I just made a gold bar! Well, it’s gold colored at least…
I’ve long been interested in both blacksmithing and metal-casting. I recently got a gas blacksmithing forge. This 3-burner is great for general heating of iron bar stock for blacksmithing.
https://amzn.to/3RU4GsA
Just turn on the gas, light it, wait a few minutes, and you have a nice heat for general work.
However, when I looked at the specs on it, they said that it should get hot enough to melt brass. I got curious, so I mail-ordered a crucible (a high-temperature pot for melting metal.) Since the forge is not the right style for melting metal, I only purchased a very small crucible, to make sure it would fit inside.
The #0 size looks like it’s about a shot glass and will hold one pound maximum of molten metal.
https://amzn.to/3vbjpGv
After an experiment or two, I found that it was fairly straight-forward to melt scrap brass. Much of the brass I used was spent .22 rounds. I first tried making some castings just by pressing objects directly into some generic sand, but then I wanted to see if I could make a real shape from scratch using 3D modeling.
Since the brass has such a nice shiny yellow color, I imagined a stereo-typical gold bar. I set to work modeling one in Fusion 360. I’ve been slowly teaching myself this software. While it is total overkill for this simple project, it does have some other features I found useful. For example, it can calculate the volume of any object I design. By assigning a material, such as brass, it pulls in density information and automatically calculates mass. In that way, I could make sure to design my shape so that it fits within the amount of metal I can melt in my tiny crucible.
I drew out the gold bar in CAD.
One reason why I wanted to make this shape is that it already has a draft. Draft is simply having an angled edge in a pattern so that an object can easily slip out of the mold. That’s why the sides of a gold bar are slanted!
A 10 degree angle on the sides of the bar looked about right.
I also embossed the lettering “24K” into the face of the bar, and made sure that had a draft as well.
Lastly, I changed the height of the bar until the calculated weight would match what I knew I could melt. The software calculated a little over 7 oz, or around 200 grams.
With my 3D object complete, I exported it to my Ender 3 S1 3D Printer.
https://amzn.to/3tsDI1C
I printed the object out of plain white PLA plastic.
Next, I heated a screw to thread into the back of the print. This would give me a handle to pull it out of the sand mold. I removed the screw and set the print in the bottom of an open wood box, which would hold some sand.
My “green sand” is a mix of fine grain sand and about 10% powdered clay kitty-litter with enough water added so that it clumps hard when squeezed in a fist. This type of sand can be packed down and becomes quite firm.
With the 3D print in place, I covered it and the bottom of the open box with chalk dust. I hoped that would work well as a release agent, keeping the sand from sticking. I loosely sprinkled sand over the 3D print until it was completely covered, and then packed it down. I then kept adding and packing sand until the frame was filled solid with it.
Next, I flipped the frame over.
Before pulling out the print, I decided to cut a small trough in the sand with a piece of pipe. I thought it might be best to not pour directly into the mold, as the weight of the molten brass might push too hard against the fine sand of the embossed lettering, ruining it. By pouring to the side of the bar and then letting the molten metal run in, I was hoping it would increase my chances of having a good casting.
I threaded the screw into the back of the 3D print, gently tapped it a few times to try to release the print from the sand, and then gently pulled it out.
This left me with an indentation in the sand the size and shape of my 3D print. However, I also noticed a small amount of sand that came out stuck to the face of the print. This would cause a rough surface, but I saw that the lettering itself still looked pretty good.
After that, it was time to melt down some brass, heat the forge, add scrap brass and a little Borax as flux, and get ready for the pour.
I poured the metal into the sand and let it come up to even with the surface. In fact, just a little further. Although I had leveled the sand-mold as best I could, it was still just a little off, producing a bit of a slant on the open top side of the casting.
Grabbing the casting with some pliers and throwing it into a bucket of water, I was very pleased with the initial result. It was the right size and shape and the lettering was clear.
At this point, I realized that I should have made the runner to the casting MUCH smaller. The pouring area had become a large hunk of metal with a LOT of surface area connecting it to the gold bar. This required a lot of cutting. I put the casting in a vice and cut off the extra with a metal-cutting blade in my cordless reciprocating saw.
After that, it was a bit of grinding to clean up the cut marks.
Next was sanding. I used a series of rough to fine sand-papers on a disc sander to take off the rough surface, smooth it down, and shine it up.
I also wanted to darken the lettering. I thought I would try ammonia vapor to do that. By simply placing a brass object in a jar above a small amount of ammonia, the vapors will react with the surface to oxidize it and make it dark. About an hour later, my gold bar was completely dark gray.
I polished the oxidation back off, but unfortunately by the time I really had the top of the polished bright, the #0000 steel wool had also polished away most of the darkness in the lettering! Worse than that, the oxidation also really showed off the scratches on the side of the bar where I put in significantly less time sanding and polishing! Oh well…
In the end, I had a nice little hunk of brass which looked like a gold bar. The drafting made it pull fairly clean from the sand mold, although I still need to improve my sand and release agent. With the exception of the bottom-left of the “K”, the text came through very well. I later painted it in with a little bit of black hobby paint, and it looked very nice.
Final weight was right around 7 oz, just as the CAD software said.
It’s VERY cool to be able to have an idea, create it in 3D computer space, and then make it into a real, tangible, and HEFTY item. While the gold bar is nothing more than a shiny paper-weight, it was GREAT practice in making and learning how to improve my next cast.
I’m hoping to be able to get a proper casting furnace which will be able to handle good-sized crucibles for making larger castings. I also need to start working on two-part molds so that I can create a casting with details on all sides, rather than a flat top only!
Casting is a blast, and I’m sure you’ll see more from me in the future!
Until then, stay charged up!
-Ben
