keep them warm under a radiator or what i did was miicrowave a wet towell till its warm and wrap the tin in it. so it gets the heat and moister. all my germed that way 100%
I dunno how much the heat is needed, I mean the dirt gets cold at night, even in the summer, so I think as long as your not freezing cold or any thing your ok. The heat may help, but the key is getting the paper towel to have the right moisture