Atomic Shrimp - More Lost Wax Casting

More Lost Wax Casting - Silicone Moulds

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The Plan

I want to make some more lard lamps, but the lost wax casting process is pretty laborious and materials-intensive, not to mention being frustrating in only producing one of any given object at a time.

The solution is to make some kind of re-usable mould. In the case of the Lard Lamp, which has lots of protrusions, and a hole through the middle, there's no easy way to make it in a one-part re-usable mould, so I plan to tackle this in stages, as follows:

Make the wax form - and from this, make a single-use plaster, clay or silicone mould.

Cast pewter into the single-use mould, to make a prototype metal form

Spend some time and effort finishing the metal prototype.

Use the metal prototype to make a multi-part silicone mould (i.e. one that comes apart to retrieve the casting).

The reason for making a metal prototype first is that it will be much easier to make the final mould based on a hard, durable, fully-finished metal form, that can have the silicone applied firmly to it without fear of distorting it, or leaving voids in the inner surface of the mould.

Am I Overthinking Again?

I might just be making things too complex for myself. Why can't I just...

Make a polymer clay form of my lard lamp (varnishing when dry, if necessary)

Use the rigid clay prototype to make a multi-part silicone mould (i.e. one that comes apart to retrieve the casting).

Pewter has a sufficiently low melting temperature that it can be cast into natural or silicone rubber moulds. But the materials to make these aren't always cheap or easily obtainable... Or are they?

A New Lard Lamp

I made a new wax form for the Lard Lamp Mk 2, learning from some of the mistakes of the first one (seen on the left).

The main difference was to provide it with only three feet, preventing the potential for wobbling, but I also made this one a bit finer and spent more time making it symmetrical.

Instead of a two-part silicone mix, I decided to experiment with ordinary, cheap as chips, silicone sealant (also known as silicone caulk).

At this stage, I had no idea how well this stuff would take exposure to molten pewter.

I added sprue pegs to the wax form, then stood it on a flattened polythene bag on a tray - knowing that the silicone won't adhere to polythene.

I started applying the silicone - it wasn't easy, as it's quite thick and viscous. Even after hardening the wax in the fridge, it was difficult to get the sealant to conform to all the little details of the wax form without distorting it out of shape.

The sealant went on really thick and not very evenly.

I fully expected this to mean that there would be voids and bubbles next to the wax, which would result in flanges and general surface imperfections.

It took a whole week for the stuff to cure - just because it was such a thick, chunky layer.

Melting out the wax was a challenge - I didn't really want to do it in the oven, in case the silicone emitted fumes.

So I settled for immersing the mould in boiling water, in an old food can over my portable gas stove.

The wax was a little reluctant to come out, but a few more repeats of boiling and tipping seemed to get it all.

The wax coming out was very odd in texture - feeling sticky and wet even after cooling. Not necessarily a good sign...

I dried the mould for a few hours next to my hot water tank, then prepared it for casting.

I wasn't completely confident that the mould was completely free of holes, or indeed that it wouldn't just burst into flames when the hot metal hit it - so I set it in a pot of sand (blocking the pouring holes temporarily with tissue to keep stray sand grains out).

As in previous experiments, I melted parts of an old pewter tankard in an empty food can over my camping stove.

It was a messy pour this time - but this isn't really a problem, as the spillages set quickly and can just be re-melted again next time.

It seemed to fill the mould OK though - and didn't spit, flare or otherwise give me much trouble. I left it to cool for half an hour.

The Moment Of Truth

I cut open the mould (this was always intended to be a single use experimental mould).

The casting was a pretty disastrous mess. A huge unwanted lump of metal in between the petals - and a generally poor finish and shape everywhere.

This won't be wasted, of course - it just goes back in the pot for the next melt.

What Went Wrong?

A close inspection of the mould remnants revealed that parts of the silicone - particularly in the middle, at the thickest point, had not cured properly - it was a sort of semi-cured paste, which had been displaced during removal of the wax, filling parts of the mould and opening up voids in other places.

What Next?

Well, this isn't a failure, it's just... data capture. I think this material has potential - The silicone itself can easily stand the heat of the molten pewter - it's just going to need a bit more ingenuity in the application.

So next time, I'm going to try freezing the wax form to make it solid, then applying the silicone in thin layers with a brush.

Comments: 8 (Add)

why don't you try filling a cup with the silicone, and dipping the object into it? you can pull it out, and let it cure, and maybe dip again for another layer if needed. You can seal up your cup with a layer of plastic wrap and a lid, and keep it fresh for the next dip.

Posted by Mona on Mar 26 2012 at 06:22

JP, that's not really the case. Lead is toxic, yes, but you're not going to kill yourself by melting a small amount of it in a tin can.

People routinely melt lead for soldering or to make movable type for printing. Only recently has the lead in these processes been replaced or made obsolete.

Long term, high-level exposure is bad, but you can do it a lot more than once before you're anywhere near a risk.

Posted by Mike (for Atomic Shrimp) on Mar 20 2011 at 23:24

There is only one teeny tiny problem with melting lead, and that's that it produces poisonous fumes when you melt it. So you could melt lead. Once.

Posted by JP on Mar 20 2011 at 22:51

MEK is (I think) Methyl Ethyl Ketone - a solvent for some silicone compounds.

Any low temperature melting metal should be fine for a project like this - tin, pewter, or 'white casting metal' (which is also basically pewter) that you can get from craft suppliers.

As long as it won't fall into the hands of small children, you could even use lead - it's not as if you're going to end up licking the thing after making it. Definitely cast outdoors for lead, though.

Posted by Mike (For Atomic Shrimp) on Mar 8 2011 at 13:21

Please excuse my ignorance but what is "MEK" I'm going to experiment with this...is there anything other than pewter that could be used

Posted by Linda on Mar 8 2011 at 12:18

I am a jeweler and I was taught a cheap way to make a mold was with silicone like you are trying to do but for the first layers of material we would mix the silicone with MEK to make it really thin then paint it on with a brush. Let it dry and do it again over and over again until you get it to the thickness you want

Posted by Blaine on Feb 2 2011 at 04:33

http://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/step3/Mixing-Oogoo/

this may solve your semi-cured problem.

Posted by John on Jan 31 2011 at 15:34

I used it for making moulds of plaster fish, but id use it for pewter with little worry about the heat. Its used to sealing exhaust pipes by poverty stricken car/bike fixers and i suspect the "Hi Temp" silicone i sealed up my stove chimney with is little more that black added to jack the price up... I read on the net that a drop of glycerine can thin it, or acrylic paint, though i haven't tried.
http://jonpenryn.blogspot.com/

Posted by jon on Jan 30 2011 at 20:17

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