PDA

View Full Version : Why are some old brass lenses so corroded, and others are not?



Jac@stafford.net
19-Nov-2017, 17:05
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.
.

Pfsor
19-Nov-2017, 17:18
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.

Jac@stafford.net
19-Nov-2017, 17:29
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.

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.
.

Pfsor
19-Nov-2017, 17:40
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.

Jac@stafford.net
19-Nov-2017, 18:12
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.

jp
19-Nov-2017, 18:28
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.

Leigh
19-Nov-2017, 20:55
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

Steven Tribe
20-Nov-2017, 02:58
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.

Pere Casals
20-Nov-2017, 04:53
Why are some old brass lens bodies are so corroded or corrupted, and others are not?
.

Bare brass is not corrosion resistant because "selective leaching" of zinc in the alloy. (https://en.wikipedia.org/wiki/Selective_leaching#Leaching_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 :)

Pfsor
20-Nov-2017, 05:56
There are several kinds of brass, CR brass is the Corrossion Resistant brass, also called DZR Brass, a carefully manufactured alloy.

I don't know what brass alloys were available in those times, just pointing that brass kinds can be more or less corrosion resistant...


DZR brass has nothing to do with old brass lenses. It is a relatively modern alloy (started after 1950' and 60').



Some metals are not oxidized by O2 at all, like Gold or Platinum so always are shining.

Sometimes you seem to be extremely well informed...:)

Pere Casals
20-Nov-2017, 06:33
DZR brass has nothing to do with old brass lenses. It is a relatively modern alloy (started after 1950' and 60').

You should know that "DZR" is more than a kind of brass, it is also property of any brass, and any ancient brass is more or less DZR, this is more or less resistant to selective leaching, for example an addition (or impurity!) of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR. Brasses of alpha or beta phases are determined by Zinc content, being less than 45% in alpha phase kind.


Sometimes you seem to be extremely well informed...

I'm somewhat informed because some decades ago I studied metallurgy in my degree, and I remember basic things like why gold shines: because electronegativity level, the highest of any metal: 2.54.

Just pointing that corrosion resistance of a metal or alloy can come from chemical or from physical barrier origin.



PD: edited to add "or Impurity !". In ancient times metallurgy had tricks that today are well explained.

Pfsor
20-Nov-2017, 07:45
You should know that "DZR" is more than a kind of brass, it is also property of any brass, and any ancient brass is more or less DZR, this is more or less resistant to selective leaching, for example an addition of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR. Brasses of alpha or beta phases are determined by Zinc content, being less than 45% in alpha phase kind.
I'm somewhat informed because some decades ago I studied metallurgy in my degree,

A pity you didn't learn that DZR brass alloys were not used for old brass lenses. And no brass was called DZR in that time regardless of your definition.
Shouldn't you complete your studies a little bit?

Pfsor
20-Nov-2017, 07:54
Just pointing that corrosion resistance of a metal or alloy can come from chemical or from physical barrier origin.


No kidding? Who would say that! :(

Pfsor
20-Nov-2017, 08:07
... for example an addition (or impurity!) of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR.

In ancient times metallurgy had tricks that today are well explained.

When they started to produce DZR brass (and call it so!) it was in the 1950' and 60'. The dezincification itself was recognised as such in about 1920. Not so ancient times... unless every time, passed by, is for you more or less ancient one, of course...

Richard Wasserman
20-Nov-2017, 08:19
I used to have an antiques business and routinely dealt with brass that was as old or older than the lenses being discussed. My understanding, based purely on empirical evidence, is that the degradation you see on the brass lenses would have more to do with how they were stored and cared for than the exact alloy used. If brass is kept clean, dry, and away from harmful acids and salts, it holds up well.

Pfsor
20-Nov-2017, 08:23
I used to have an antiques business and routinely dealt with brass that was as old or older than the lenses being discussed. My understanding, based purely on empirical evidence, is that the degradation you see on the brass lenses would have more to do with how they were stored and cared for than the exact alloy used. If brass is kept clean, dry, and away from harmful acids and salts, it holds up well.

That's correct - that's why I said "Add to it different air quality (humidity) and you have differently corroded brass lenses." But the opposite is true too - different alloys have different reactions to the same air quality, so it 18 and 20 minus 2...

Pere Casals
20-Nov-2017, 08:29
A pity you didn't learn that DZR brass alloys were not used for old brass lenses. And no brass was called DZR in that time regardless of your definition.
Shouldn't you complete your studies a little bit?

Any kind or brass has more or less "DeZincification Resistance", in the XIX or today. A particular highly DeZincification Resistant brass alloy is called DZR. Those XIX lens barrels that are corroed probably had low DeZincification Resistance. Those that arrived OK possibly are lower than 45% in Zinc and contain more than 0,02% As impurity or addition. Don't you think?





No kidding? Who would say that! :(

It is what I was teached. To give you an example you can easily understand consider COR-TEN Steel, https://en.wikipedia.org/wiki/Weathering_steel

All metals having an electronegativity that allows their oxidation but are corrosion resistant this is because their superficial oxid layer is protective. Example is AISI 304 Stainless Steel, and Copper, and Tin, and Silver... if you make an scratch then you see the metal, as some time passes a very thin oxid layer covers it, becoming darker, that darkening comes from surface oxidation, and this layer becomes protective as it makes a physical barrier that does not allow O2 to reach the metal under.

If you polish that layer then air O2 oxidizes again the newly exposed surface, making a new protective layer, like if it was paint.

Pfsor
20-Nov-2017, 08:52
... if you make an scratch then you see the metal, as some time passes a very thin oxid layer covers it, becoming darker, that darkening comes from surface oxidation, and this layer becomes protective as it makes a physical barrier that does not allow O2 to reach the metal under.
If you polish that layer then air O2 oxidizes again the newly exposed surface, making a new protective layer, like if it was paint.


I think I have had my dose... Thank you.

Pere Casals
20-Nov-2017, 09:14
Richard, storage conditions and lacquer would be important, as you say, but also consider that some kinds of brass, like "naval brass", can withstand large amounts of abuse without degradation, while some lower priced brass (lower copper for example) can be easily ruined.

Regards

Myriophyllum
20-Nov-2017, 13:47
Hi,

I think one problem with old brass is that some people find it a good idea to polish all lacquer (zapon or similar) off. They don't apply any new lacque. The blank brass now looks shiny for some time, then gets corroded and looks terrible. Depending on brass type and storage, of course.

Best
Jens

Ted R
20-Nov-2017, 14:32
Brass is not a pure metal it is a mixture (alloy) of copper and zinc. In addition there may be small amounts of other constituents for special purposes.

During the period that vintage lenses were being made (>100y ago) the varieties of brass available were not under strict control and the makers of lens barrels may have been vague in the specification of the material. Over time the various batches of brass used by a single maker may have had different corrosion resistance properties.

All brass corrodes, especially when in the presence of atmospheric contaminants (think coal burning Victorian city air)

Finger marks on the freshly cleaned surface if allowed to persist will become permanent stains.

To remain shiny brass requires constant removal of the surface dullness by polishing.

To eliminate the need for polishing the clean surface may be given a clear coating, one type probably was shellac laquer.

Wear and damage may penetrate the coating and permit corrosion to occur.

Richard Wasserman
20-Nov-2017, 14:54
Jac, I used to buy lacquer (in 55 gallon drums) just for this purpose. It works well and is easy to apply, or remove if you need to.