Of course, before asking you this question, I looked up on Google with keywords such as "bronze", "corrosion", "unrust", "acid", etc. I did not find the answer I was looking for, but I found articles about a dreaded affliction called "bronze disease".
Instead of being a light patina on a bronze object, the "bronze disease" is a patch of clear and non shiny fungus-like "moss", which attacks the metal by a self-sustainng reaction producing hydro-chloric acid (there is no bacteria working there, it is purely chemical).
To my horror, I realized that some of my precious arrow-heads are suffering from this disease (see example below).
The websites say it is difficult to get rid of, and that I should first dip the heads into distilled water, before starting more technical treatments.
Have you yourself cured some artifacts from this "disease" ? If so, following which method ?
Dave
Romain, this is purely hypothetical, but you could maybe put the arrowheads into bicarbonate soda to neutralise the acid formed. Maybe.
Dave
Bede
I have a museum curation article at home on how to permanently fix bronze disease. A friend in Japan had problems with some arrow heads and he followed the instructions. He said it fixed the problem.
Are you interested in the steps involved?
Bede
Stephen,
I do not think your recommendation is appropriate for an object with bronze disease. This is an ongoing chemical reaction that must be stabilized before any other treatment is applied.
Any lacquer coating will affect the resale value of any antique bronze object where patina is a criterion. This might not apply in this case.
A bronze item with bronze disease would have no resale value anyway.
jack farrell
Romain; It is truely serendipitious that you have made a post about Bronze disease. I recently took an ancient Greek point to the local History museum for the same problem and the diagnosis was the same, Bronze disease. I was given a reprint from NRLC, National Research Laboratory for Conservation of Cultural Property, web site. Their treatment involves Zinc powder. I have yet to try any remedy but will also try Stephen's as a comparison. Without doing anything, I risk loosing some good old stuff. If not properly cared for it would have been better to have just left it where in the the ground! -j-
Romain
Jack,
First, let me thank you for ameliorating my English ! I did not know the word serendipitious but I found on Google that "Serendipity is finding something unexpected and useful while searching for something else entirely".
Good word !
The recipees I found up to now to fight bronze disease are the following :
-cook the artifact in an oven for about one hour. It will suspend the attack for a while, but it will restart as soon as humidity is back. (Web)
-dip the artifact in pure distilled water for about 2 weeks. It seems contradictory to the advice above but distilled water has properties that normal tap water doesn't. Something to do with ions. Change the water every few days. (Web).
-dip the artifact in a solution of sodium sesqui-carbonate. I don't know what it is. (Web)
-coat the artifact with Incralac sealant (Stephen's way above). Maybe it will stop the reaction from going further.
-I'm still waiting for Bede's way.
Romain
I just found this on Bronze disease :
Synergistic effects of corrosion inhibitors for copper and copper alloy archaeological artefacts Stavroula Golfomitsou and John F. Merkel
Abstract Benzotriazole (BTA) is one of the most widespread treatments used at present for the stabilisation of bronze disease. Unfortunately, BTA does not always work effectively when applied to heavily corroded copper and copper alloy archaeological artefacts. Combinations of corrosion inhibitors are extensively used in industry to retard copper corrosion. When two or more corrosion inhibitors are used, the inhibition efficiency of the mixture may be improved. This paper presents data that indicate the mixture of BTA with other inhibitors for chloride-containing corrosion products improve corrosion prevention under accelerated testing conditions. The six selected compounds. 5-Amino-2 Mercapto-1, 3, 4-Thiadiazole (AMT), Benzylamine (BZA), Ethanolamine (ETH), 1-Phenyl-5-Mercapto-Tetrazole (PMT), Potassium Ethyl Xanthate (KEX) and Potassium Iodide (KI) were tested individually and in combination with BTA. Factors such as concentration of the solution, time of immersion and solvent used (de-ionised water, and ethanol) were examined. The inhibitors were applied initially on artificially corroded copper coupons and accelerated corrosion tests were assessed. The treated coupons were analysed using Scanning Electron Microscope (SEM-EDS) to examine the effects of such combinations. The results indicate that the combination of BTA with AMT improves the inhibitive efficiency at lower concentrations with ethanol or deionised water as a solvent. Finally, the combinations of BTA with AMT and BTA with PMT were applied on archaeological objects from the excavations of Kaman Kaleh?yuk in Turkey and Mochlos in Crete, Greece. Treated objects have been monitored on an annual basis to investigate their long-term performance.
It doesn't help much as I don't know where to find BTA in Beijing, but at least it confirms the good action of distilled water.
Romain
Something more feasible, it combines the soda of Dave, the distilled water from the Web and the sealant from Stephen:
Bronze disease is a light green powder that comes off easily and often forms pits deep into the coin. In the second example, the reddish-brown is copper that has been freed from the bronze matrix, producing the "scarring" that is characteristic of BD.
The best treatment of BD is prevention. Do not remove the patina on ancient coins. BD will often rapidly attack coins that had a patina removed by over cleaning. Keep your coins in a dry place and use desiccant if you live in a humid area. Unfortunately, if you have a large collection, someday you will probably have to deal with at least one case of BD.
What to Do
1) With running water and a stiff nylon brush, scrub the entire surface free of "green fuzz", allow to dry.
2) Use a magnifying light and a sharp needle to remove and open any obvious green spots still visible. With care these will not be obvious later.
3) Pre-pare a bath of 5 parts baking soda [sodium bi-carbonate] to 8 parts washing soda [sodium carbonate]. You may go by weight or simple dry measurement, i.e. tablespoons full. Store the mix in an air-tight container.
4) Use de-mineralized or de-chlorinated water for better results.
5) Mix 2 tablespoon of the soda mix to three cups of water; add coins and heat the mix to boiling, reduce heat for a minimum of 5 minutes. Set aside and soaking to continue for an extended period of time. This should be at least 24 to 36 hours, for thick coins like sestertius allow at least 72 hours. After the initial soak, rinse with clean water and give the coin/s a light scrubbing.
6) Repeat step 5, twice more.
7) After the last soak and scrub, dry the coins, and soak in 100% isopropyl alcohol, for about 20 minutes. This will help draw out more water from the coin fabric. Dry thoroughly, and seal with a paste wax, well rubbed into the coin.
8) It is recommended that all bronze coins be checked periodically, as I have found "clean" coins suddenly break out 2 years after purchase.
9) While this procedure should not effect a true green patina. It will remove any artificial coloring or re-patination.
So I must find 2 different types of soda. Could you tell me what is the difference between isopropyl alcohol and normal alcohol ?
Fox
:P
Romain,
I am visiting Beijing 1st or 2nd week of July and would be nice to get together.
My source for the following comments is ?Conservation & Restoration for Small Museums? published by the Western Australian Museum in 1981.
After discussing some detailed tests for bronze disease, which they describe as cyclic corrosion in a humid environment, they detail a four step process to remove the problem.
Step 1 is degreasing using a detergent or a soap. Other products can be used with very dirty objects. Methylated spirits and trichlorethylene are suggested. Mild abrasion using cotton buds is also useful for removing some accretions.
Step 2 involves removing corrosion. This must be done sensitively so that patina is preserved. A mild citric acid bath is described. All traces of the acid should be removed by soaking in water and washing. Acetone is used to complete the cleaning. This step might be skipped if the patina is intact and no visible signs of bronze disease are present.
Step 3 is removing chlorides from the object. This is done by immersion in a 2% solution of sesquicabonate ( they suggest 10 grams of sodium bicarbonate and 10 grams of sodium carbonate in one litre of distilled water) for a considerable time. The solution is changed weekly for the first month and monthly until the fourth month. The process produces a green brown stable patina. They suggest a further process if bronze disease reappears.
Step 4 is protecting the surface, either using Incralac as Stephen suggests or mirocrystalline-polyethylene wax. However, they imply that a patina which forms on the bronze is generally stable except in harsh conditions and does not need any coating.
The above comments have been paraphrased by me and I may have made some mistakes through lack of understanding of the chemistry. The arrowheads have a patina which is being disfigured by bronze disease. I think that following the book?s instructions will certainly help. The essential points are cleaning the surface and neutralising the chemical actions of the chlorides.
Stephen Selby
I think Bede's post addresses Romain's post #3 in this series. I should really split the post, but it has got too entwined by now.
Bede is talking about combatting 'bronze disease'. You have been commenting on freeing up the crossbow mechanism to make it work. I have been dodging between the two.
Romain
Alas, there is nothing wrong with your monitor settings, this is the actual color of my arrowheads today...
After dipping them in distilled water for a few weeks, there was a real improvement, with patches of "illness" totally disappearing.
Then I got my hands on a bag of sodium bicarbonate. I put a little amount in my distilled water and forgot about the whole thing 3 weeks.
Today to my horror, I saw that they turned deep blue with patches of bright pink.
I put them back into distilled water to wash them, but I'm afraid it will take a few millenia before they return to their nice green shade that they had before...
