I've done this successfully, twice. But I waited 21 days from the day I was sure she was broody. If she isn't ready, she could kill the chicks you put under her.
Thank you, scratch n pec! The lash egg pictures look very similar to what I have. As long as she's acting normal I'm not going to worry. Thank you for finding the info for me.
I found a strange mass in my nesting box this morning. I'm not sure which hen it came from. It was very hard and rubbery. Graphic pics are below (hopefully they show up). And all are still alive. I would have expected something like this to kill her.