Introduction: Halloween Full-size Bubbling Cauldron Prop

About: There are some things you should just NEVER do.....

Halloween Full-size Bubbling Cauldron Prop

"Double, double toil and trouble; Fire burn, and caldron bubble." (- William Shakespeare, Macbeth). Every Halloween display needs a cauldron, but they are hard to find... When was the last time you saw a cauldron salesman going door to door?

So the only option is to make it! Of course it has to be life- (I mean) death-size and bubbling with green goo!

This creation is a full-size cauldron with a removable clear 6-inch liquid-tight tray on top that can be filled with any liquid you desire and back-lighted any color from below. The tray is plumbed with multiple air outlets that provide a constant stream of bubbles. It sits on a glowing fire 'suspended' from its chain and tripod.

The cauldron consists of:

The Pot (Cauldron proper)

Liquid Tray

Support Base


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Step 1: Paper Mache the Pot (Cauldron)

Photo 1:  The cauldron is based on the shape of a large (~3ft dia) balloon picked up from a party supply store.  The balloon is paper mached with four or five layers of newsprint dipped in liquid starch.  Each layer is allowed to dry before applying the next.  After the paper mache ball is dry the top is cut off and the balloon is removed.

To give the cauldron support and allow space for the back-light and air pump a large plastic trashcan is cut down in height and placed inside the paper mache ball.  The height of the trashcan is selected to allow enough space from the top edge of the trashcan to the lip of the cauldron for the height of the Liquid Tray.  

Photo 2:  In order to provide inconspicuous access to the inside of the cauldron from the outside, a pass-through was provided in the bottom of the trashcan and cauldron.  This was done with slip-joint ABS plastic pipe with fittings that accept threaded plugs so that the trashcan could be partly filled with water in case in the future I ever wanted to use water and dry ice in it.  (Only a slight amount of water to keep the weight down.)

It is intended that the weight of the cauldron will be supported by the base that it sits on.  The cauldron bail / handle is only intended to steady it and complete the appearance.  Securement of the bail is provided by two threaded eyebolts slipped over two u-bolts that pass into the side of  the cauldron.  The u-bolts are tied to the trashcan by 12  gauge sheet metal brackets (framing ties) that are bent to shape and bolted to the trashcan.  This ties the bail firmly to the trashcan but not in a way that could support the weight of the cauldron with liquid in it.  The base does that.

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.

Step 3: Make the Liquid Tray

Photo 1:  Instead of filling the entire cauldron with liquid when used, which is not practical and would be extremely heavy, the cauldron was fitted with a liquid-tight tray.  Even though the tray is fairly shallow it would have to support a substantial amount of weight of whatever liquid was used -- several gallons. 

The tray was constructed of clear acrylic sheet that had a texture on both sides. The texture made sure that if a clear liquid was used it would not be possible to see down into the trashcan.  The bottom of the tray was 3/16 inch thick and the side was 1/8 inch thick acrylic.  The side acrylic sheet was heated until flexible and bent into the circular shape to form the wall.  Small reinforcing triangles (where is a laser cutter when I need one?) were added between the bottom of the tray and the side to give the side additional rigidity.  Vertical 'I-beams' sized to fit down inside the trashcan were added under the bottom of the tray to increase its strength.  The tray was assembled using Methylene Chloride cement to create solvent bonds fusing he pieces of acrylic together.  

When the tray was installed there was no way to grab it to remove it from the cauldron so two 'finger handles' were added to the tray to be able to lift it out.  They were just tall enough to get a finger or two in and still be located under the liquid level.  These u-shapped 'loops' were made from half-inch acrylic and fastened to the tray bottom at opposite sides.  In addition to being cemented in place these handles were also secured by tapping a thread into them and being additionally secured by a machine screw from beneath the tray. 

Step 4: Add the Bail / Handle and Stand to the Cauldron

Photo 1: The bail consists of a 1/4 inch solid steel rod that has threads cut on one end. The rod is covered with half inch aluminum tubing. Both were bent together to form the semi-circular bail. The bail was attached to one side of the cauldron to the eyebolt by a threaded union. Since the other side could not be threaded into the union (I suppose a turnbuckle could have been used but I did not want to have the look of a turnbuckle as part of the bail) so I ground down about an inch of the rod to a smaller diameter and slipped it into the union then drilled a hole through both and installed a cotter-pin to secure it in place. 

Photo 2: The stand is just a plastic flower pot that had the bottom cut off. A circle of plywood was cut to match the diameter of the flower pot and the sides were duct taped to it to give added strength. V-shaped openings were cut up the sides to allow 'fire light' from a red incandescent lamp that would be located inside the base to shine out onto the logs stacked around it. The stand is intended to carry the majority of the weight of the cauldron and liquid. But the bail also needs to be used to steady the cauldron as the liquid tray makes the cauldron top heavy.

Photo 3: Alternate base is a cut off top of a black 5-gallon bucket. A hole is drilled through the back for power and also to connect a string of orange Halloween lights that are used behind stacked fire wood.

Step 5: Form Tray Wall to Meet Lip of Cauldron

Photo 1:  When the tray was installed there was a small gap of varying width between the lip of the tray and the lip of the cauldron (can be seen in step 4 photo).  To close this gap the acrylic side wall of the tray was heated with a propane torch until soft and bent over into contact with the lip of the cauldron.  It was held in position with masking tape until it cooled completely.  If you try this, use thick heat resistant gloves for this step.

Step 6: Add Tray Bubbles

Photo 1:  View looking up through the tray bottom from underneath.
To provide the bubbling 'goo' the bottom of the Tray had a multitude of air holes that would provide a constant flow of air (and resultant bubbles) from the aquarium air pump located in the bottom of the trashcan.  Elbow connectors for the vinyl air tubing were made by taking solid half-inch acrylic blocks and drilling two holes at 90 degrees so that they met inside the block.  One hole was about a 1/16 of and inch in diameter for escaping air and the other large enough to accommodate a small acrylic tube that the vinyl tubing would fit over. The small acrylic tube was glued into the block making sure to keep the air path open.

Photo 2:  Bottom view of tray.
Holes were drilled at strategic locations throughout the bottom of the tray and the blocks lined up and cemented into place.  The vinyl air tubing was connected to all the blocks and run back to two multi-output valved aquarium air manifolds.  These allowed connecting all the air lines together as well as providing some fine tuning of the air flow to insure that all bubble locations were bubbling equally.

Photo 3:  Testing the Liquid Tray.
I originally thought that I would use Karo corn syrup in the tray thinking that the thick viscous liquid would provide some larger slow forming / popping bubbles, but it really did not perform very well.  It was sticky, an added expense, and hard to clean up.  I later settled on filling the tray with water along with powdered corn starch mixed in.   The corn starch added opacity to the water so that it diffused the bottom back lighting and made it so the bottom of the tray and 'finger handles' could not be seen.  The tray was only filled up about half way when in use.

The sides of the tray were spray painted flat black to match the exterior of the cauldron.

Photo 4:  The finished effect with air running and back lighted from below.  The back light can be any color in case you prefer your goo a color other than green.  Color changing lights would also be an option if desired.

Step 7: Set Up and Enjoy

To set up the Cauldron the base is positioned on a firm level surface and the red low wattage (25W) bulb is installed in a porcelain socket in the base (not shown) which is plugged into an extension cord. The cauldron is set into place on the base and the bail / handle attached with some tension to a chain or rope descending from a secure attachment point above, to add stability. 

A tripod can easily be made by taking 3 sticks of metal EMT electrical conduit and drilling a hole through each of them about an inch from the end. Lay the conduits next to each other and thread a bolt through them and secure with a nut. The 'legs' of the tripod are then spread out and flared a little, bending the bolt somewhat. The conduit is spray painted black.

An extension cord is run up through the base and through the bottom of the cauldron to power the green light and air pump.

The vinyl air tubing from the tray manifold is connected to the air pump and the tray is laid into place.

The air pump is energized and the tray filled half way with water. Corn starch is added and mixed in until it is nice and cloudy.  The lights are turned on. I found that during daylight the green light is not bright enough to make the goo look green so I added some green food coloring to the water/corn starch mix.

There you have it a bubbling cauldron of green goo ready for your Halloween visitors!


PS - Take a look at my fog cauldron

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