View Full Version : Identifying Aluminum

07-28-2010, 08:11 PM
Is there are simple way to identify aluminum? I have some parts which I believe to be cast aluminum, but I am afraid they might be zinc or some other soft metal. How can I test them? I know that they are nonmagnetic and soft, yet capable of breaking in a brittle manner.

Weston Bye
07-28-2010, 08:56 PM
Weight. Measure the volume of the metal in question and weigh it. Look up the weight of metals in Machinery's or on the Internet. Zinc is considerably heavier than aluminum.

Todd Tolhurst
07-28-2010, 08:57 PM
If you can measure the density of the parts, you'll certainly be able to distinguish aluminum from zinc. You'll need to weigh the parts, and you can measure the volume of the parts by water displacement.

07-28-2010, 09:02 PM
As stated check the weight, Zinc is almost 3 times the weight of alum.
by size.
Zinc is 440 pounds per cubic foot.
Alum. is 165 pounds per cubic foot.

07-28-2010, 09:03 PM
What if it's Al with a little bit of Zn? That might be indistinguishable from some other Al alloy.

I've wondered this too. I know there are some chemists on this board. Are there any simple assays you can do to narrow down what alloy you are dealing with?

At the very least, I'd like to be sure that I'm not grinding cobalt or welding zinc.

Lew Hartswick
07-28-2010, 09:31 PM
Well my chemistry class that did the analysis of "unknowns" has been
about 55 years ago but I bet a simplified version could be come up
with. The big problem is going to be having all the required reactants
around and the ability to measure the precipitates mass accurately.
It certainly isn't a "shade tree mechanic" type of operation.

07-29-2010, 10:25 AM
First off, if the parts are cast aluminum, it certainly WONT be pure. Silicon is a favourite casting alloy additive. As a general statment, very few metals are used in their pure state, with the exception of electrical copper. You will grind cobalt whenever you sharpen carbide cutters, since it is the commonest binding agent in cemented carbides. Zinc alloys are probably the second highest tonnage casting alloys after aluminum, (think "pot metal" and zamak.)
I suggest that you spend a bit of time with Wikipedia searching for "copper and its alloys," aluminum ditto, iron ditto.
There are no simple tests for metals, unless you first have an idea. example;- if it is some shade of yellow, it is some brass or bronze alloy, therefore mostly copper, but what else? If it is light for its size, it is likely aluminum of some alloy, BUT maybe magnesium. If it is magnetic, it is mostly iron, but maybe nickel or cobalt-it depends.

07-29-2010, 11:54 AM
Is there are simple way to identify aluminum? I have some parts which I believe to be cast aluminum, but I am afraid they might be zinc or some other soft metal. How can I test them? I know that they are nonmagnetic and soft, yet capable of breaking in a brittle manner.

Melting a small chunk could give some idea. Most Zinc castings usually seem to melt quicker than Aluminum. That's a generalization. Alloys of each could probably be found to defy that statement of course.

When you say "...capable of breaking in a brittle manner" are you meaning at room temp? Heated up to a certain temp, but well below melting, aluminum becomes what's known as "hot short", at which time it can be easily crumbled up into small chunks. Zinc does it too, to some degree, but in my experiences (limited), it was usually right on the verge of melting at that point

But weight is the best differentiator between the two. Tho, depending on the shape and voids within a given casting, it's sometimes hard to guess just how much actual material you're holding, therefore hard to say if it's "heavy" or not.

I'll sometimes take my pocket knife and whittle out a small sliver. Both can be cut fairly easily that way, but zinc will "feel" different; softer and sort of gummy.

I'd suggest getting some known samples of each, and heft them, cut them, note any differences in surface oxidation, etc., and observe the differences.

07-29-2010, 06:10 PM
I think it's cast Al alloy and I want it to be that.

When I hit it with a hammer, at room temp, it bends and smashes all over the place but it also is pretty easily to make it fail in a brittle manner. I can have 1/2 inch by 1/8 inch cantiliver 1 inch long, hit with small hammer and it breaks off without hardly bending.

Haven't gotten around to measuring the density yet.

07-29-2010, 06:18 PM
you got magnesium as well ..that looks the same ..and im told dangerous to machine.

mag is brittle

07-29-2010, 09:01 PM
Oooooh. Don't want to melt that.

THe things have a grey coating that looks like anodization. Does anything imitate anodization? Like Magnesium?

07-30-2010, 05:29 AM
From memory magnesium is also gray and light. You could always try scraping off a small sliver and lighting it - if it's magnesium you'll certainly know...

I seem to recall that Zn has a melting point of around 400 degrees C while Al is more 600 to 700 degrees C, so another way may be to try melting a piece (exact temperature will depend on the alloy).

From my time mucking around with pipe organ pipes, I know that the guys casting the metal (primarily a tin lead alloy - historically pipe organ material is just called 'metal') would cast some small pieces (1/2" spheres - like a fishing sinker) of the various alloys 10:90, 20:80, 30:70 and so on. When they wanted to determine an unknown alloy they would then cast a piece with the same mold and use a balance beam to determine what it best matched.


Your Old Dog
07-30-2010, 07:53 AM
This might be a good reason for you to start a collection of known aluminum so you can do comparison test. For instance drop one chemical that reacts to all of your samples and see which one compares best with the unknown? If you want to get into casting it might be a valuable skill.

07-30-2010, 11:29 AM
To test to see if your stock is aluminum is not difficult. Find some sodium hydroxide (lye) and mix a 10% solution with water. Technical grade NaOH is preferred but drain cleaner or oven clean will work. Clean an area to remove all oxide or surface finish. Apply one drop of solution and allow to react for ~ 10 seconds, then rinse with running water.


Compare with the photo above, or better yet with known aluminum alloys prepared with your NaOH solution. Magnesium or zinc parts won't react.

07-31-2010, 01:51 AM
The safest test is vinegar. No reaction on Al. Bubbling and darkening on Mg and Zn.

07-31-2010, 05:12 PM
I determined it to be Al, mainly by weight. I have started a melt.