I wanted to experiment with printing on a glass plate, but I did not want to re-level my bed each time. The glass I used is LOTS from IKEA, a 30x30 cm mirror tile, 3 mm thick, that I cut to 22x24 cm. My bed had enough play so I could push it down 3 mm. These holders will lower the bed by 3 mm, so the print-head is level with the glass, and hold the glass in place. Print with support (Touching buildplate), and 100% infill. For more on printing on glass, look here: http://umforum.ultimaker.com/index.php?/topic/3404-printing-on-glass/