Thermite

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Problem Statement

I don't really know how to start this one. The problem is of a very personal nature. A man, Larry Campbell, whom shared many of the same passions as I, passed away in January of 2018, from the same flu that also had me confined to bed for several weeks. The 'problem' is that a good friend died. A good friend who often annoyed me. This fact is hard to come to terms with. The last time I saw him I probably said something snappy to him, because I didn't always approve of how he handled shared resources. A bitter pill to swallow, as I had and have enormous respect for the man. My intent with this project is to create a memorial using a small amount of Larry's ashes that is worthy of such an amazing man. I only wish he could have joined in while he was still alive.

To truly honor someone's life, use their death as a reason to grow. Learn new things. Help someone out. Explore something you never would have otherwise. Grief is powerful, use it in a positive way and do something they would have been proud of. Otherwise it will consume you and fill you with regret.

Larry was extreme. He loved metalworking. He loved the C64 in his youth, and he loved all methods of high tech fabrication. His primary frustration was that 3D printers don't easily print metal. There are many ways of doing lost PLA casting to aluminum -- but what Larry wanted was steel.

I want to use thermite mixed with a small amount of Larry's ashes to cast a Japanese style dagger with his C64 BBS named, Banzai, embossed on the handle. I want to 3D print the dagger, create a mold, then fill it with liquid steel from thermite.

Log of Experiments

Plaster and Dextrin

Initially and naively I created a mold from plaster. I used a 3D printed calibration cat as the test piece, and added three vents by welding 1.75mm PLA filament to the high points of the print. I submerged the print into 50% plaster and 50% playground sand. I used a pen to create the main sprue about 0.5" long. The PLA was removed by placing the cured plaster mold upside down in the oven at 500 degrees F with a ramp rate of 50 degrees every 10 minutes.

50g of thermite using laboratory grade Fe2O3 and laboratory grade purified atomized aluminum was mixed with 2% dextrin and pressed into a 3" flower pot with a pencil sized central vertical cavity. An aluminum foil diaphragm covered the bottom of the terracotta pot. The dextrin was added in an attempt to create a solid 'brick' without air as well as a source of carbon for the creation of steel in stead of raw iron. A small amount of metal poured into the mold, but it was clear that the mold was mostly empty, so a second batch of 100g of the same mixture, but not moistened, was placed into a second flower pot and ignited. The mold was still not full, but was destroyed to asses the yield. A total of 35 grams of iron made it into the mold, and it was clear that the inlet was too small and several droplets of iron sat on top of the main sprue.

The lower half of the calibration cat's shape was apparent, but massive amounts of frothing in the iron were observed as well as an extreme sulfur stench similar to the smell of a used ested model rocket engine tube. The sulfur was a byproduct of the plaster decomposing due to temperature, over twice the decomposition point. It was clear that plaster was not an appropriate mold material.

Thermite Yield Experiments

I decided at this point to attempt to quantify the amount of iron that should be expected in an attempt to efficiently size the charge to fill the mold. Stochiometric analysis shows that Fe2O3 is 70% iron. Therefore, the yield should be 70% & 75% = 52.5% iron. In other words, 100g of thermite contains 52.5 grams of iron, which is clearly a theoretical maximum. The thermite-to-mold yield from the previous experiment, 35 grams, is 150*.525 = 78.75g, 35/78.75 = 44% of the theoretical maximum, which honestly is not too shabby considering the iron particles that remain in the slag and adhered to the sides of the terracotta pot crucible.

Cut Star Thermite

The amount of iron beads covering the crucibles from the previous experiments led me to believe that the iron was pooling on top of unreacted thermite, which was ejecting the puddle as it reacted. Therefore I attempted to create 'channels' for the iron to flow towards the bottom by creating 50g of 1/4" thermite cut 'stars'.

Pressed without dextrin

It became clear to me that the dextrin itself may have been the source of the gas which ejected the iron puddles. Therefore, 50g of thermite was pressed using a 3D printed espresso tamper into the bottom of a flower pot as densely as possible. The results were the best yet.

Frustrations with commercial 'red iron oxide'

As my supply of laboratory grade Fe2O3 dwindled, I purchased commercial grade red iron oxide. It became IMMEDIATELY clear that the purity of the commercial grade iron oxide was nowhere near as good as the lab grade. The commercial grade sample refused to sustain combustion when pressed densely. When fluffed, it combusted extremely slowly, taking over 10 seconds to finish, whereas the lab grade materials burned in a fraction of a second. The iron yield for the longer duration burns was negligible, likely due to both the stochiometric offset from the desired as well as a limited maximum temperature due to the extended duration of the burn. Upon inspection under a microscope, it was clear that the samples contained sand and clays. The color was also much more brown than the lab grade oxide, likely also due to quartzes and clays. Apparently selling iron rich dirt qualifies as pure iron oxide....