Wait about 15 minutes using a friend to help out. Then pull it off carefully, let it dry and reinforce it on the outside again with plaster gauze to close up any holes, let it dry thoroughly. Then use non-hardening plastic clay on top of the plaster base to make the mask. Again, use petroleum jelly or use baby oil as a fellow mask maker has suggested on the plasticine as a release agent. I also alternate the colors of paper in between layers so that I can see if I ve covered the whole area evenly.
Extra layers can t hurt, and it just makes it stronger.
The brown grocery bags are thick and when I use it I can cut down to 6-9 layers of mache'. Drying time varies for mache' depending on heat, humidity, and the number of layers. I have found that if I let layers dry and then add more layers, the last drying period is shorter. Finishing the mask -After the layers have dried completely, you can pry the mask from the base. I then get busy decorating, but will first gesso, sand, gesso, until I have it the way I want.