Jump to content

Yikes! My gudgeons are sweating!!

Recommended Posts

Can anybody tell me what is happening here?


I finished installing my pintles and gudgeons a couple of weeks ago. Had a bit of a issue with the blackening rubbing off when I oiled over them. I am ok with the rudder pintles, it looks weathered, the gudgeons came out differently. My plan has been to go back and touch up the gudgeons and try to match them to the pintles. Work has kept me from sitting down and working on the problem till this morning. Now I see what appears to be sweat on the gudgeons as well as some green corrosion starting. 


Anybody have any idea what the "sweat" is a and what I need to do about it? It is hard to see in the photo but in the areas where the blackening has rubbed/flaked off there is little tiny drops of what looks like water or glycerin. I will try to get a better photo. for now there is this one.



Edited by src
Link to comment
Share on other sites

To add to Druxey's excellent questions, if it is brass, did you pre-treat the metal before you blackened it? Some brass will have a protective coating on it to keep the brass looking bright. This coating has to be removed from brass before blackening. Pre-soaking in a mild acid, rinsing with water, and then cleaning with mild soap and then another rise will usually do the trick. Since I hard solder my metal work, I have picking solution, which is a stronger acid. A soak in this for just a few minutes will remove a coating. After that, I wear nitrile gloves to rise and clean the item to keep my hand oils from contaminating the object.



Link to comment
Share on other sites

Hi Druxey, 

thanks for getting back to me. I used 1/64x1/16 brass strips from K&S. I annealed, formed, silver soldered then filled with a #6 jewelers file and scrubbed with  a nylon brillo pad where I could. Cleaned with Soap and water with distilled water, rinsed with distilled water. soaked in muratic acid then rinsed in distilled water. Blackened in Blacken-It diluted about 50-50 with distilled water. Glued on with 5 minute epoxy as well as "nailing" with blackened nails and some epoxy.





Link to comment
Share on other sites

Sounds as if you did everything correctly, Sam. The only thing I can spot is that Blacken-It should be diluted about 8:1 to work well. However, that shouldn't have given you the result that you've shown. Definitely a metallurgist's or chemist's specialty question, I'm afraid.

Link to comment
Share on other sites

Hey Sam

Boy oh boy, I always get the fun questions. :dancetl6:

In a nut shell, I haven't got a clue, so I'll give the standard chemistry teacher answer, "Looks like metal contamination to me"


Seriously though, as an analytical chemist based in organic (pharma) seperations, my metallurgical skills aren't so great. Its something not covered to much in class and what little I know of metal chemistry is self taught and from my stint in quantitative analysis class.


So, what is it.

First I assume all metal is sourced from the same piece so we can assume the metal ratios are the same (Brass being a mix of copper and zinc basically, but depending on the brass other metal can be added for differing properties).

My first gut instint is to say it was a processing problem, but I don't understand how it effected one piece and not the other (this indicates contamination is most likely the problem, not the metal). Did the same reaction happen on the other side of the ship the same?

The strange reaction started when the oil was added and you state it caused the blackening to come off. In the picture it appears that neither the pintle or gudgeon are blackened anymore. Is this correct? How long was the blackened surface allowed to cure before oiling, although this shouldn't matter.


Blackening is a reaction that occurs with -2 anions typically (O2- and S-2, are most common). Natural aging occurs when the metal is exposed to the oxygen in the air naturally but most blackening reactions are a Sulfide reaction using what old school chem called Liver of sulphur. From my study of Liver of sulphur it isn't an exact science and can contain many differing salts to achieve the oxidation reaction. I call it oxidation because the process of oxidation to a chemist means it loses an electron (any anionic ion can oxidize, stuff on the right of the periodic table). Oxygen typically does this and is the most common oxidizer, but if you look at Sulfur (S) its right under O on the periodic table and so will have similar properties (as does Selenium...). So the blacken stuff is an oxidation of the copper in the brass and is supposed to be a stable coating, although this isn't the case here.


The sweat appears to be a reactant so it is still reacting with something. Does the sweat have a bluish or greenish colour (oxides and sulfides are typically black, where sulfates and chlorides typically have blue/green colourations)? What type of oil did you use BTW?


My gut instint still says contamination but it doesn't quite look right. It looks to me as though the black was removed from the pintle but remained on the gudgeon, then later peeled, which is giving it the mottled look with the sweat only appearing on the more brass looking spots.


Most brass is coated to prevent oxidation but annealing would burn off such a coating. Thats where many problems like this occur, where a coating is breached and then the oxidation occurs under the edge of a coating (think paint blisters on a car that rusts after a scratch in the paint). But this shouldn't have been an issue. Common sense is telling me the metal was contaminated between the cleaning and blackening process which caused the oxidation to not adhere. Were they ever touched between these steps and with what?

Although the oil seemed to be the catalysing agent.


The real problem here is the inconsistent reaction between the pintle and gudgeon. I hate to say this but "Not knowing I couldn't say with any degree of accuracy" which is chemical speak for "I don't have a clue". There are just so many variables. How long was there between blackening and oil?


Sorry I seem to have asked more questions then answered. My gut says contamination though. Most likely under the blackening, which the oil caused to become more reactive or become a faster reaction. I see nothing wrong with your procedure, is there any place in the steps where contamination could have happened, especailly between cleaning and blackening?

Link to comment
Share on other sites

Hey Dr Per,

Thanks, although metallurgy is the weakest part of my chemistry. Metal analysis is a different story as that was what I did mostly in TA'ing quantitative analysis as a grad student, which is all old school wet chem. Lots of titrations and Oxidation/Reduction reactions.

Of course modern technology such a AAS and ICP/MS make old school methods obsolete. (BTW - AAS is atomic absorption spectroscopy and ICP is inductively coupled plasm mass spec)


My first gut instinct would be to say the pintles and gudgeons were of differing metal as yes brass is a mix of many possible metals, with each giving some wanted attribute. IIRC lead is used sometimes for softening brass (malleability and ductility), tin is also common (IIRC copper and tin make bronze). In my quant class I had about 100 differing steel sample and the students had to determine the % of Mn and Cr in their given sample.

Sam stated though that the metal was all K&S stock so that gut instinct probably isn't correct.


When I see uneven reaction such as this I usually think contamination. Its so easy to contaminate something, but the oil seemed to play a part also in the post blackening stage, which is strange (maybe coincidence).

Brass etching is a good example of oxidation/reduction (redox reactions) being used to remove metal rather then form a protective coating. Redox reactions using metals typically remove metal as it's an electronegativity cannibalisation so to speak.


for example the Ferric Chloride (FeCl3) used in etching is a Fe+3 cation salt, which gains the copper metal electrons and the copper metal atom forms a copper ion which in turn then forms some salt with whatever anion (chloride Cl- , perhaps), so

          Cu + Fe+3 --> Cu+2 + Fe

Basically the cationic metal of the salt becomes metal and metal become a cationic ion - i.e. copper + iron chloride --> copper chloride + iron (although iron may not completely reduce to Fe but rather Fe+2 , this can occur in multi-valence metals)


The Redox reactions that occur with anionic components typically form salt coatings rather then metal replacement. For example stainless steel is created when the Cr percent in the steel reaches a certain pecent. This allows a high enough Cr content in the surface of the steel to allow a fast forming Cr2O3 which prevent the iron from forming rust or iron oxide. Copper Oxide is the first stage of oxidation of copper with air (think brown penny) then a later slower reaction happens when copper carbonate forms (think statue of Liberty) from the CO2 in the air. The carbonate is actually protective and stops further oxidation, just as in stainless steel.


Sams Sweat indicates a reaction is still going on, I just don't have a clue why. One crazy idea, was the piece rinsed in tap water after the acid bath? Was the Tap water well water or city water? I know Fe in water can play tricks in reactions, This is why Bourbon is made using iron free water, typically that has ran through limestone (think Jack Daniels :)) That probably got the attention from those dozing off.

Seriosly though, Ive seen Tap water ruin many experiment and in my research I used distilled water that was then quadruple filtered with 4 different reverse osmosis stages. Distilling can leave behind organics. But water doesn't answer why one piece and not the other? but Sam stated that the blackening was removed from both (if I'm reading correctly) which could be a friction of wiping rather then the oil. Salt on the surface under the blackening could have also reacted with the oil (salt and oil don't mix typically). Just so much doesn't make sense.


I'd be interested in knowing what type of oil was used (although it shouldn't matter). Mainly was it natural vs synthetic (think tung oil vs Wipe on poly), Also how long was the cure time between blackening and oil and also how long after the oil application before the blackening came off and the sweat appeared? was the blackening stable with no rubbing off or flaking before oiling?


I just love troublesooting :dancetl6:

Link to comment
Share on other sites

Below is the PM I just sent Keith.


A couple of things occurred to me as I was responding.

  • Minor detail - parts were filed to #4 cut file not #6. I dont think this played a part
  • The gudgeons were blackened separately not at the same time. I know because I remember trying to match the color on the gudgeons to the pintles.
  • I believe all the parts were made from the same brass stock but I cant swear to it. I bought a full box of 1/64x1/16 brass strips from an online hobby store, I need to look up the store but I want to say omnimodels.com. I think I may have had a package from my LHS but it was still K&S brand
  • I am 90% certain we started a new bottle of dish soap between blackenings.
  • Somebody asked about flux, yes I used Ottoflux from OttoFrei.com
  • all excess solder was filed off and I had a nice bright shiny bras part when I started the cleaning process
  • All my water was distilled water - I have done a fair amount of chemical wood staining in the past and learned the hard way about tap water and chemicals. ;)
  • The oil used is Tried and True Boiled linseed oil - a true boiled oil; no chemical dryers used, its non-toxic they claim you can use it on wooden bowls but I have never tried it.

The rest of the process is detailed below




Thanks for responding. My gut was telling me that you would give me an answer like that. No worries. 

I cant say for sure whether the pintles and gudgeons were from the same brass stock, I want to say yes of course. I have bought numerous brass pieces over the last several months. It is very possible they came from two different sources although my memory is that they all came from the same box of brass. I have found its easier to buy a whole box of brass online rather than one or two sleeves. 7 bags of two strips was almost the same price as 2 bags at my LHS. But like I said I have bought brass several times over the last year both online and at my LHS so I really cant say for sure.


Thinking back the gudgeons were blackened separately but the process was the same. With the exception of the soapy water and first and final rinses the containers had the same solutions in them


As far as touching the parts, once they were cleaned in soap and water I used latex gloves and tweezers to tie them to some 2 lb fishing leader. 


The entire process was

  • Anneal
  • form
  • silver solder
  • Mark and drill the "rivet" holes
  • File with #2 then #4 cut swiss files 
  • rub with purple nylon scrubby pad where I could
  • tie o fishing leader
  • Wash with hot distilled water
  • rinse with distilled water
  • soak in 50/50 muriatic acid  - diluted with distilled water - agitated every 10 minutes or so for an hour
  • Quick dunk in distilled water and baking soda
  • final rinse in distilled water
  • soak in 50/50 blacken-it and distilled water for 10-15 minutes agitating every couple of minutes and watching for color

Not sure what happened between blackening one set of parts to the next but now I am thinking something contaminated one of the solutions. They are kept in tupperware style containers - glad disposables. 


We did change dish soaps come to think of it, I wonder if that may have anything to do with it?


Thanks again for your time


BTW, loving the Pink Floyd themed build!

Link to comment
Share on other sites

Jay, I dont think I will try, those "rivets" are brass rod hammered over to form a head and epoxied in also. On a side note I wish I had seen the posting here about  making nails, I could have made mine on less than half the time it took to heat and mushroom over.


Based on Keith's observations, if I can get the reaction to stop I will try to airbrush some color over them. It was something I planed to do anyway. I was ok with the pintles, from normal viewing distances they looked aged and I kid of liked the look. I was going to try to get the gudgeons a bit closer and call it a learning experience. 


Link to comment
Share on other sites

I'm with you, Sam: I don't think I would have pulled them either--probably cause more damage than the "sweating." Painting sounds like the way to go, esp. if you can neutralize the reaction. Hopefully Keith will have a trick up the sleeve on how to neutralize what's going on. Hope it all works out for you and your build, Sam!



Link to comment
Share on other sites

  • 4 months later...

Can anybody tell me what is happening here?


I finished installing my pintles and gudgeons a couple of weeks ago. Had a bit of a issue with the blackening rubbing off when I oiled over them. I am ok with the rudder pintles, it looks weathered, the gudgeons came out differently.

Looking at your picture, I am almost sure that the 'green' is caused by residual acid in the part. I cannot imagine that the wood has anything to do with that???


When you blacken brass, or any other metal for that matter, the compound is based on nitric acid, and then some. Unless those pieces are completely acid free they might continue to 'rust' afterwards.


The fact that you probably treated both parts at the same time and they came out looking like that is a mystery to me. To cover up the process with oil is something we will never know about?????


You might try to touch up the parts with some 'base' like a mild solution of sodium bicarbonate (baking soda]. Let it sit for a while and see if there is any visual reaction on the pieces. If there is, acid left overs would be the reason.

Continue with the baking soda and rinse (if possible) afterwards.

The base should have no affect on the wood or finish that I know of.

Good luck.

Link to comment
Share on other sites


My apologies, I should have posted a follow up to this when I finished. 

I ended up scrubbing everything down with a nylon scrubby pad to bright brass and then washed with backing soda. after that I used some black paint with a brown tint to it, airbrushed it on and called it a day. Not 100% happy with it bit MUCH better than corroded green.




I am still not positive what went wrong, guessing that I didnt get all the blacken it off or neutralized.


Thanks everyone for your help and input.



Link to comment
Share on other sites

  • 3 months later...

Hello Sam, 


I think I might have something to contribute on this mystery. I have been experimenting with Blacken-it (will post my results someday soon), and recently ran into an issue with sweating on some brass test pieces. In both cases I prepared the pieces with a muriatic acid pickle, followed by a deionized water rinse, then an acetone bath, and another deionized water rinse. Both pieces were then placed in undiluted Blacken-it and agitated until the desired colour was obtained. In both instances I DID NOT rinse with water (or water with baking soda), after using the Blacken-it. I simply forgot to do this because I was so happy at finally achieving an even, non-flaky finish. I buffed them with a soft cloth and left them on my work-bench, happy with a good result. 


Here is where it gets interesting. 


One of the parts was left on the base of a flat steel table lamp, the other on my rubber/plastic cutting mat. When I came down to my workroom the next day I noticed that the piece on the table-lamp was damp - when I wiped my finger across it, the black surface smeared off like ink, leaving a copper-tinged streak across the metal. The other piece looked perfectly fine. A day later, however, the piece on the mat also started to sweat, but to a much lesser degree.


My conclusion is that the chemical reaction was still ongoing in both pieces, and had been sped along by contact with the steel table lamp in the first piece (a sort of electrolysis, I assume). I think this is because I forgot to neutralize the pieces prior to buffing them. 


Sam, I noticed in your post that you didn't rinse the pieces to neutralize the reaction at the end of the blackening process. Many use water for this, but perhaps a weak solution of baking soda and water might be prudent. 


Just my two cents - the mysteries of the blue liquid are still eluding me. Someday I'll do a post about my findings - when I finally figure out a process that results in good, stable, results. 

Edited by E&T
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Create New...