Hi Ted,
You really covered all the bases in your question.  IF you protect your shells
from their three arch-enemies (light, moisture, and to a lesser extent, heat),
all of which you alluded to, then they will remain fresh-looking for many
years.  I don't think of fungus as an additional category - if you effectively
protect your shells from moisture, you won't have fungus problems.  Fungi are
living, cellular organisms, and just like us, they need water to grow - not a
lot - just a bit of dampness will suffice, but fungi cannot grow on truly dry
substrates of any kind (Note that dessicated foodstuffs, like flour, cereal,
etc. do not get "moldy" as long as they remain dry.  Moist foodstuffs like
bread and fruit readily become moldy - and so does cereal if you moisten it).
Fungi are problematic in shell collections primarily in warm, humid (typically
tropical or sub-tropical) climates.
There is sort of a little Catch-22 here.  Sunlight (actually the ultraviolet
radiation in sunlight) inhibits the growth of fungi, but it is also
destructive to shell pigments.  Darkness protects pigments, but encourages
fungal growth.  So, since shells have to be protected from light, it is
especially essential to maintain low humidity in order to control fungi.  The
various sealants that have been mentioned (oils, silicone, etc.) help by
forming a barrier against moisture.  A shell should be thoroughly dry before
applying such substances.  If there is water in the pores of the shell when
sealants are applied, you may seal the moisture in, and the moisture itself
will act as a barrier to the effective penetration of the sealant.  Such
sealants should be applied liberally to the dry shell, rubbed (on a smooth
shell) or brushed (on a rough shell) into the shell, and then thoroughly wiped
off the external surfaces.  If applied too lightly, they won't penetrate into
the shell matrix effectively.  If not wiped off thoroughly, they will make the
shells oily or sticky to the touch, leave oily spots in drawers and on labels,
and attract dust like a magnet.  On rough textured or heavily sculptured
shells, where it may be difficult to effectively wipe off the excess sealant,
a fine spray of water can physically remove the excess from the shell's
exterior, without removing sealant that has penetrated into the porous matrix
of the shell.
Heat, as such, is not a problem when shells are stored under normal indoor
environmental conditions, provided the humidity is low.  However, given the
presence of moisture, fungi will grow faster at warmer temperatures (which is
why we refrigerate moist foods).  Also, shells stored under unusually hot
conditions, for example in an uninsulated attic or car trunk where the
temperature may reach 150-200 degrees fahrenheit or higher on hot summer days,
may fade somewhat, even in the absence of UV radiation.
 
Paul M.