Full Version: Malachite Encrustation On Chinese Artefacts
Recently, I have had a chance to see a Russian dagger being treated by removing a heavy Malachite Encrustation after being unearthed from a place in Khazar. I wonder would this phenomenon is common in Chinese Artefacts? One of the tricks I heard from people around museum is to use sulfamic acid to remove many kind of oxides and malachite encrustation. Can anyone describes better this process?
QUOTE (Boleslaw I @ Apr 5 2008, 03:39 PM) 
Recently, I have had a chance to see a Russian dagger being treated by removing a heavy Malachite Encrustation after being unearthed from a place in Khazar. I wonder would this phenomenon is common in Chinese Artefacts? One of the tricks I heard from people around museum is to use sulfamic acid to remove many kind of oxides and malachite encrustation. Can anyone describes better this process?
Do you have a reference to the story? It would be very interesting - assuming it is in English!
QUOTE (William O'Chee @ Apr 5 2008, 08:21 AM)
Do you have a reference to the story? It would be very interesting - assuming it is in English!
Oh! It is like this. My teacher brought some pictures he took from an archaeological site. The item is dated around VI century or VII century and supposed to belong to a rich man during the Age of Khazan Empire. Yet, it is covered with malachite encrustation. I have seen some Chinese artefacts too, but these are bronze, not iron, so I am waiting for someone like Kenneth give opinion.
I don't understand why the malachite would be removed, as it is only aesthetic I assume, and the surface both protects the object and preserves historic & enviromental information.
That being said, I have not looked at removing patina in depth but have followed some discussions. There are a range of chemicals that are suggested as removing patina, and these are chosen carefully so as not to cause alterations in themselves.
I'll ask Jim on Antiquities Science group for the 'best' method as it was discussed there a while ago. I believe even sonic cleaning was attempted by some members during experimentation, and some ancient coins were cleaned quite well via a long process.
Of the removal of patina however, unless it is obscuring some detail or the piece is something of little worth, then it is rather a shame to remove the end result of a very long process from a surface.
That being said, I have not looked at removing patina in depth but have followed some discussions. There are a range of chemicals that are suggested as removing patina, and these are chosen carefully so as not to cause alterations in themselves.
I'll ask Jim on Antiquities Science group for the 'best' method as it was discussed there a while ago. I believe even sonic cleaning was attempted by some members during experimentation, and some ancient coins were cleaned quite well via a long process.
Of the removal of patina however, unless it is obscuring some detail or the piece is something of little worth, then it is rather a shame to remove the end result of a very long process from a surface.
Hi Boleslaw I,
I put together some info from the Yahoo group.
Here are a couple of posts by knowledgable members of the Antiquities Science group. Both seem to prefer mechanical cleaning to chemical. Roger however did manage to strip a Song coin of heavy encrustation as part of an experiment and this is an extract from the discussion when he posted a series of photos of each stage:
Roger Seganti
-------------------------------------------------------------------------
Melanie Roy is a proffesional who works in the scientifical appraisal of artefacts, and here was her suggestion to a member who wished to clean an object:
Melanie Roy posts:
--------------------------------------------------------------------------
I have used both mechanical and chemical cleaning to remove patina previously, though very very rarely, mainly to reveal some hidden detail. I leave as much as I can intact, and for instance applied weak acid to only >10% of one objects surface to reveal detail & gold gilding under a patina. I spent 2 weeks just brushing a little diluted citric acid (i.e lemon juice) to one Han artefact on a specific spot, and had good result removing a patina on another object using citric acid, about 20% of the surface cleared to reveal some very excellent but hidden art underneath.
I am really reluctant to remove patina and disagree with it in principle however. Leaving as much as possible intact is best, for this reason only a fraction of one side of an object is enough to reveal the quality of detail below.
Mechanical cleaning, I find too difficult, as patina is fused to the bronze itself (the actual line between patina and original metal is blurred) and removal mechanically can damage the bronze. I have had a few experiences like that, which the patina going below the orginal metal surface, and use lemon juice to slowly reduce a patina for that reason. Roger & Melanie seem to have had more success with mechanical means. Citric acid also however runs the risk of activating bronze disease where copper oxide is converted to copper chlorides (IIRC the chemical process) and while I haven't had this happen it is a second reason I would be reluctant to alter a stabile and intact mineral patina. It formed over thousands of years after all.....I feel that messing with it is not appropriate without a pressing reason!
For more info on chemical cleaning I have a Pdf. called "a study of reagents used in stripping bronze" (1978) which uses several chemicals, including citic acid, so that might be a good start.
Send me an message via my profile with your e-mail address and I will sent the file/files.
I have another called "aspects of Bronze patina & its treatment" 1968 & a 1974 paper on Malacite with is more geology & its properties, plus one called "The conservation of bronze" 1964 which discusses issues over cleaning. This is the Hanna Jedrzejewska paper that Roger mentioned, and explains why even mechanical removal of patina can damage the original surface of a bronze.
These papers will answer any questions you have relating to the process of corrosion, its effects on the object, and the chemical properties of corrosion.
If you like, sign up to the Yahoo group "Antiquities Science". http://groups.yahoo.com/group/Antiquities_Science/
It would be a good group to put any such future questions to as I may have missed some details beyond those I took from Melanie & Roger.
I put together some info from the Yahoo group.
Here are a couple of posts by knowledgable members of the Antiquities Science group. Both seem to prefer mechanical cleaning to chemical. Roger however did manage to strip a Song coin of heavy encrustation as part of an experiment and this is an extract from the discussion when he posted a series of photos of each stage:
Roger Seganti
QUOTE
"I used a low concentration of Muriatic acid ( Muriatic Acid is also know as Pool or Masonry Acid ).
MA is basically about (some concentrations change in different countries) 30% HCl Hydrochloric Acid and 70% Water. Here in Italy the Muriatic Acid is made of about 10% HCl and 90% water. I soaked the coin for about 30 seconds mainly to see if there was any reacting with Calcium carbonate (Calcite) it did also clean a little.
I never use electrochemical or electrolytic methods in cleaning bronze artefacts, they both tend to strip away too much.
I always clean mechanically (under microscope) corrosion products, you have a better control in cleaning and I usually like to leave some corrosion products or some nice crystals if found. It takes some time, but this way you don’t completely strip the artefact.
(Rogers notes result: some bubbling around the azurite crystals and a slight loss of the superficial malachite, the crystals that bordered the cuprite and bare metal remained)
Roger adds: I find Hanna Jedrzejewska studies on this argument very interesting
"The Conservation of Ancient Bronzes"
She advices always to leave a small area of the original corrosion products on the artifact.
David A Scott in
Copper and Bronze in Art
Page 357
Gives this reference and opinions on Jedrejewska's studies.
“Although thermodynamically less damaging than chemical cleaning, mechanical cleaning may remove evidence of materials, such as burial conditions and corrosion chemistry, adhering to the surface of an object. This evidence is lost forever when these products are removed. In a paper dealing with the mechanical cleaning of some Egyptian bronzes, Jedrejewska (1976) made a clear case for leaving some bronze uncleaned to serve as archeological records in their own rights. For those bronzes that are cleaned, she advocated that a small area of the surface be left untreated, if at all possible, to serve as a record of the true state of original patina. Over the last twenty years this wise advice all too often has not been heeded by conservators”
MA is basically about (some concentrations change in different countries) 30% HCl Hydrochloric Acid and 70% Water. Here in Italy the Muriatic Acid is made of about 10% HCl and 90% water. I soaked the coin for about 30 seconds mainly to see if there was any reacting with Calcium carbonate (Calcite) it did also clean a little.
I never use electrochemical or electrolytic methods in cleaning bronze artefacts, they both tend to strip away too much.
I always clean mechanically (under microscope) corrosion products, you have a better control in cleaning and I usually like to leave some corrosion products or some nice crystals if found. It takes some time, but this way you don’t completely strip the artefact.
(Rogers notes result: some bubbling around the azurite crystals and a slight loss of the superficial malachite, the crystals that bordered the cuprite and bare metal remained)
Roger adds: I find Hanna Jedrzejewska studies on this argument very interesting
"The Conservation of Ancient Bronzes"
She advices always to leave a small area of the original corrosion products on the artifact.
David A Scott in
Copper and Bronze in Art
Page 357
Gives this reference and opinions on Jedrejewska's studies.
“Although thermodynamically less damaging than chemical cleaning, mechanical cleaning may remove evidence of materials, such as burial conditions and corrosion chemistry, adhering to the surface of an object. This evidence is lost forever when these products are removed. In a paper dealing with the mechanical cleaning of some Egyptian bronzes, Jedrejewska (1976) made a clear case for leaving some bronze uncleaned to serve as archeological records in their own rights. For those bronzes that are cleaned, she advocated that a small area of the surface be left untreated, if at all possible, to serve as a record of the true state of original patina. Over the last twenty years this wise advice all too often has not been heeded by conservators”
-------------------------------------------------------------------------
Melanie Roy is a proffesional who works in the scientifical appraisal of artefacts, and here was her suggestion to a member who wished to clean an object:
Melanie Roy posts:
QUOTE
".....acid solutions for stripping encrustations are not wise, as they
will often reduce the copper-based corrosion products to metallic
copper, leaving a layer of this metal on the surface of the artifact.
We have worked with many thousands of bronze artifacts, ranging in
value from a few hundred dollars to several hundred thousand
dollars. Regardless of value, the method we find most reliable and
safest, for the artifact, is to gently remove the encrustation
mechanically with hand tools. We avoid power tools.
The one chemical we will occasionally use (stress occasionally) is
ammonium hydroxide. There are a great many copper-based corrosion
products that will dissolve in this chemical without leaving behind
anything we do not want, and without excessive risk of attacking the
uncorroded core metal. Be aware that this same chemical is used in
metallography to etch copper-based alloys, so do not leave it on the
metal! Carefully apply it to the corrosion products, and use damp
swabs to remove any spill from the exposed metal. Give the chemical
a little time to work on the encrustation and then you can use hand
tools or a soft brush to work on the encrustation. I must stress
that this is not a method we recommend, as all chemical cleaning
comes with risks to the artifact, and to the human. If you choose to
try NH4OH you must work in a well ventilated area, wear gloves,
splash goggles, etc.
The ammonia {note: some people use household ammonia} you buy for household cleaning is
not what we use. The industrial solution we use can harm you if handled improperly!
......Long story short, with roughly 30 years of experience in handling
artifacts, we almost invariably chose mechanical cleaning of bronze
over any chemical methods. It may be time consuming, but the results
are better in most cases.
will often reduce the copper-based corrosion products to metallic
copper, leaving a layer of this metal on the surface of the artifact.
We have worked with many thousands of bronze artifacts, ranging in
value from a few hundred dollars to several hundred thousand
dollars. Regardless of value, the method we find most reliable and
safest, for the artifact, is to gently remove the encrustation
mechanically with hand tools. We avoid power tools.
The one chemical we will occasionally use (stress occasionally) is
ammonium hydroxide. There are a great many copper-based corrosion
products that will dissolve in this chemical without leaving behind
anything we do not want, and without excessive risk of attacking the
uncorroded core metal. Be aware that this same chemical is used in
metallography to etch copper-based alloys, so do not leave it on the
metal! Carefully apply it to the corrosion products, and use damp
swabs to remove any spill from the exposed metal. Give the chemical
a little time to work on the encrustation and then you can use hand
tools or a soft brush to work on the encrustation. I must stress
that this is not a method we recommend, as all chemical cleaning
comes with risks to the artifact, and to the human. If you choose to
try NH4OH you must work in a well ventilated area, wear gloves,
splash goggles, etc.
The ammonia {note: some people use household ammonia} you buy for household cleaning is
not what we use. The industrial solution we use can harm you if handled improperly!
......Long story short, with roughly 30 years of experience in handling
artifacts, we almost invariably chose mechanical cleaning of bronze
over any chemical methods. It may be time consuming, but the results
are better in most cases.
--------------------------------------------------------------------------
I have used both mechanical and chemical cleaning to remove patina previously, though very very rarely, mainly to reveal some hidden detail. I leave as much as I can intact, and for instance applied weak acid to only >10% of one objects surface to reveal detail & gold gilding under a patina. I spent 2 weeks just brushing a little diluted citric acid (i.e lemon juice) to one Han artefact on a specific spot, and had good result removing a patina on another object using citric acid, about 20% of the surface cleared to reveal some very excellent but hidden art underneath.
I am really reluctant to remove patina and disagree with it in principle however. Leaving as much as possible intact is best, for this reason only a fraction of one side of an object is enough to reveal the quality of detail below.
Mechanical cleaning, I find too difficult, as patina is fused to the bronze itself (the actual line between patina and original metal is blurred) and removal mechanically can damage the bronze. I have had a few experiences like that, which the patina going below the orginal metal surface, and use lemon juice to slowly reduce a patina for that reason. Roger & Melanie seem to have had more success with mechanical means. Citric acid also however runs the risk of activating bronze disease where copper oxide is converted to copper chlorides (IIRC the chemical process) and while I haven't had this happen it is a second reason I would be reluctant to alter a stabile and intact mineral patina. It formed over thousands of years after all.....I feel that messing with it is not appropriate without a pressing reason!
For more info on chemical cleaning I have a Pdf. called "a study of reagents used in stripping bronze" (1978) which uses several chemicals, including citic acid, so that might be a good start.
Send me an message via my profile with your e-mail address and I will sent the file/files.
I have another called "aspects of Bronze patina & its treatment" 1968 & a 1974 paper on Malacite with is more geology & its properties, plus one called "The conservation of bronze" 1964 which discusses issues over cleaning. This is the Hanna Jedrzejewska paper that Roger mentioned, and explains why even mechanical removal of patina can damage the original surface of a bronze.
These papers will answer any questions you have relating to the process of corrosion, its effects on the object, and the chemical properties of corrosion.
If you like, sign up to the Yahoo group "Antiquities Science". http://groups.yahoo.com/group/Antiquities_Science/
It would be a good group to put any such future questions to as I may have missed some details beyond those I took from Melanie & Roger.
QUOTE (Kenneth @ Apr 7 2008, 09:30 PM)
Hi Boleslaw I,
I put together some info from the Yahoo group.
Here are a couple of posts by knowledgable members of the Antiquities Science group. Both seem to prefer mechanical cleaning to chemical. Roger however did manage to strip a Song coin of heavy encrustation as part of an experiment and this is an extract from the discussion when he posted a series of photos of each stage:
Roger Seganti
-------------------------------------------------------------------------
Melanie Roy is a proffesional who works in the scientifical appraisal of artefacts, and here was her suggestion to a member who wished to clean an object:
Melanie Roy posts:
--------------------------------------------------------------------------
I have used both mechanical and chemical cleaning to remove patina previously, though very very rarely, mainly to reveal some hidden detail. I leave as much as I can intact, and for instance applied weak acid to only >10% of one objects surface to reveal detail & gold gilding under a patina. I spent 2 weeks just brushing a little diluted citric acid (i.e lemon juice) to one Han artefact on a specific spot, and had good result removing a patina on another object using citric acid, about 20% of the surface cleared to reveal some very excellent but hidden art underneath.
I am really reluctant to remove patina and disagree with it in principle however. Leaving as much as possible intact is best, for this reason only a fraction of one side of an object is enough to reveal the quality of detail below.
Mechanical cleaning, I find too difficult, as patina is fused to the bronze itself (the actual line between patina and original metal is blurred) and removal mechanically can damage the bronze. I have had a few experiences like that, which the patina going below the orginal metal surface, and use lemon juice to slowly reduce a patina for that reason. Roger & Melanie seem to have had more success with mechanical means. Citric acid also however runs the risk of activating bronze disease where copper oxide is converted to copper chlorides (IIRC the chemical process) and while I haven't had this happen it is a second reason I would be reluctant to alter a stabile and intact mineral patina. It formed over thousands of years after all.....I feel that messing with it is not appropriate without a pressing reason!
For more info on chemical cleaning I have a Pdf. called "a study of reagents used in stripping bronze" (1978) which uses several chemicals, including citic acid, so that might be a good start.
Send me an message via my profile with your e-mail address and I will sent the file/files.
I have another called "aspects of Bronze patina & its treatment" 1968 & a 1974 paper on Malacite with is more geology & its properties, plus one called "The conservation of bronze" 1964 which discusses issues over cleaning. This is the Hanna Jedrzejewska paper that Roger mentioned, and explains why even mechanical removal of patina can damage the original surface of a bronze.
These papers will answer any questions you have relating to the process of corrosion, its effects on the object, and the chemical properties of corrosion.
If you like, sign up to the Yahoo group "Antiquities Science". http://groups.yahoo.com/group/Antiquities_Science/
It would be a good group to put any such future questions to as I may have missed some details beyond those I took from Melanie & Roger.
I put together some info from the Yahoo group.
Here are a couple of posts by knowledgable members of the Antiquities Science group. Both seem to prefer mechanical cleaning to chemical. Roger however did manage to strip a Song coin of heavy encrustation as part of an experiment and this is an extract from the discussion when he posted a series of photos of each stage:
Roger Seganti
-------------------------------------------------------------------------
Melanie Roy is a proffesional who works in the scientifical appraisal of artefacts, and here was her suggestion to a member who wished to clean an object:
Melanie Roy posts:
--------------------------------------------------------------------------
I have used both mechanical and chemical cleaning to remove patina previously, though very very rarely, mainly to reveal some hidden detail. I leave as much as I can intact, and for instance applied weak acid to only >10% of one objects surface to reveal detail & gold gilding under a patina. I spent 2 weeks just brushing a little diluted citric acid (i.e lemon juice) to one Han artefact on a specific spot, and had good result removing a patina on another object using citric acid, about 20% of the surface cleared to reveal some very excellent but hidden art underneath.
I am really reluctant to remove patina and disagree with it in principle however. Leaving as much as possible intact is best, for this reason only a fraction of one side of an object is enough to reveal the quality of detail below.
Mechanical cleaning, I find too difficult, as patina is fused to the bronze itself (the actual line between patina and original metal is blurred) and removal mechanically can damage the bronze. I have had a few experiences like that, which the patina going below the orginal metal surface, and use lemon juice to slowly reduce a patina for that reason. Roger & Melanie seem to have had more success with mechanical means. Citric acid also however runs the risk of activating bronze disease where copper oxide is converted to copper chlorides (IIRC the chemical process) and while I haven't had this happen it is a second reason I would be reluctant to alter a stabile and intact mineral patina. It formed over thousands of years after all.....I feel that messing with it is not appropriate without a pressing reason!
For more info on chemical cleaning I have a Pdf. called "a study of reagents used in stripping bronze" (1978) which uses several chemicals, including citic acid, so that might be a good start.
Send me an message via my profile with your e-mail address and I will sent the file/files.
I have another called "aspects of Bronze patina & its treatment" 1968 & a 1974 paper on Malacite with is more geology & its properties, plus one called "The conservation of bronze" 1964 which discusses issues over cleaning. This is the Hanna Jedrzejewska paper that Roger mentioned, and explains why even mechanical removal of patina can damage the original surface of a bronze.
These papers will answer any questions you have relating to the process of corrosion, its effects on the object, and the chemical properties of corrosion.
If you like, sign up to the Yahoo group "Antiquities Science". http://groups.yahoo.com/group/Antiquities_Science/
It would be a good group to put any such future questions to as I may have missed some details beyond those I took from Melanie & Roger.
Thanks, that is suprising. I myself do not concur with removing TOTALLY the malachite encrustation. I think the encrustation on the dagger must be quite thick, so it must be taken away. Such an artefact as your Ji with heavy earth encrustation, plant root remains and crystallisation is a treasure, an achievement of more than thousand years formation. I enjoys your post in another forum about cerrusite. It is amazing!
There is also some accounts in the discover of Scythian tomb in Siberia. Anna Svetlana, a specialist in metal conservation also said the same thing as you when she exclaimed how hard it is to remove malachite and earth encrustation from a ornament sword hilt, which made by gold. Her task is too challenging that she must perform everything by hands, since chemical use might do harmful to the precious artefact. I would sign in Yahoo group as soon as possible. This kind of topic is very interesting.
For many reasons, some artefacts with extremely heavy earth encrustation did not undergo any process of cleaning. In order to reveal its internal details, scientists perform X-Radiography. I also read about this technique it is very useful to detect hidden pattern in metal artefacts. Science now facilitates everything.
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