Once you finish your sketch, it's in an extremely fragile state.
Most of your charcoal, pastel and graphite 'dust' that creates
the shades are just sticking to the paper. With time and age, they
start falling off essentially decomposing your sketch. Also,
the paper surface is prone to atmospheric oxidation which can
result in your sketch taking on the brownish tinge. It's quite
important to choose the quality of paper that you wish to sketch
on. Always try to use an acid free paper which can ward off the
oxidation for a long time.
To give your sketch that extra life, it's always advisable to
spray it with a coat of transparent fixative which can help
bond the particles to the paper. These are special fixatives
and require special attention during application.
DONOT ever try with a cheap fixative or apply it from too close.
One careless deed can wipe off months of your effort. I use
a Windsor & Newton artist's fixative and it has worked well for
me.
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