Ok...I am wanting to have a group of 100+ kids do fossil impressions for an upcoming science night at my school. It will be ages K-5, and want it to be pretty easy, quick, economical. I plan to put out various materials and allow the students to make impressions that they can take home. I DO NOT want casts of the impressions. Need to know a good material to use. That I can get a lot of, that won't dry too fast but will dry enough for the kids to take them home that same night. Any ideas? There so many types of clay and such out there...checking around. Thanks.