How do I clean dust from an old mirror while preserving writing?

Vinegar and old newspaper and q-tips. Use the q-tips dipped in vinegar to clean in between the writing. Spritz vinegar on newspaper (or a 100% cotton cloth or microfiber cloth) and clean the mirror.

Once you have cleaned it i would box frame it.

I would use a hair dryer to blow the dust off then on really bare places do what Naomie said.

Connie, have you tried those air cans for computer key boards? May help. Do you have a small area where you could try just water? If the lipstick is very old there may not be enough oil in it to resist the water. But if it does have enough oil in it, use the compressed air, gently clean with water (try spraying it and then blot it dry. NO RUBBING) and after it is dry spray it with a finish. Good luck.

What a wonderful memory. I am touched and charmed that you thought to preserve what was possibly just a simple random gesture.

I have no additional cleaning ideas...but agree that you should box frame it.

Thank you all for your suggestions!! I will try them out 😊

Hope you are successful in cleaning it and then presering it. Nice!