Why are some old brass lens bodies are so corroded or corrupted, and others are not?
I understand that the brass is not necessarily an indication of lens degradation (or is there a coincidence?).
Just wondering why so many look terrible. Composition?
I have two old lenses that look like new. Strange.
Thanks for any help.
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Different brass alloys (with a different amount of zinc in it) have different atmospheric corrosion resistance. Add to it different air quality (humidity) and you have differently corroded brass lenses.
Is a larger percent of zinc better or worse? Were lens bodies, for example, of largely brass content but with zinc alloy coatings, or visa-versa? So many lens bodies are eroded to a terrible condition I wonder why just a minority are not corroded.Originally Posted by Pfsor
I am prompted to ask because a member of this group once commented in a negative tone that a particular lens appeared to be 'polished',
In my modest experience very many old brass lenses ppearance cannot be improved by polishing. The ugliness goes to the bone, so to speak.
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It's the zinc which is water soluble so more of it causes greater corrosion. But it also depends on other metals presence. Tin makes brass more resistant again the zinc corrosion in it. High humidity is bad, especially when condensation occurs.
So, is it true that a pure brass lens body is likely to be less susceptible to the rot we see on so many? Is a solid brass construction a good thing or a random circumstance of history, a sign of little use, good storage, a deviation from ordinary materials during manufacture or something else?
Why would some denigrate a pristine looking old brass lens because it looks good?
I do no think we can polish a wreck of a brass lens to look like new. The layers are gone.
NO? Expert correction is welcome.
Along with humidity, I'd bet cleaning/polishing products cause serious corrosion too if they are not removed after cleaning and the metal protected.
I'd certainly accept the theory that zinc is part of the corrosion issue; boats have easily changeable sacrificial zinc to prevent other metal parts from corroding.
There are many different brass alloys, some more susceptible to corrosion than others.
Brass objects can be protected (more or less) by applying a thick coat of lacquer.
That will degrade and disappear with handling, and must be replaced after cleaning.
But brass is not the only "yellow" metal.
Relevant to this discussion is alloy 954 Aluminum Bronze, an alloy with 83% copper and about 10% aluminum.
Other metals make up the remaining 7%.
It is a high-strength alloy with a yellow color slightly lighter than brass.
Its main advantage is that it will not oxidize in normal or corrosive atmospheres.
It's quite strong, usable for many demanding applications such as shafts and bearings.
It's the only "yellow" alloy that I stock in my machine shop.
-Leigh
If you believe you can, or you believe you can't... you're right.
A couple of points:
- there is some difference in the type of type of lacquer used. T,T &H (Cooke) have been described as having a robust finish. This too, can be a disadvantage as the engraved information becomes a weak spot with a "blur" spreading outwards. The rest of the finish is often perfect!
- early surface attack is definitely due to zink leaching out. I have cleaned some where the surplus copper forms a thin copper sheen on the surface. Pitting comes later and I think is due to very damp (wet?) conditions - think garden shed or attic! Copper is attacked at this stage and is shown by verdigris and colouring of lens surfaces.
- generally speaking, brass holds up well to aging. I have never seen signs that the exact composition of the brass makes any difference. Not like aluminium in the early days! Thin brass areas do crack if not treated well. Early brass lens hoods were extremely thin and, apart from distortion, develop edge cracks and the soldering comes loose. Early Waterhouse cut-outs in sleeves (both ex-works and modifications) often have a narrow bridge which breaks.
Bare brass is not corrosion resistant because "selective leaching" of zinc in the alloy. (https://en.wikipedia.org/wiki/Select...aching_of_zinc)
There are several kinds of brass, CR brass is the Corrossion Resistant brass, also called DZR Brass, a carefully manufactured alloy.
Also there is Naval Brass which includes a 1% Tin.
Most metals/alloys are oxidized with air contact, but it can happen that (depending on the particular metal or alloy) the outer surface oxidized layer is more or less protective for the the material under surface. If with oxidation the resulting material has less volume than the original metallic mesh then it happens than O2 penetrates in the resulting cracks. If not the oxidized layer on the surface is very protective and only the outer surface darkens color with exposition to air.
I don't know what brass alloys were available in those times, just pointing that brass kinds can be more or less corrosion resistant...
Most of corrosion resistant metals are oxidized by O2 from air, but the outer oxidized layer protects the inner material. Some metals are not oxidized by O2 at all, like Gold or Platinum so always are shining. Polished Silver shines until the oxidized surface (and protective) "black" layer is created again. I mention silver case because our beloved grains of metallic silver
DZR brass has nothing to do with old brass lenses. It is a relatively modern alloy (started after 1950' and 60').
Sometimes you seem to be extremely well informed...
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