No, thats always gonna be a risky one, especially if its been on there a while. Just use plenty of hot soapy water for lubrication and get a bohemian bladed safety scraper and work from the outside in i.e don't just send the blade straight into the mortar/ cement. Have a hose on stand by to completely rinse the whole lot off after a couple of passes to demonstrate to the custy that, this will be the standard. Push into the glass don't be afraid, be firm enough and go in one direction only, forward obviously. Don't use the yellow stanley scraper. Cover your hole with the custy, if they are expecting a miracle they are setting you up.