Numismatic Part: Slabs on occasion have something on them that it would be nice to remove. Generally it's some adhesive from a label. Past experience has shown me that isopropyl alcohol is often good for that job. I use the 70% stuff that is also labeled "rubbing alcohol". BUT A WARNING I also know that the 90% stuff will "melt" the plastic. And at the moment I have some. Non-Numismatic Part: So here's a question for the chemists among you. How much distilled water do I have add per unit volume to dilute 90% to 70%?