Gray Matter
How do you turn molten metal into perfect spheres? Just pour it off the roof
By Theodore Gray Posted 09.04.2008 at 12:39 pm 6 Comments

Real Deal The Sparks Shot Tower in Philadelphia was built in 1808. John B. Carnett

About 230 years ago, molten lead that rained from the sky—historically something to avoid at all costs—became a clever new way to manufacture an important commodity: shotgun ammo.

Precisely round pellets fly straighter, but casting each in its own 1/8-inch mold isn’t exactly mass production. In space, making them would be easy. In zero gravity, surface tension pulls any liquid into a sphere, the shape with the least surface area for a given volume. This is as true for molten metal as it is for drops of water floating inside the space shuttle. On Earth, the closest thing to zero gravity is freefall—falling objects experience weightlessness right up until they hit the ground.

Steady Drops: A vibrating motor shakes the stream of molten lead falling out of this inverted oil can, the modern way to make precisely sized metal beads.  Mike Walker
Thus, the “shot tower,” first built in 1782 by English plumber William Watts. He added three stories onto his three-story home and cut holes through all six floors. Then he poured molten lead through a sieve at the top and let the drops fall into a pool of water. The water cushioned and further cooled the now-round balls so they wouldn’t deform on impact.

Replicating this is easy, except for the six-story height. I used my hydraulic lift to get about 40 feet up, but that wasn’t enough; the lead was still molten when it hit the water, and I didn’t get good pellets. (I decided not to climb to the top of my windmill with a pan of molten lead. Sorry, you’ll have to live without photos of perfect modern drop shot.)

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6 Comments

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andrewl
09/05/08 at 8:40 am

How is it not obvious to you that what readers will want most after reading your article is photos of the final product?

How spherical are they? How uniform are they?

What a letdown!

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DiGMEH

from Montreal, Quebec

09/05/08 at 2:07 pm

Hah, nevermind the final product.. What worries me is people actually trying it out. Get on a tall building and try it. But then you have another idea: what do you do with the already made "ammo"... A use needs to be found...
Trouble ahead :P
- DiGGY

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Schlomoe99
09/06/08 at 12:21 pm

Incidentally, this is the technique used to make Dippin' Dots, the expensive ice cream that comes in tiny, frozen pea-sized spheres. When making it they allow the ice cream to freefall as tiny liquid droplets through a super-cooled (probably -30^F) air chamber. They freeze on the way down and reach the bottom almost perfectly round.

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jsnbr80
09/18/08 at 10:52 am

What's obvious is that andrewl did not read the article. He could get high enough for the shot to cool properly, thus when it hit the water it deformed.

As for a use, sling shot ammo.

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mit_geek
10/02/08 at 2:29 pm

what is suggested is what is possible only in space.when it comes to earth,the results are unpredictable as dealt with various metals,at varying lab conditions.well it's not a mass production at all as described.none is a fool to invest huge on "virtual space creation"to form perfect metal spheres.also i feel that's impossible also.so far,latest casting techniques have produced the most perfect spheres in mass production.i find the technique a triffle needless in terms of money and feasibility.

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romaniamac1
10/21/09 at 4:30 am

wow, that is briliant

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