Chemical smoothing of lost-foam casted aluminum by sodium hydroxide and vaselin

I’m trying to cast aluminum by the lost-foam technique, a clever way to change a styrofoam model into an identical piece of aluminum! Currently, I’m using as rough as 0.2-1.0 mm sand, which results a quite ugly surface onto my aluminum. I’m going to buy a bag of smoother sand, but I still wanted to figure out a way to avoid the coming filing and sanding of my lost-foam casted aluminum bodies. So, if you want to try my idea, remember that I don’t take any responsibility of your acts and I cannot guarantee the proper functioning of my method nor its safety. It is your own choice to apply these instructions and take a risk that something bad can happen! If you are unsure or inexperienced on handling dangerous chemicals, do not try to follow these guidelines.

The surface of a lost-foam casted aluminum body will be full of hills and valleys, bigger or smaller. If the valleys are protected from the reach of caustic sodium hydroxide (NaOH), the highest spots will flatten down and the surface texture of your aluminum will eventually become smoother. To try this idea, I made a strong (and dangerous!) 15g/100ml NaOH solution by slowly dissolving the NaOH pellets into pure water being mixed constantly. If the NaOH is added too carelessly, the solution might start to boil, make a spill, or even break the mixing glass by the excess heat! I rinsed my aluminum test piece with water, quickly dipped into the NaOH solution for cleaning its surface, and then carefully rinsed it again with water. A small blow of hot air by a heat gun to dry the piece, and the slightly warm aluminum can be dipped into vaselin. As the vaselin melts, its hydrocarbons will fill every surface irregularities in your aluminum. The excess vaselin is wiped off by a couple of paper towels which cannot reach into the the deepest valleys, but the most highest spots will become exposed. Then aluminum piece is dipped into the strong NaOH solution and the formation of (explosive!) hydrogen bubbles indicate the corrosion is in progress. This reaction is also exothermic, so remember to goggle your reaction to avoid thermal runaway. After a few minutes, remove the aluminum and rinse it with pure water, dry it quickly with a heat gun, apply a fresh coating of vaselin, wipe the excess off, and put the aluminum back into the NaOH solution. If the formation of bubbles is low, try to remove more vaselin or make a new NaOH solution. Repeat the process as many times as you like. I didn’t manage to get rid off all the surface irregularities in my test object, but the major texture due to my coarse sand smoothened out quite a bit, only after a couple of rounds! Here are some photos from the process I tried, enjoy!


Here is the test piece from my lost-foam aluminum cast (left, at the beginning), smoothened by the topological vaselin protected NaOH corrosion (right). Not all the irregularities went away, but clearly the midsize porosity texture smoothened out. With finer and more homogeneous sand this could work pretty well! Longer hydrocarbons could also work better due to stronger adhesion onto the aluminum.


Weighing more NaOH and rubbing a fresh layer of vaselin on my lost-foam casted aluminum piece. Btw, the vaselin can be burned away, good thing to do before anodizing for example, but the heating might change the physical properties of your aluminum.


I used the watering can and the long aquarium tongs to dilute the NaOH solution away from the object after each corrosion step. If done outside, remember to give a good rinse of water to dilute the caustic NaOH! Aluminum is found everywhere in nature, why the dissolved aluminum should not be harmful to environment.


This solution is enough fresh, which can be seen from the vigorous formation of hydrogen bubbles.

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