Adventures in Aluminium Lost Wax Casting (Part I)

Plaster Molds after the wax was removed
Plaster Molds after the wax was removed

A friend of mind built a DIY forge to melt aluminum cans and play with casting things. They didn’t mind if I played along… but to cast aluminum you need sand and plaster molds. Thankfully that’s something right up my alley!

Or at least in the same neighborhood. 😉

Previously I’ve made plaster positive casts from 100% Silicone Caulk molds,  but these would need to be negative casts, so I decided it was time to play with lost wax casting.

As always, click for much larger photos!

New Sculptures

Silicone Caulk Molds of Sculpey Sculptures
Silicone Caulk Molds of Sculpey Sculptures

Since I was planning on casting these in aluminum, I needed to create some new sculptures. I wanted something much bigger than the horse magnets so I made a pair of dragons. I wasn’t sure how much detail the aluminum would capture, so I made two different levels of complexity.

(These turned out to be too hard to cast for beginners, so I made a corgi and a cat later that were much simpler forms.)

I made the silicone caulk molds using the same soapy water method as before, but I’ve transitioned to using GE’s Clear Silicone I caulk, since the Lowe’s stopped selling the first brand. As a sidenote, don’t try this with GE Silicone II since there’s something in that mixture that doesn’t work as well.

As always, after making the silicone molds you should wait a few days for them to stop off-gassing before use. Otherwise your house will smell of warm vinegar when you pour in the wax and anyone else living in the house will not be pleased. I have no idea how I would know this. *innocent whistling*

Casting Parafin Wax in Silicon Caulk Molds

ArtMinds Parafin Wax
ArtMinds Parafin Wax

Most of the websites that talk about lost wax casting are also talking about wax carving, which I didn’t do much of. Thus instead of trying to get a sculpting wax or mix beeswax and parafin, I just went with a 100% parafin wax.

This had some downsides. The parafin wax will burn, so I had to make sure not to get the heat gun too close, and it will seep into the plaster molds (as I learned later) and cause air bubbles. It’s also very very soft, so I had to handle the resulting casts carefully so they didn’t deform.

I was able to recapture most of the wax after use, so I’m still working away with the same blocks I started with. Plus parafin is a heck of a lot cheaper than beeswax or sculpting wax, so it was a lot easier to rationalize playing around with! 🙂

Heat Gun Melting the Wax into the Silicone Molds
Heat Gun Melting the Wax into the Silicone Molds

Note: Make sure to work inside some sort of container when doing this. Otherwise you are likely to get even more wax all over the stove, which is easy to cleanup but will leave your kitchen smelling like candles the next time you cook dinner.

Getting the wax into the molds was actually more (and less) complicated than I had thought. My generic draft heat gun was plenty hot enough to melt it easily, although I needed to be careful with the distance so it didn’t start to smoke.

I started off shaving small pieces of wax into the molds and then using the heat gun to melt them, but that ended up blowing the liquid wax around. So I my husband came up with the little tinfoil ramp, which sort of worked. We tried a few configurations of this before I ended up just holding a large piece of wax in one hand and melting it so it dripped into the molds.

Fancy Soda Can Wax Crucible
Fancy Soda Can Wax Crucible

As always, MacGyver mode kicked in and I cut down a soda can, put a spout in it, and then made a handle out of some wire. The wire loops once around the can and then has a bent loop on the top and bottom to keep it firmly in position. This let me hold the can with one hand, melt the wax in it, and then pour the wax into the mold with very little spilling.

I was worried the wax would cool too quickly, but if you warm the mold up with the heat gun before you start pouring it’s not an issue.

You have to wait a while for the wax to set, but once they are opaque they are normally okay to demold. You could also toss them in the freezer once they are at this point, just to make them a little firmer, but since I was planning on smoothing them out I didn’t do that.

Making a Sand and Plaster Cast of the Wax Sculpture

Prepping the Wax Mold for Plaster Casting
Prepping the Wax Mold for Plaster Casting

This is where things got a lot more experimental!

For the first attempt, I was using a pure plaster mix and tried ‘gluing’ the wax casting to wax paper. The first time I simply melted the wax on the back of the sculpture and stuck it down (which came loose) and the second time I melted a puddle of wax on the waxed paper and then stuck the sculpture in that (which stayed).

I was using a small box to cast in, which seemed large enough at the time, but turned out to be much too small. However, it’s a good example of my ‘use anything at hand’ method of making molds. As long as there is a liquid-proof layer between the plaster and the container, you can use almost anything to cast in! 🙂

Plaster Casting of Wax Molds
Plaster Casting of Wax Molds

Because the wax floated in the first attempt and didn’t bond as well as I would have liked in the second, the casts had some plaster between the edge of the cast and the wax. This was easily broken out with a butter knife in both cases, so it wasn’t much of an issue.

The wax paper also peeled off the plaster cast easily, but it had broken down quite a bit and the underlying cardboard box got a little damp. I ended up putting the box in a plastic bag and then putting the wax paper into that for future attempts.

The second round of castings used a much larger box, but with the same grocery bag + liner. This time I used tin foil as the liner and used wood glue to attach the wooden block to it. I had zero flotation problems this time around and once the mold was set I was able to just pop out the wood and wax with a butter knife.

Creating a mold from tinfoil and boxes
Creating a mold from tinfoil and boxes

This meant no wax seeped into the plaster and hopefully that there will be a lot less air bubbles the second time around. *crosses fingers*

These molds also used a sand and plaster mix that I created by mixing the plaster with water as normal, then adding a little more water and a lot of sand. I tried starting with the plaster and sand and adding water to that, but it didn’t seem to set as well. The mixture is 50/50 and although I was using a slightly coarser grain of sand it seems to have captured detail relatively well.

Getting the Wax Out of the Plaster Mold

Melting the Wax Out of the Plaster Casts
Melting the Wax Out of the Plaster Casts

To get the wax out of the molds I used the heat gun to melt it, which worked pretty well, as you can see. However what it also does (which I didn’t realize at the time) is make the wax melt into the plaster mold.

As you can see in the photos, I moved from using just the generic tinfoil pan to putting in a layer of parchment paper. The parchment paper works wonderfully for capturing the wax and holds up to the heat gun without getting too hot.  I shifted to using it as my default surface instead of the bare metal because it made wax reclamation a heck of a lot easier.

The two molds that I made using this method (which are in the picture at the top of the post) didn’t work out as well as I’d hoped in my first casting run. The designs were both too complex and the wax created gas bubbles as it burned out of the plaster.

Popping the wax mold out of the plaster
Popping the wax mold out of the plaster

When I went back to the drawing board I was looking to make something larger, thicker, and also inset so that over-pouring wouldn’t mean we had aluminum all over the ground.

As a happy side effect of this, I also found a way to remove the wax without melting it, so my lost wax process is less ‘lost’ and more ‘temporarily submerged’. In theory I could have simply baked the plaster molds in the kiln to burn out any leftover wax (as well as raise the temperature of the mold so the metal didn’t harden as quickly) but not I might not have to…

Mounting the Wax on a Wooden Base

Wax Casting from Silicone Mold
Wax Casting from Silicone Mold

After fiddling around with a few ideas I settled on sticking the wax sculptures to the same wood I use to make the stablemate display bases. This would inset the design a good half-inch into the mold and should be enough.

I coated the wood with wax a few times and then rubbed it off, so that there was only a thin coat of wax left. My theory was that this would help the wood to not stick to the plaster and sand mixture as it dried, and thankfully it worked!

I used the same method to attach the sculpture to the base as I did the wax paper– I melted a small pool of wax to the top, softened the back of the dragon and then stuck the two together. After that I went in an used sculpting tools and a little bit of added wax to make sure the connection was secure.

It turned out having the wax attached to the wood also made it a lot easier to modify the sculpture. I didn’t have to worry about putting too much pressure on it, or how to hold the dragon so I didn’t leave finger prints. I was also able to use the heat gun to smooth the castings slightly without worrying it would stick to whatever it was resting on.

Aluminum Casting: Take 1!

Plaster Molds after the Aluminum Casting
Plaster Molds after the Aluminum Casting

My first set of molds met liquid metal…. and totally didn’t work. (But it was still awesome to watch! 😀 )

The thinner of the two molds cracked into pieces when we demolded it and had an interesting burn pattern to it. This was the drier of the two molds, the larger one was still slightly damp so I’m not sure if that was also a factor.

I not sure how many ‘muffins’ of aluminum we used for these two test casts, but they only took a few moments to pour. Waiting for them to cool down was the hard part… I could tell something had gone a little wrong from looking at the backs, but I had no idea there were as many bubbles before we flipped them over.

Aluminum Casting Fail
Aluminum Casting Fail

Looking at the casts you can see where the air bubbles popped up and I used this in my modifications of the flying dragon design. Although some of these might have been from the wax burning or the leftover water in the plaster escaping, they were a good place to start.

I’m torn on if I want to try the more complex standing dragon again. The mold is in one piece, but isn’t inset and you can see on the cast where it bubbled up over the edge and could have possible spilled. *ponders*

Either way this was a lot of fun and I can’t wait to get a chance to test out the second generation of the molds!

(…First I need to help fix the smelter, which we accidentally broke, but that’s for another post! 😉 )

Martha Bechtel

My name is Martha Bechtel and I write fantasy and science fiction stories, paint small model horses silly colors, cast resin and plaster magnets, code random code (and Wordpress plugins)... Come on in and join in the fun!