Sorry for the late response .I put my cups in then I poured sand around the tray.Then I mixed a quart of water and mixed quite a bit of Elmer’s glue until I basically had watered down glue so it would completely soaked down into the sand. I plugged it in and set it on low heat for a couple hours and then turned it off and let it sit overnight and by the time I got up the next day it was good and hard I could pull my Pyrex cups right out. It will take 400 degrees with no problems.