How do I identify this metal?

July 18, 2011 8:30 PM

How do I identify the metal from which that this nameplate is made? It's from a WW2-era lathe. Is behaves somewhat like lead, but is harder.

My father and I are restoring a South Bend 9 model A, a really classic lathe, for my future use. My current work is restoring the various data plates on the thing that show you how to operate it, gear ratios, etc. At first glance they looked like aluminum, but upon closer inspection I'm fairly sure they're not.

Pictures, all with reflecting light to show the luster and with white/black objects to give approximate color:
* reflection on polished raised letters
* reflection on dull gray body
* back

And details:
* the object shown used to have a paint-filled body with the polished letters standing proud above the paint. The paint was almost entirely gone and I was able to lightly flake off what was left in the corners. It was cleaned with denatured alcohol (which just got off some gummy residue in the corners). I swiped it with lacquer thinner, too, but nothing came off.
* the polished raised letters are about as shiny as polished aluminum, but the material is softer than it should be; I've welded and formed a fair bit of aluminum and this feels wrong.
* everywhere there's a scratch or wear (such as on the edges of the letters) the material has turned a dull gray. This rules out it being aluminum.
* it's too hard to be lead (I've worked with rolled lead sheet and lots of solders) but the corrosion/color looks just like it, honestly.
* the lathe body is stamped JAN (Joint Army-Navy) and was manufactured in 1943.
* All other South bends I've seen used brass plates with this exact same same chemical milling (etching) style. I'm guessing the scarcity of materials during the war necessitated this plate being something cheap and available.
* it leaves a metallic smell on my hands after I handle it for a while. It's not "stinky penny" smell, but definitely metallic.

My best guess is tin, but I'm not sure how to narrow it down. I've kept up my electrical engineering skills, but my chemistry and materials classes have receded from memory. I have the usual battery of household chemicals available for testing, up to hydrochloric acid and copper sulphate. I've other tools like power supplies, welding tools, and such, if that helps.

This is pretty much an academic exercise, but given the neat, traceable history of the lathe (e.g., it was used post-war to make jukebox parts!), it would be nice to know the material so it can be part of the machine's story.

Help me, MetaFilter, you're my only (metallurgical assay) hope.

posted by introp to science & nature (12 answers total)

That's tin like crazy. But try this: http://castboolits.gunloads.com/showthread.php?t=1038
posted by tumid dahlia at 8:35 PM on July 18, 2011 [1 favorite]

Probably tin, but I wouldn't rule out zinc, either.

If it's zinc, it's very susceptible to hydrochloric acid.
posted by kaszeta at 8:53 PM on July 18, 2011

Tin, or intaglio pressed, nameplate pewter. You might repair or fashion new plates from pewter sheet, manually paint them, and then carefully rub to expose the remaining high areas, but it's near artist work, to look decent. Originals were probably made with steel master dies on intaglio presses. You can still have this done, as originals, if you're willing to stand the cost of making dies, and lithographic grade pre-press artwork. In which case, 100 or 1000 plates will cost you very little more than 1.
posted by paulsc at 8:59 PM on July 18, 2011

We're fairly sure the plates weren't pressed, but etched. (We know that's how the brass ones were made; the process is well-documented on South Bend fan sites.) Like the brass ones I've seen, there's no evidence of any stamping deformation on the back.
posted by introp at 9:04 PM on July 18, 2011

"... Like the brass ones I've seen, there's no evidence of any stamping deformation on the back."
posted by introp at 12:04 AM on July 19

Intaglio pressing, properly done, doesn't "print through." US Federal Reserve notes are still intaglio pressed, on both sides, to give them a unique tactile surface feel that holds up in wear. But even single sided intaglio pressing, such as is sometimes used for "engraved" stationary, oughtn't print through, much less through nameplate pewter sheet blanks!

The main advantage of the process is precise deposition of "ink" in the voids created by pressing, without further registration problems.
posted by paulsc at 9:11 PM on July 18, 2011

I like the idea, but it looks like intaglio pressing only deposits ink on the surface, thus creating the raised effect. These have physical metal standing proud over the base, by about 0.030" or so. Stamping a raised effect like that will leave telltale impressions on the back. (Unless the manufacturer of the plates went through the trouble to flatten the back, but why would they add that cost?)

However, this is all excellent input. The idea that they're pewter is interesting; pewter reads as being harder than straight tin (good), but it requires a little copper from the war effort (bad). I wonder if there's a way to test for copper content.
posted by introp at 9:36 PM on July 18, 2011

How about doing a simple density test (mass/volume) and compare it to a known list of metals and alloys?
posted by TDIpod at 10:40 PM on July 18, 2011

In answer to the question How do I identify the metal from which that this nameplate is made? Try Energy-dispersive X-ray spectroscopy or X-Ray Diffraction. Do you know anybody at your local University?
posted by gallagho at 12:08 AM on July 19, 2011

Any chance this is "pot metal"? That's what I would lay a modest amount of money on.
posted by maxwelton at 3:02 AM on July 19, 2011

As you say, the Southbend Model A is a classic machine. Surely someone has documented this by now. For that matter, Southbend still exists. Ask them.
posted by DU at 5:16 AM on July 19, 2011

1- It looks like tin to me. Maybe pewter. They probably wouldn't have used fresh copper to make the alloy, rather, they probably melted down other things that had the copper already in it.

2- I'm not sure if it is called intaglio when it is done to metal, but it looks a little like that's what process was used. They created a die as if it was going to be embossed (with voids for where the print would go, but instead of a corresponding reverse with raised parts for where the letters go, it is just flat.

The idea being that the original blank is, say, .050. After you run it through the press, the non letter areas get smushed down to .040, and for where the letters go, the metal gets smushed "up" to say .060.

(Analogy: take some playdough and smush a standard wooden letter block into it. The playdough fills the void. In this case, the die is the opposite of the letter block, ie, the voids are where you want the print to go.)

(Second analogy: the same way they make coins, except with a blank/flat reverse.)

My justification for this is that the flat area of the plate looks very flat, and the raised letters of the word "off" are sort of rounded over. If it was chemically etched, the letters would be dead flat, and the flat area would be less so. Further, the plate appears to have a bit of a wave to it.

The process would take a fairly ductile metal and work harden it.
posted by gjc at 6:10 AM on July 19, 2011

The zinc, pewter, and pot metal ideas are all good ones. The comment about the university reminded me that I used to ride with a woman from Virginia Tech who was a metals guru.

So:
* probably not high-zinc because the glossy letters don't tarnish gray when laid bare
* probably not a classic pot metal because it they typically heavily tarnish and powder as the years go by
* definitely not pure tin since it would've fallen apart from tin pest (which was new to me and fun to read about); it would have to have some amount of something like antimony, which is why they put it in pewter
* so it's almost certainly a tin alloy like pewter, only serious testing (chemical, hardness, X-ray fluorescence, etc.) would tell me if it has hardeners like lead or copper. Given its age, it is probably safe to assume it has both, though who knows in what quantities.

Thanks for all the help!
posted by introp at 11:08 AM on July 19, 2011