In article , my_2¢
writes:

.can HNO3 be
"cooked" to a higher percentage (exposed to heat, thus reducing the
water content? I was told that under no circumstances to get rid of
the higher percentage grades as they cost quite a bit more money. But
if they can be reduced, why pay for a higher percentage? I am quite
aware of the "kitchen" methodologies of extraction w/ methylene
chloride and destilation but I always wondered why a higher percentage
was simply not produced via evaporation?

Doing a Google search for fuming nitric acid on
http://www.ch.ic.ac.uk/rzepa/mim/env...itric_text.htm
I got:

100% pure, anhydrous nitric acid is a colourless anhydrous solid. What we call
'concentrated nitric acid' is actually a solution of 68% by weight HNO3 in
water (16M), and is often pale yellow as a result of photochemical
decomposition which gives NO2. By dissolving even more NO2 into the pure
material produces red 'fuming' nitric acid, which is an extremely powerful acid
and oxidising agent using in the semiconductor industry for cleaning silicon
wafers.