I bumped into this , don't ask me how , I don't know anymore :-)...
PREVENTING DAMAGE TO ORIGINAL ARTWORK ON PAPER FROM AGING AND CONTAMINATION

There are two main enemies of original art on paper during long-term
storage or display: Chemicals that are CONTAINED IN the paper, and
chemicals from OUTSIDE objects that come in contact with the paper.

THE ENEMY FROM WITHIN is the remaining acidity or alkalinity from the
manufacture of the paper, or the build-up of acidity due to the aging of
materials in the paper. Nasty chemicals are often used in the making of
paper products from wood and a few other fibers. Some of these
chemicals, as well as parts of the plant material, can remain in the
paper, causing it to chemically change with age and turn yellow or
brown, due to a buildup of acidity.

Ideally, you want paper as close to a pH neutral condition as possible.
That means that it's not acidic or alkaline. A paper that is pH neutral
or has some alkalinity will tend not to discolor with age. Some papers
are buffered, meaning that they have alkaline chemicals added during
production that tend to offset any acidity that might build up with age.

Some papers are known as 100% rag. They are made from fibers other than
wood, such as cotton, wool, flax, synthetic fibers, and more. Not all
100 percent rag papers are completely archival, but most are. On the
other hand, some common copier papers made from wood are fairly pH
neutral when new, but assume paper made from wood will become more
acidic with time.

If you want archivability in paper, be sure to find out about it before
buying it. And double-check for balanced pH with a testing device, such
as a pH testing pen.

EXTERNAL CONTAMINATION can come from self-adhesive tape, glue, acidic
paper, humidity, skin oils, temperature, aerosols and ultraviolet or
strong light.

Use only stable, acid-free or pH neutral materials that might come in
contact with or be stored near paper.

The sticky coatings on adhesive tape contain volatile chemicals that can
work their way into the paper and stain it. Plus, the adhesive will
eventually dry with age and come loose, crack or crumble. Never allow
masking tape, cellophane tape, duct tape, drafting tape or any
self-stick tapes on or near the front or back of the art. Also avoid
spray adhesives.

You can use pH neutral cloth or paper tape that has a dry, water-soluble
adhesive. This kind of adhesive will not ooze chemicals into the paper
of the artwork and can be removed cleanly with a moist cloth. Use as
little as possible, and use only on the far edges of the paper.

The best way to secure originals on a backing board is with archival
corners. That way, no adhesive touches the art. Using corners made of pH
neutral paper and secured with archival tape is the best possible
mounting technique. (For added safety, place a sheet of pH neutral paper
between the artwork and the mounting board and/or the matte overlay.)
(In my experience, matte board is usually pH neutral on the white back
surface, but it should be checked for acidity after a few years.)
Archival paper tape can be easily folded for use as archival corners.

DISPLAY COVERINGS: to prevent UV light from aging the paper or fading
colors, use UV light blocking glass (or other high-rated UV protective
hard plastics), as long as any coatings are on the outside, away from
the art. Be sure to clean the clear cover with mild soap and water
before placing it against the art. Avoid direct sunlight and fluorescent
lights. Avoid reflected sunlight, unless using a very good UV protective
covering.

FOR INFREQUENT DISPLAY/STORAGE, Mylar (polyester) is recommended because
of its stability. Polypropylene, polyvinyl acetate, or acrylics (such as
Perspex, Lucite and PlexiGlas) are almost as good as mylar, but should
be replaced every few years and must be kept away from heat. Vinyl, PVC
or other soft plastics must be avoided. The softer and more flexible the
plastic, the easier it is for solvents to leak out. Chemicals evaporate
out of the plastic or accumulate on the surface of the plastic and cause
damage or discoloration to artwork as they soak into or chemically
combine with the artwork. Avoid lamination or photo albums with
"magnetic pages."

¶ 10:21 PM