, in years, would 99% vv 92% make?
We've been all around that question in previous threads, Robo. In a nutshell, the answer depends on the artwork and all factors of the environment.
Your question is something like asking how long a particular suit of clothes would last, compared to another suit. Or how long a particular chair would last, compared to another type of chair. Experts in each field probably could say confidently which of the two choices would last longer in identical conditions of use. But how much longer? Who could say?
Whether we're talking about picture glass, or suits, or chairs, or any other durable goods, the implication of such questions is that the better quality might not be worth the extra price. In some cases that must be true, but it probably is not true generally, and it certainly is not true when longevity is an issue. All questions aside, how is it wise for any seller to recommend buying a lesser product?
99% is obviously almost maximum, but isn't 92% still 'high'?
Well, 92% UV blocking is higher than 91%, but 93% is still higher. When blocking UV radiation matters in preservation framing, has anyone suggested that 92% is 'good enough'? As far as I can tell, preservation authorities these days consistently recommend 97% or better blocking in the range of 300 to 380 nanometers. For that, compare products such as
Schott MiroGard Protect and
Museum Glass.
Has anyone in the industry, other than ArtGlass and Claryl, suggested that blocking as much UV radiation as possible is not important in preservation framing? If they offered a 97% to 99%UV filtering product, they would probably recommend it.
When preservation is not a goal of the framing, then blocking as much UV radiation as possible is not an issue. In that case, ignore the UV recommendations and buy non-UV blocking glass. Compare prices for
ArtGlass, MiroGard, Claryl, and AR Glass. Leave Schott MiroGard Protect and Museum Glass out of that comparison, because none of those products is comparable to any 97%+ UV blocking glazing.