Step 3: Sculpt a vase
We're going to take advantage of the fact that in the first few minutes in the slow cooker, the cup both softens and expands. This behavior can be manipulated by applying a constant force to some parts of the cup, making it collapse inwards instead.
Take a coffee cup, and put a rubber band around it near the top. Put it in the bowl, replace the lid, and boil for a couple of minutes (watch it, don't time it!). Remove once suitably shape-changed, and take off the rubber band. Done!
Try putting two bands on, or put an odd-shaped object inside to act as a mold (I ran out of used coffee cups, so haven't actually tried the latter). This project is great for experimentation to see what effects you can achieve.
Tip: get the rubber band as tight as possible. Strong, tight bands will create the most interesting effects.
COASTER19 advises that if you have a pressure cooker, you get best results from looser bands, with the cup upside down, and a 7-minute cooking time.
Valaynetine reports that some sorts of polystyrene cup don't change shape at all. The ones I tried - all the same sort - were a soft, relatively bendy type of cup, so that's probably the style to use.
calischs from InstructablesTV made a short video based on this instructable, and cut the rim off then immersed the cup in boiling water with the help of a metal weight. It seemed to work well. Check it out, it's only 46 seconds long!
rojo.balloon made a styrofoam hat from a bowl.
Maurice1985 made an instructable (and made lots more vases than me!) showing how you can fill these with concrete to make a more permanent vase.