Ask them to spray the inside of their bottle with a very light coating of vegetable oil. Students should then spray the item and the top of the sand with oil. They should put about six ounces of plaster into a mixing bowl, slowly add water, and stir with a tongue depressor until the mixture is the consistency of pancake batter. Be careful not to add too much or too little water. Have participants pour all the mixed plaster into their liter containers, covering the first item.
Students can smooth the layers by gently jiggling the bottles back and forth.
Be sure that students clean out their mixing bowls to avoid getting dried plaster in the next batch. Allow this layer to dry for 5-10 minutes. Have participants continue this layering process until all four items are buried. Allow 24 hours for complete drying. Have students gently remove their plaster layers one at a time, pouring the sand layers out as they go.