I had searched for a vintage mirror for a while, and the only ones I could find were more than I was willing to pay for them. I also was trying to use some of the things I already had. I had a mirror that I purchased from Ikea a few years ago. Here is one that’s similar (I forgot to take a before photo, so I took one while I was in Ikea).
I actually let it set for several hours. Next, I used a scraper and scraped the backing off of the mirror. You will want to scrape until you have basically scraped off the backing and mirror. Just remember, if you scrape off too much, you can’t put it back. So, I started with small amounts. It just depends on how much black, aged look you want. I wanted it to be light and spotty.