We clean a lot of windows after construction is done. One of the most common questions clients ask us is, “You’re not going to scratch the glass, right?” We’re happy to say, “No, we don’t scratch glass.” But not all window cleaners can say the same. Some know glass can scratch if you use a scraper. Others might know which glass is safe to scrape and which isn’t. But do they know why some glass is okay and some scratches easily? Here’s the explanation: Most glass used today is tempered. Tempered glass is heated during its making, which makes it stronger and safer to use. Here’s something glass manufacturers don’t like to talk about: Properly tempered glass won’t scratch if you use a good razor blade. Tempered glass is very tough! A skilled window cleaner with a quality razor blade can safely clean off paint, adhesive, mortar, and other debris from the glass after construction. Razor blades are the best tool for this job, giving the best results. But here’s the catch: For tempered glass to stay smooth and scratch-free, it needs to be cleaned before it’s heated. Glass manufacturers are supposed to clean raw glass using special machines before tempering it. The process should go like this: The glass is cut, cleaned on both sides, and then tempered. If done right, the glass is strong and smooth, and it can be scraped safely with no worries. The problem is that manufacturers don’t always take care of their cleaning machines. The brushes that clean the glass can get dirty and full of tiny glass bits and dust. When this happens, the glass isn’t cleaned properly. Tiny glass particles and dirt stay on the surface, and then the glass is heated. During tempering, these particles stick to the surface and create flaws. These flaws, called “fabricating debris,” are often invisible. The average person might not notice them. But if an untrained window cleaner uses a scraper on this kind of glass, the flaws can cause scratches. As the scraper moves, it drags the stuck particles across the surface, leaving scratches behind.