There are a number of ways to preserve pencil drawings. This question is explicit in distinguishing that the drawings were purchased, which introduces another consideration.
A common way to preserve pencil drawings is with a fixative. How can I protect my pencil & charcoal drawings once complete? goes into recommendations and discussion on that ...
Using a fixative, as described in previous answers here, is a standard way of protecting one's own work. On an acquired piece, you're free to do whatever you want with your own property. However, it would likely negatively affect the market value or historical value of the piece. On a valuable or historical work, fixative might be viewed similar to "...
not sure if that is the best option, but on dry chalk paints I use common/regular hair lack in rattle can.
It does fix the last chalk layer enough, dries fast enough, and do not smell that much as synthetic lack I use for car repairs etc.
As it dry really fast the next chalk layer can be made within seconds.
However repairs to previous layers are a bit hard.
Welcome to the community!
I don't know if you want to do this, but there's something called "fixative spray".
In Walmart for $6 you can buy it, it's so effective, you can even edit after spraying.
Just let it rest after 1 hour and you'll be fine! That's the solution! the substitute is hairspray, that's a cheaper option, but you won't be able to ...
There's this thing called "fixative spray" this spray allows you to edit after spraying!
From my experience, this spray is the best, it strongly protects your drawings from smudging and yellowing! I really recommend it! hopes this helps! luck!