Cut egg cartons into 2-inch pieces and place into a large bowl.
Cover completely with hot water and soak for 1 hour.
Blend soaked pieces and water until mushy, about 2 minutes.
Strain through a fine mesh sieve and then through cheesecloth. (Don’t squeeze too hard, you still want some moisture).
Mix strained carton pieces with glue, oil, flour, and salt until smooth and not sticky – like a firm but moldable clay (this may take some trial and error depending on different variables like how much water might be left in the cartons. If still sticky, add more oil. If it’s too soft and mushy, add more flour).
Now your clay is ready to create whatever sculpture you wish!