Step 2: Foam the Cauldron

Photo 1:  The space between the paper mache shell and the trashcan was filled with expanding urethane foam.  This is pour-in-place two-part rigid urethane foam.  It comes in two parts that are mixed in equal amounts by volume to expand to 30 times the original size and sets rigid (final density is ~2lb/ft^3).  The foam was poured in several batches until it was almost up to the lip of the trashcan. 

Photo 2:  A shelf to help support the Liquid Tray was formed by finishing the fill of the foam up to the top of the lip of the trashcan.  To insure a flat surface as the foam fills the void, a 'capping form' of wax paper backed by cardboard was taped into place between the trashcan and the paper mache in order to contain the foam as it expanded.  After each pour of foam around the perimeter of the trashcan the form was relocated for the next pour.  

The top edge (lip) of the cauldron was finished  and strengthened by taking piece of large diameter vinyl tubing and slitting it down the length so it could fit it over the edge of the rough paper mache lip.  The tube was shot full of 'Great Stuff' foam sealant and slipped over the edge of the paper mache and allowed to bond and harden.

After the flat surface for the Liquid Tray was complete it was time to fill the space under the overhang of the cauldron lip.  This was done by building forms as before with wax paper and cardboard that were tapped to the lip of the cauldron and supported from expanding inward by pushing nails into the shelf foam.

To seal the urethane foam from liquid spills it was coated with polyester resin which was mixed with catalyst per the manufacturer's instructions and applied to the exposed foam of the shelf and also over the outside of the paper mache.  In the future I anticipate fiberglassing the exterior to add strength and to resist damage from handling.

The cauldron was spray painted flat black.

Did you have any trouble with the liquid flowing back into the pump and clogging it?
No problems with back flow as the holes in the liquid tray are fairly small and the air pump has a substantial output; made for larger aquariums. However, if you are concerned it is easy to put in an air check valve and prevent any problems. <br> <br>Thanks for the question.
Thanks everyone for the nice compliments!
Congratulations on being a finalist in the Halloween contest!!! Can&rsquo;t wait to see if you win! Good luck!
Very smart! Love this - totally want to create something similar this year, Thanks for the inspiration!! :)
Wow your really talented, It's impressive to see all your work come to life in your pictures. I enjoyed reading how you made this.
Oh like this. I wish I had the patience to undertake this project.
What an amazing illusion! It looks so real!
So cool I wish I thought of that!
this was an excellent instructable!
Hello there, this is a great unstructable! <br> <br>Could you please answer a couple of questions: <br> <br>How do you smooth layers of toilet paper papier mache? <br> <br>How do you make your newspaper papier mache so smooth? <br> <br>Thank you.
Thanks for the question.<br><br>Only newspaper was used to make the shape of the cauldron. If you take a single sheet of newspaper, cut it into strips and dip them in liquid starch (the type used in the laundry for making washed shirts crisp; i.e. starched). when they are pretty saturated with the liquid starch they smooth down quite well. <br><br>When I pull a strip out of the liquid starch I 'squeegee' it through my fingers to remove excess liquid then place it and smooth it down. I have had very good results. There is a optimum time to let the newspaper sit in the starch, too short and ti will not stick or smooth well, too long and it falls apart too easily. Some trial and error works well here.<br><br>You have to be sure not to use too wide of strips as the curvature of the balloon will cause 'puckers' on the sides of the strip since is will not lay flat.<br><br>Good Luck!<br><br>
adding to favorites, this could be really cool if we do another witch scene next year.. Great &quot;ible&quot;
Awesome! Great Halloween decoration!
Thanks! It was fun to make.

About This Instructable


180 favorites

Bio: Of course it won't blow up in my face.....
More by wannabemadsci: Ouch! Screw in Head Costume Build Decorative Lights to add Atmosphere Omelette in a Zip Bag
Add instructable to: