OK, here is the situation. We have a Golden Comet hen that went broody on us. My Khaki Campbell ducks have been laying for a month or so now. In order to give her something to do, we put one of the Khaki Campbell eggs under her (yes we have a drake so it could be fertile). Daily we take out any chicken eggs that happen to be under her and leave the duck egg. Today we found she had moved to another box and left the duck egg unattended. The egg was very cool to the touch (50 degrees today). How long can an egg go uncovered before it becomes unhatchable? This is day 12 for the duck egg.