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I've noticed on bronze pieces that there are three basic kinds of patina. A dark brown patina which I take to be copper oxide, a blue/green/gray patina which I take to be coppercarbonate/sulfate, and a deep striking green patina that at times can look like marble.
Does anyone know what this dark green patina is and how it's formed?
The blue/green patina can be easily faked. Should there always be a layer of brown patina underneath it?
Are there any books/websites that explain patination?
Any help would be appreciated.
If anyone knows of a really good resource about patination please contact me and I can place the link on this page.
The basic process I'm describing below applies to buried bronzes; the process is a little different for objects left in the open air.
Bronze is an alloy of copper and tin, and initial patination occurs as copper ions react with oxygen in the burial environment. First, a brownish layer(tenorite, CuO) is formed. Cuprite (Cu2O) then forms over the tenorite, which has more of a reddish/pink tint due to its higher copper content. In a burial context, these layers actually penetrate the metal, and attempting to remove them can substantially damage the object. (You'll notice that you rarely see bronze objects stripped down to the core metallic bronze color because doing so usually wrecks the item in question.)
The various shades of green and greenish-blue that you've noticed are secondary corrosive products that develop on top of the tenorite & cuprite. This layer can be composed of various carbonates, sulfates, and chlorides, depending on what is present in the soil, and it can become quite thick and irregular. Skilled cleaning of these secondary deposits can reveal lots of details that were previously obscured because the deposits generally don't penetrate the metal substrate.
You should be able to see that the patina is layered,
and it can certainly flake if suitably thick and irregular. The layering is the major key -- most fake patination methods fail to place an adequate tenorite & cuprite layer, leaving only a flat greenish color from the use of various simple, hydrated salts.
There are some very high-grade fake Egyptian bronzes on the market now which possess a cuprite-like red layer on an acid-browned fake tenorite. The key when looking at bronze is to scrape into it and look for actual infiltration of the corrosion into the epidermis of the metal. This cannot be faked.
When allowed, iron corrodes very quickly, but source, style and culture play a very important role in authentication.
Stone is a different kettle of fish altogether. Toolmarks are the key when assessing stone. If evidence of modern high-speed grinders can be seen, or the style doesn't match the period or culture or the material is wrong for the period or culture then a piece can be debunked effectively.
These factors are all learned through extensive research and possessing many objects in-hand over many years. If you have some ancient bronze or iron or lead or silver, scrape into it laterally in an inobvious place on the underside and look very carefully at the layers which reveal themselves.
With stone, the first test is to make it wet and smell it. You'll smell a very sweet and deeply rank and musty smell - kind of like an old, dirty floor mop. I suggest using distilled water when testing with water, as it has no bromine/chlorine or other contaminates in it which may leave staining when dry.
General tests for restoration
If you don't know how to look for restoration, here's a couple of relatively easy and efficient methods:
1. Put a pin in a cork, heat the exposed end until it is red hot and prod the object in various suspect areas. Restoration and repairs will melt/hiss/smoke/smell or do something funny. Intact areas will remain unchanged as long as you are gentle. This works on Roman glass too. It will not tell you if something is genuine or fake - just whether or not there is repaired (and concealed) damage.
2. Spray some water over the object, using one of those spray-mist-bottles which miraculously fill themselves up next to the ironing board. If the surface is porous (i.e. not waxed etc) repairs will show up amazingly clearly and the water quickly evaporates without causing any damage.
More about general considerations of fakery from Rolf HERE
The smell test
Moistened ancient pottery and sometimes faience as well, has a distinctive musty smell.
But take note of Ben's comments at the top of this page!
With clay, I believe the smell is a function of activated microbial excretion left by anaerobic micro-organisms which consume hydrocarbons present in the clay and excrete fatty-acids. Anaerobes as opposed to aerobes, as there is probably not much oxygen inside a dense pottery matrix, but I could be wrong. The other argument for anaerobes is that the extent of consuption of the actual structural material of the pottery would require an extended period of time. Aerobes work very quickly and anaerobes work very slowly.
The microbial excrement eventually becomes dessicated through lack of moisture, having consumed what moisture exists in the clay in order to carry out their life-cycle, which means no smell when dry. With fakes, they smell when they're dry. Not just when wet, like ancient potter and stone.
Human body-odours are caused by much the same process, only the microbes consuming sweat in our warm, dark regions are aerobic and work much more quickly. The smell in body odour is the smell of the fatty acids, or microbial excrement. Foot-smell and underarm smell are quite similar, but our noses discern them differently. But we're getting off-topic.
When the microbes consume the molecules of hydrocarbon in the clay's fabric, they consume the food and moisture, excrete fatty acids, reproduce and continue as such until they exhaust their supplies. They then die off, creating a food source for the more resilient microbes, and, by the time a piece of ancient pottery makes it's way to one of us, the internal fabric of the pottery is much different physically than it was when first manufactured. It is much lighter in weight and vacuuous for moisture.
If you wet ancient buff clay, you'll notice that the water disappears into the clay immediately. That's because of the lack of moisture in the pottery due to the process described above. Much of this research is in the beginning stages for me. I've been thinking about it for quite awhile and never been offered a convinvcing solution as to why pottery smelled the way it does, why it makes that "tuck" sound when lightly tapped, why it sucks water the way it does. Microbes. It's the only answer that makes any sense to me. It holds true for the surface of ancient stone. Microbial intrusion and consumption of minerals in the surface effectively erodes the epidermis of ancient stone. That prevents refraction of light from the geodedic structural cleavage in the epidermis of ancient stone. The epidermis of the stone is dulled, effectively.
The water test
I had some specific questions regarding the "water test" for determining if the teracotta is ancient:
There would be some variables that would/could effect the results, more specifically:
Sorry if I am sounding obsessive-compulsive but I am very excited about this way of perhaps determining "ancientness" but would like a bit more info so that I am getting the best possible results.
Take water with room temp.
Put the object for 3 to 5 minutes in the water. Ancient pieces takes the water very quick.
An air conditioned room or not, this do not harm the effct - only if you let it dry in the sun! New ceramik is dry within 30 Minutes to 2 hours, ancient keramics hold the humiditie for one day to a view days.
You know exactly when the object is dry if you weight it before the test (some grams different are no problem). More importand - you see it on the surface. An ancient piece is wet for a long time and that is to see on the dark outside when it is wet.
On polished or glased objects this test doesn´t work!
If you have an tell Halaf idol that is polished on outside and didn´t take water, make a second test! : Go on top of a builing thats 100 meters high, let the Tel Halaf idol falling down and look how long it needs to crash on the ground! (just kidding).
Remember, some non-burned clays have the same effect and takes and hold water as same than ancient pieces. Some fakers makes pieces from only dryed clay so if you can form the clay after make it wet, it is a fake!
Clay that's been fired at a high temperature with minute sized organics, such as wood fragments no more than "wood dust" (almost hydroscopic in size) mixed in fluidly with the clay during the kneading process, effectively burns out of the fabric leaving minute cavities for the water to slip through. This, combined with the high-firing of a thin clay hardens the clay allowing less water to penetrate the actual fabric but giving the illusion of evaporation.
That's what you're seeing with ancient items, too. It's not actual evaporation. It's a function of adsorbtion - it enters the clay, but not on a cellular level, just pores which exist in it. Hence, it is a function of adsorbtion as opposed to absorbtion (science people, correct me if I'm wrong here) being witnessed as opposed to evaporation.
The validity of the water test is to be doubted
From Roger S
3rd April 07
Thanks for sharing the info and pics on the water test. (See link below here)
I can add this small experience on a “water test”.
Last summer I carried out a small absorption test on some pottery objects.
I began the experiment by soaking 10 pieces of ancient and 10 pieces of modern terracotta objects for 2 hours in a bucket of room temperature water.
During the 5 days of testing the RH varied from 65% to 74% and the temperature was constantly 25°C. During the experiment I also used some early 1900 roof tiles and brick fragments, and these also passed the “smell test” like some ancient artifacts.
As you can see from the data there is a slight difference between a few modern and ancient items but some dried up equally.
Considering items of roughly the same weight we have.
1) N° 7 Roman vase (100,4g) dried up in 65 hours like the modern brick fragment N°8 (106g).
2) N°5 Greek Pottery fragment (55,8g) dried up in 65 hours like N°6 Flower Pot fragment (54g)
N° 10 Gallo Roman Flagon that was slightly glazed, dried up in 48 hours so this type of testing definitely cannot be applied on glazed or slip coated artifacts.
In my opinion to have some good data, (if this type of testing can be applied??) you would need to compare the artifacts, with modern terracotta of the same weight and fired at about the same temperature.
I would consider this type of test not to relevant because there are so many factors that can change the chemistry in pottery, and most important the firing temperatures. The porosity of terracotta is related to the temperature in which it has been fired. This high porosity/absorption can be also found on low fired modern terracotta.
You can find tons of these early 1900 bricks, I have had some experience in working these bricks (in construction) the well fired bricks usually darker and nearly vitrified are very hard, sometimes even difficult to break with a 5kg hammer.
The low fired usually dull colors light yellow-brown to light orange are very porous they bubble even 15 minutes in a bucket of water, and can be shaped with sandpaper and broken with a piece of wood.
So technically a small figurine or amulet can be easily shaped from these bricks and these WILL pass the two “water tests”.
Last week at a Fair I seen students from a Restoration School make friezes, relief’s and may items from modern terracotta bricks using chisels, files, sandpaper etc.
These bricks don’t have inclusions like ancient coarse-ware pottery.
Indeed, and as Tom said above:
Remember, some non-burned clays have the same effect and takes and hold water as same than ancient pieces. Some fakers makes pieces from only dryed clay so if you can form the clay after make it wet, it is a fake!
Personally I am trying to concentrate my research on microscope, UV and petrology inspections, and I would consider these more reliable than the two “water tests”.
Just a few personal thoughts on water testing methods.
30th March 07
More comments about this
I have never fully understood the smell test, but I can explain a little about wetting clays, where there is a limited but important amount of information to be learned by how quickly it drys.
Terracotta fired at the temperatures most ancient Terracotta were fired at, was initially very impervious to moisture, and if wetted slightly (a dab of moisture on you finger tip) would not have absorbed the moisture much and which would then have to dry slowly by simple evaporation.As terracotta age the clays weather and become more absorbent.
Once they are a couple of thousand years old, they become highly absorbent and if wetted slightly (again just a dab of moisture on your finger tip) will absorb the moisture very quickly and also dry very quickly (sometimes in only a couple of seconds).Thus wetting them slightly, can provide you with clues as to if they are old or not.
But there are a couple of problems with this test.
1) New terracotta that are just dried rather than fired, will absorb and dry in a similar way to ancient weathered Terracotta. However, you may notice a slightly tacky feel as you moisten it with your finger tip, which you should not feel with fired Terracotta.
2) New terracotta that is fired at very low temperatures (such as over an open wood fire) will also absorb moisture and dry at a similar rate to ancient weather terracotta, and thus will give you a false positive for age. I have seen brand new fakes of pre-Columbian items react that way. But low fired Terracotta has a different feel to it than higher fired terracotta, and with experience it is possible to know which one you are dealing with.
3) An ancient item with a surface burnished to a high gloss, or with any type of glazing on it, will fail the test in spite if being genuine.Because of these potential false positives or negatives, the wetting test should be used only as a guide, and not a stand alone test to prove either authenticity or falsehood. It is just an indicator to be used with other methods before a final conclusion can be drawn.
However, there is a second slightly different types of wetting test, useful in spotting restoration. For this test you need a spray bottle of the type window washing fluid comes in, with which you lightly spray the entire surface of the object (just enough to moisten it a little).
Clay temporarily changes color when moistened and then returns to it's original color when it drys. Different types of clay, or the same clay but with a different amount of weathering (ie a different age) will change color differently.An object intact with no restoration should all be one type of clay with an roughly equal amount of weathering, and so the color change should be roughly consistent over the entire object. If there is some differential due to differential weathering, the boundary between the zones should be graduated rather than abrupt.If you spray an object and get very distinctly different color changes on various parts, and especially the lines of change a distinct sharp lines, you are probably looking at a restored object containing a mix of new and old parts. As a general rule, the original old parts will dry faster than the newer parts,.
But please note the comments above about potential causes of false positives and negatives due to burnishing and glazing.If when you spray that object that is otherwise absorbent and fast drying, but you see very distinct narrow bands that are non-absorbent and very slow drying, there is likely to be glue from recent repairs just below the surface on those bands.
WARNING - ALWAYS BE VERY CAREFUL ABOUT THE AMOUNT YOU WEB AN ANCIENT CLAY OBJECT. Some genuine ancient figures, and most clay tablets, are just dried clay and if you wet them heavily you may end up with a small pile of mud were the object used to be.
Ancient pottery will usually seem to dry out very quickly; thats because the surface moisture is adsorbed more quickly. It acually remains wet inside for much longer than new pottery.
Another similar test which is oft quoted is to see if the tongue sticks to the terracotta when it is licked. Before using this test, please consider that if your object is genuine it may have been in a tomb for the last few thousand years; if it is fake then it may well have been in a dung heap or sewer much more recently!
The smell test is very simple really. If your object smells old and dank like a thousand years of being in the ground, then that is a good sign. If it smells like a row of flower pots in a garden centre, it is not! Again, a few drops of water can help to bring out the smell.
That's a good point; a very litle water is needed and you need your nose right up to the pot.
From Tom Wegener
Words derived from considerable experience and expertise !
With Tom's permission; slightly edited.
What can be indisputably evaluated?
- Faience, ceramic
- Bronze (alloy of copper and tin), silver
- Fabric und Papyrus
- Wood, when painted, plastered or covered with other organic material
What cannot or with great difficulty be evaluated?
- reliefs, when there are no organic traces available (i.e. paint))
- stone pottery
- unpainted wood
- objects made of stone, like granite, sandstone etc.
- gold or gemstones
This ushabti was offered on Ebay.com in 2001, it was sent to us for examination by a German collector. It shows some of the typical tell-tale signs of a fake. The piece was sold with a material analysis of which the executing institution in hindsight turned out to be property of the seller. A restitution was denied by the seller, even though a counter-expertise was produced by the collector in the form of a TL-analysis
Layed on glazing without glaze damage or just the smallest sign of staining or age-required wear in the glazing surface. The entire brown surface with yellow glass inclusions is absolutely homogeneous on the material, very hard (contrary to antique faiences which are easily crazed with a probe or sharp scalpel under a microscope) and also without any glaze damages.
False inscriptions with the name (without cartouche) of Seti I although stylisticaly it concerns an ancient piece (if it would be genuine!).
Generally these falsifications deal with faience that is much to light and it's "sound", as soon as one taps it with a hard object, is high, just like a newly baked ceramic pot.
Smaller variations with the same false characteristics.
Methods of authentification
Authenticity of antique bronze and silver objects
Bronze and silver antique materials represent absolutely no difficulty today in regard to their authenticity recognition! So far, the most common and at the same time most useless methods were:
in which the composition of the alloy was analyzed and which didn't have the smallest thing to say about the authenticity of a bronze! Good counterfeiters derive the exact percentage from literature about the appropriate bronze or use fragments which are molten and being cast again.
examines the crystal structure of the patina. However it's possible to grow subtle and rough crystals by means of climatic chambers. Thus this analysis is also completely useless.
here the radioactive isotope of Blei-210, which is contained in copper ore and has a radioactive half-life of 22.3 years, is being measured.
Unfortunately the Blei-210 is only in traces present in the ore, thus only a relatively short time of origin can be proven, but was already known to be falsified in the 18th century as well.
We now arrive at the method, which was developed and/or found by Mr. Tom Wegener, and which can be used for antique copper alloys as well as antique silver!
It's common knowledge that the patina on fake pieces is artificially made and applied on the bronze with reagens like chloride and amino-acids. With genuine pieces, which lay in the soil for centuries or thousands of years however, a proper process of transformation takes place in the ore, which does not only influences the surface, but penetrates the highest layers of metal or in some cases mineralizes the entire bronze through and through. In such cases for example, the inclusions incapsulated in the bronze can be shown, in which one opens up a very small surface on the blank metal with an ultrasonic chisel, and looks for these inclusions under the microscope. With new castings one also cannot cast these patina inclusions into the cast, because these are separated in the melt and it always developes as a clean homogeneous bronze. Since this effect also cannot be falsified in the future, one can assume that so far it absolutely represents the safest procedure for the authentication of bronze! The same also applies for silver, only here the inclusions concerned are horn silver.
Macro image of a genuine Egyptian bronze with the surface opened up to the metal (darkly coloured bronze with incapsulated inclusions in the metal surface).
This picture shows you that effect of mineralization of the upper layers of metal which can't be falsified!
The price of this examination, including documentation and macro photographs, as well as retouching of the opened place is 180.00 euro including taxes.
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