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I am attempting to identify brass casings vs aluminum casings. I'm been trying to figure a reliable way to differentiate between the two. I've thought about

  • Color: This may work, but I fear that dirty optics and targets may not work so well.
  • Weight: I'm not really sure how to implement this in my context.
  • Conductivity/Resistivity: Perhaps an inductive sensor? But aluminum is very close to the same value as brass.
  • Ultrasonic properties: This may not work with varying levels of dirt. But I really don't know much about it.

I prefer non-contact, and a solution that is impervious to contamination, but I'll take any suggestions given.

It seems that an inductive sensor would be the way to go. I'm okay with rolling my own, perhaps a couple of tuned coils which the target passes through?


Okay, thanks to Charles for getting me on the right track. It looks like I'm needing an eddy current sensor.

These are shell casings, and when run through the machine will all be of the same caliber, so the uniformity should be good. For material identification, distance from the coil is important, and since the target is not always in the same position, I'll run the casings through the coil center, which should be immune from position errors.

It looks like some aluminum alloys overlap the brass alloys in resistivity, but I bet that the alloys used in cartridges are very consistent. I do have another bit of information, and that is the actual identification of the head stamp. Headstamp coupled with resistivity should get me a very accurate indication of what type of metal the cartridge is made from.

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  • \$\begingroup\$ Why not research how different metal detectors work? Some of these can easily differentiate between brass and aluminium. \$\endgroup\$ Commented Jul 25, 2016 at 3:18
  • \$\begingroup\$ Actually, I did, but they all seemed to lump aluminum and brass into the same category. Differentiating from steel seems to be easy, and is what most of them seem to do. Perhaps I'm not using the best Google-Foo. Do you have any leads? \$\endgroup\$ Commented Jul 25, 2016 at 3:33
  • \$\begingroup\$ If the aluminum ones are much lighter, I think that is going to be your best bet. What kind of cases are these? Cases as in ammunition? There are DIY devices for separating cases and dropping them into a shoot one at a time. After that, you weigh (on a load cell), and then drop left or right depending on weight. Doesn't seem that hard. But it is easy to say "doesn't seem that hard." \$\endgroup\$
    – user57037
    Commented Jul 25, 2016 at 3:37
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    \$\begingroup\$ Both aluminum and brass alloys have ranges of resistivity, but the resistivity of aluminum seems to be about 33 to 40 percent of the resistivity of brass. You need to know something about the range of alloys that you might have and the variations in material thickness etc. AC magnetic fields will repel samples and DC magnetic fields will impede their motion. There is a lot that you can do with that. \$\endgroup\$
    – user80875
    Commented Jul 25, 2016 at 3:53
  • \$\begingroup\$ Are these shell casings? Do you have to deal with different sizes? \$\endgroup\$ Commented Jul 25, 2016 at 4:31

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run them through rollers and down a chute so they're all moving the same speed then pass them horizontally through a strong magnetic field, the lighter and more conductive aluminium casings will be slowed more by eddy currents than the denser brass ones and fall into the near bucket. The brass will be slowed less and fall into the far bucket

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Archimedes had just this problem over 2000 years ago. King Hiero II had supplied the pure gold for a votive crown to be made, and was suspicious that the metal had been adulterated with silver by a dishonest goldsmith while making the crown, and gave Archimedes the problem of proving or disproving his suspicion without damaging the crown. Archimedes noticed when taking a bath that his body, regardless of its irregular shape, displaced its own volume of water, and so took to the streets naked shouting "Eureka!"

Alli has a density of about 2.7 times that of water, brass is about 8.0. With such a huge difference, a density discrimination by noting the change of weight when immersed in water should be very easy.

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Are the samples to be measured of uniform physical characteristics? Size, shape, weight, etc? I would imagine that magnetic properties and/or their reaction to ultrasound would be varied enough to do a reasonable differential identification.

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  • \$\begingroup\$ Size and shape: yes. The type of material is mixed, so weights will be different. \$\endgroup\$ Commented Jul 26, 2016 at 14:03
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Eddy current is probably best, but if you wanted to pursue the color method, color sensors have become very sophisticated. For example, the TCS34725. You can easily eliminate a lot of effects of dirt on the sensor or illumination. Very simple I2C interface.

enter image description here

Of course if they decide to start anodizing the aluminum brass color it may not work so well.

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I'd expect brass and aluminum both to be variable in color, hardness, and both are diamagnetic. So, I'd moisten a swab with hydrochloric acid and wipe the metal. It won't do much to anodized aluminum, but it turns brass red (attacks the zinc, leaving a copper-rich surface). Unanodized aluminum will etch, but won't turn red.

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If it's acceptable to place a probe in good physical contact with the object and to take a few seconds over the measurement, you could discriminate the two metals on the basis of their thermal conductivity (figures in W m-1 K-1 at 25 °C):

Aluminium        205 
Brass            109

You could do this by applying a fixed rate of heat input (i.e. power) and measuring the rate of temperature change. If you have examples of brass and aluminium samples to calibrate the technique with, you don't need to actually calculate what the thermal conductivity is, only to decide empirically what the threshold is between the two.

Note this method might be more difficult if the samples are hollow with thin walls and the wall thickness is not consistent - it can still be done but the measurement would need to be quicker and more sensitive and would then be more susceptible to variations in the quality of thermal contact between the probe and sample.

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