Based on the type of metals used in the production of products, they can be divided into three archaeological groups with clear morphological characteristics.
1 – products made of iron, cast iron, steel and their compositions – the archaeological object has a surface of a characteristic red, brown color, consisting mainly of iron hydroxides, limonite, goethite, etc., characterized by the presence of these minerals and sedimentary rocks / sand, clay, organic inclusions and mineralogical concretions/ on the modified, metamorphosed surface of the object itself, with or without an iron crystalline core. An archaeological substance can repeat on an enlarged scale / epitaxial growth / a form typologically similar to the object or form a difficult-to-describe conglomerate with it.
2 – products made of copper and copper-containing metals / bronze, brass, tombac, etc. / - the archaeological object has a surface of a characteristic green-blue color, consisting of basic copper oxides and minerals azurite, lapis lazuli, atacamite, etc., mineralized surfaces and crustal layers Compared to iron archaeological objects, they usually have a more identifiable shape and dimensions close to the original ones.
3 - products made of high-grade silver and silver-containing alloys - an archaeological object made of sterling, high-grade silver has a slightly mineralized surface of dark gray or light gray color, consisting of silver sulfide and chloride. In low-grade silver products with a high content of copper, tin and other alloying additives, copper-containing minerals and chlorargerite are present in the mineralized surface; such objects have large distortions of the original shape and, as a rule, large structural changes (1).
A special group should include relatively corrosion-resistant metals, such as high-grade gold and its alloys (electrum). Platinum and platinum group metals.
Due to the specific nature of corrosion processes - tin, zinc, lead and their alloys.
For all metals, despite the difference in chemistry, dynamics and originality of corrosion processes, it is necessary to note the general physical and technological properties of materials that determine their structural strength and corrosion resistance: Mechanical compaction of the crystal lattice during forging, rolling, drawing. Compaction of the outer layers of the metal and hence better corrosion resistance of thick-walled castings, despite selective corrosion and the multicomponent composition of the metal. There is a direct relationship between the rate of structural degradation of the material and the packing density of the atoms of the surface layer of the metal, the homogeneity and presence of dislocations in the crystalline structure of the metal, the degree of its polishing, roughness (Boilby layer). For Slavic archeology and silver treasures, the fact of natural embrittlement and aging of the silver-copper system outside of corrosive conditions is interesting (1)
and many other factors.
Stages of research and scientific
conservation work
1. Scientific preparatory. Evaluative. Due to the complex morphology of both the archaeological object itself and the complex stratigraphy of mineralized surfaces, it is necessary, using research methods, to clarify the typology of the object and its structural features, the presence of a solid metal core and its boundaries, the nature and characteristics of corrosion and mineralization, the presence of composites (the most representative type research is the interpretation of the results of electron microscopy (SEM), combined with spectrometry of archaeological samples (XES) and Auger microscopy, etc. Sometimes the only method that gives a reliable picture of the structural features of the studied samples is metallographic, microstructural studies using a metallographic microscope. that in this scientific and practical area of research, enormous experience has been accumulated and there is a colossal amount of information available to researchers.
2. Scientific documentation. Drawing up a topographical diagram and plan - a map of work on conservation measures: washing and removal of mineralized layers, nodules and inclusions; stabilization of the monument; full disclosure to the metal core or partial to stable protective oxides, such as the “noble patina” on copper; passivation, inhibition, protective coatings or impregnations, and possibly deep preservation of the entire mineralized or metamorphosed object without penetration into it.
The lack of a complete understanding of the archaeological object, the nature of its destruction or a joint expert opinion of an archaeologist, a specialist researcher and a restorer regarding the condition of the object and possible methods of carrying out work is sufficient for not carrying out conservation and restoration work
Practical conservation work
1- Cleaning – rinsing in water. It is carried out in distilled water at room temperature with the addition of a wetting agent (3-5% methanol or ethanol) in order to prepare for pickling, helps to remove light corrosive deposits and biological inclusions. Calcium deposits are removed in a 5-10% solution of sodium hexametaphosphate using brushes or swabs. The chemical activity of water during prolonged soaking for 1-2 days is sufficient to destroy adhesive bonds and remove organic inclusions and weak mineral deposits; this is greatly facilitated by the addition of 10% potassium, sodium tartrate or ethylenediaminetetraacetic acid salts (EDTA, Trilon-B, Chelaton). It is possible to repeat the washing several times, alternately removing weakened mineralization products with a brush or stack, taking special care for thin-walled and brittle objects. Note: - washing in water or aqueous solutions of salts is impossible in case of complete or partial destruction of the metal, especially thin-walled ones, as a result of selective or intergranular and other types of corrosion due to the possibility of loss of the original layer of jewelry and especially fine decoration (gilding, niello, notching , filigree, enamels, varnishes), and sometimes even the base metal itself. In these cases, washing is preceded by a stage of consolidation or fragmentary strengthening of the object. 2- washing is difficult to perform if the archaeological object has undergone field conservation using synthetic and natural waxes, polymer synthetic water-insoluble or partially soluble resins, varnishes or other materials that make it difficult to use water as a solvent. In these cases, solvents are used that correspond to the preservatives being removed: purified gasoline and kerosene (saturated and unsaturated hydrocarbons) for paraffin and wax-containing coatings, acetone, toluene, ethanol (ketones, alcohols, ethers), etc. for resins, synthetic resins, adhesives, varnishes, as well as organic preservatives and adhesives, such as shellac, dammara, copal. When using all types of solvents, especially volatile ones, it is desirable to use a stepwise method of influencing the preservative - from a light solubility test, exposure to solvent vapors in a closed container or “Petenkofer bag”, to immersion in the solvent and soaking for a long time. It is necessary to work on full-scale samples and obtain a scale for the dynamics of solubility of polymeric or organic materials, especially taking into account the possibility of “swelling” (7), rather than complete solubility of some polymeric, especially degraded, materials.
2- In all cases of using solvents to remove preservatives, one should proceed from the safety of these operations for the preservation of the object itself, as a single spiritual, historical, scientific or artistic whole. All stages of cleaning or restoration work are carefully documented(4).
3- Stabilization of an archaeological object - this means carrying out various preparatory work before the actual conservation, the purpose of which is to create in the structure and on the surface of the archaeological object physical and chemical conditions favorable for conservation with its reliability. Often stabilization measures directly depend on the chosen or existing methodology for carrying out conservation work and their technological parameters. It should be noted that strictly mandatory PH testing for chemical acid-freeness or neutrality of all materials and working surfaces, at all stages of conservation work, the use of certified restoration materials There is always a danger that preparatory work (draining, heating, degreasing, etc.) may negatively affect on the strength characteristics of the object (5). Create prerequisites for accelerated aging of materials, both the archaeological object itself, and accelerate corrosion processes that change the morphology of the surface (for example, epitaxial growth due to the accelerated formation of hydroxides at high humidity or recurrent corrosion under a film coating (6). The possibility of structural degradation should also be taken into account materials previously used for conservation, if any in the structure of the object. When all kinds of risk factors during stabilization are difficult to control, methods of smoothly changing parameters with stepwise control of characteristics are used. For dehydration, buffer hydrophilic materials are used (paper pulp, cation exchange resin, anion exchange resin, silica gel, etc. .).For moistening, the method of remote moistening is used. For regeneration, for example, varnish, they use long-term exposure of the object in solvent vapor (Petenkofer package). Special techniques: vacuum heating, freezing, deionization in a gas-discharge chamber (low-temperature plasma ionizer), laser technologies and others are used in the presence of strict laboratory data from preliminary studies in favor of the use of such techniques and, as a rule, are approved by restoration councils with the participation of leading specialists - restorers, archaeologists, and researchers. Carrying out conservation work at the final stage - an archaeologist or restorer carrying out conservation work must always remember the Main Rules of Restoration Activities: “Save” and “Do No Harm”, which are associated with the basic methodological principle of restoration and conservation activities - “any work with an object is restoration- conservation practices should culminate in conservation measures. This principle formed the basis of conservation activities due to the existence of the second law of thermodynamics (WLT) and the phenomenon of entropy. Any impact on an open system, which is any object of material culture, causes a fluctuation in the possible equilibrium of the system and, ultimately, an increase in entropy or the degree of disorder of the system. Ultimately, accelerated structural degradation or aging of the object’s materials occurs, weakening molecular and interatomic bonds, leading to its complete destruction. Therefore, the degree of isolation of an object from the external environment, along with the internal dynamic component of the aging process, are the main measurable factors that make it possible to control the aging process or, more precisely, not to accelerate it. What is actually the task of conservation practice is to isolate the system from the external influences of negaentropy and achieve a state of equilibrium in the system.(8) That is why, having optimally prepared the structure of the material and, having reduced the redox, energy exchange processes on its surface, they move on to isolating it from the external environment using insulating coatings that are sufficiently gas, moisture and energy impermeable. Such coatings can be polymer film, organic film: oil film, wax, organosilicon up to pure silicon dioxide on the surface, etc. The choice depends on the structural features of the object and the severity of the effects of negentropy environment. It is generally accepted that conditions with low humidity up to 35-40% and possible humidity fluctuations of no more than 10% are suitable for long-term storage of a metal archaeological object.
Scientific research recent years show that creating optimal climatic conditions during storage, exhibition, transportation - insufficient measures to maintain the stability of archaeological objects in cases with spontaneous uncontrolled degradation processes ending in self-disintegration - total destruction of the structure. In these cases, exceptional conservation measures are applied:
placing an object in an environment with an inert gas, creating an internal frame that strengthens the structure of the object, using impregnation with liquid polymer solutions with their subsequent hardening or organosilicon polymer solutions, up to the creation of transparent monoblocks. These exceptional measures in no way cancel one of the most important restoration and conservation principles - the reversibility of all restoration processes, dictated by the relative fragility of the restoration materials themselves. The need to secure an object of special spiritual, scientific, cultural and historical significance, to protect it from the negative consequences of possible restoration errors. Due to the imperfection of human knowledge and its supposed constant scientific development. What is done well today may be done better tomorrow.
NOTE:
1 Extrapolation calculation shows that the rate of copper release along grain boundaries is 10 microns per year at room temperatures (Schweizer and Meyers, 1978), taking into account the corrosion dynamics of the Ag-Cu alloy, we can talk about oxygen embrittlement of all copper-containing silver artifacts as the main problems of archaeological silver, in addition to the well-known problem of the corrosive activity of chlorides.
2 The historical fate of an archaeological find is complex and is often determined by the real value of the monument, which turns into an object of desire for both the conqueror and the collector. God forbid you end up in the wrong place at the wrong time. This is very important for the survival of both people and their man-made works. For example, Slavic and Old Russian archeology has long noted the abundance of highly artistic finds in treasures of the 11th - 13th centuries. throughout the territory Ancient Rus', especially in the strata of urban settlements of the North-East and South-West. Many monuments show traces of fires, associated structural changes and damage, which perfectly confirms the peculiarity of the period in the archaeological material internecine wars and Tatar-Mongol conquests (see N.P. Kondakov “Russian Treasures”). The fate of the “Treasures of King Priam”, found by Heinrich Schliemann in 1873 during the excavations of Troy, in Greece, is very remarkable. A huge treasure in terms of the number of finds, and priceless in terms of scientific significance, which, in addition to two tiaras, alone, gold rings, contained over eight thousand. It did not go to Greece, and was lost for many years to the world scientific community. So far, very scattered and incomplete, the treasure has not appeared in Soviet Russia, in the Pushkin Museum. Only thanks to the durability of the main material of the products - high-grade gold, it has reached us in a good state of preservation. Here it is worth mentioning the happy fate of the finds. Metropolitan of Kiev and All Rus' St. Alexy (1292-1378), as chronicled sources mention, found enamel pellets in the remains of St. Michael's Golden-Domed Monastery, some of them became part of the decorations of his future sakkos, TK-1, Armory Chamber of the Moscow Kremlin.
3 Dr. Scott David A. Scott. Ancient metallic artifacts, metallography and microstructure, 1986, CAL, Smithsonian Institution, Washington, DC, USA.; Plenderleith H.J. and Werner A.E.A. The Conservation of Antiquities and Works of Art, 1971, London, Oxford; Dowmann E. Conservation in Field Archaeology, 1970, M & Co. etc.
4 Most completely unified government requirements to the principles of conservation of archaeological objects and collections are reflected in the British Standards (Standards in the Museum Care of Archaeological Collections. 1992, Museums & Galleries Commission) and the UKIC recommendations (British Institute of Conservation, Guidance for Conservation Practice, 1983).
5 Consolidation or strengthening, strengthening the structure of an object in individual parts or as a whole, is strictly necessary in case of the potential danger of the archaeological object losing information fields: parts of the decor, inscriptions or other paleographic features.
What can happen both in the process of decapitation (layer-by-layer removal of corrosion and mineralization products), and in the process of natural structural degradation of the object during storage, before and after conservation and restoration measures. In a strict sense, it is the main activity during field conservation of an object. See conservation - consolidation
6 Film preservative coatings, as a rule, require a dried and heated surface, a roughness sufficient for adhesive contact, and chemically neutral. The structure of the object should not contain excess unbound water, be electrochemically passive, and not contribute to the separation of the film insulating coating due to incomplete reverse osmosis during gas formation and recurrent corrosion processes - i.e. stable.
7 During field conservation, butyl-phenol impregnating solutions, polyvinyl acetate, acrylic, and organosilicon were often used for consolidation. At the same time, according to general appearance surface of an object, it is difficult to determine their presence in the structure. This is what makes it necessary to have strict documentation of the progress of all conservation work during in situ field conservation.
8 Due to VNT, the entropy Si of a closed system cannot decrease (the law of non-decreasing entropy) dSi > or = 0, where i is the internal entropy corresponding closed system. In stationary (equilibrium) systems dSo< 0 т.е. изменение энтропии отрицательно, нет её оттока из системы. Но есть приток в систему так наз. "негэнтропии", обратной величины. Если постоянно dS >0, and the growth of internal entropy is not compensated by “negentropy” from the outside, then the entire system moves to the nearest equilibrium state of the stationary system, when
dS = 0 while maintaining the dynamic component of internal entropy. Achieving such an equilibrium state of the system is the main task of all conservation and restoration scientific and practical activities.
Total entropy change open system equals dS+dSi+dSo.
9 In world conservation practice, when stabilizing archaeological objects made of iron, the use of aqueous and alcoholic solutions of tannin to create an inert and stable layer of iron tannate on the surface, chemical and electrochemical passivation of surfaces, inhibition, etc., has proven itself. See - "Practical academic courses restoration."
So technical deadline The shelf life of polymer film coatings, with the exception of some organosilicon coatings, is four to five years, after which restoration is carried out - removal of the previous ones and application of new protective coatings.
Bonus for those who read: http://wn.com/bainite
Smirnova D.I.
All metal products, with the exception of gold and platinum, are subject to corrosion to one degree or another. Corrosion is the destruction of metal caused by environmental influences. Destruction usually begins at the surface of the metal and gradually spreads deeper. At the same time, the metal changes its appearance: it loses its shine, the smooth surface becomes rough and becomes covered with chemical compounds, usually consisting of metal and oxygen, metal and chlorine, etc. The nature and rate of corrosion depend on the composition (alloy) of the metal and the physicochemical conditions of the environment. In the soil, in the presence of sodium chloride, the chlorine ion of which, especially in the presence of water, carbon dioxide and humic acids (found very often in the soil), etc., quickly leads to the destruction of iron, chlorine compounds with iron are first formed, which in the presence of air and moisture, in turn, again give new compounds with iron hydroxides. This process occurs quite quickly in the soil and can then continue under museum conditions.
On iron objects coming for restoration, there are observed different kinds corrosion: uniform surface, point and intercrystalline - between crystals.
Surface uniform corrosion is formed under the influence of complex chemical reagents, in most cases on metal exposed to the open air, and spreads evenly over the entire surface of the metal object in the form of a film of oxides. If this film, called patina, covers the object with an even, smooth layer, then it prevents further penetration of gases and liquids into the metal and thereby prevents further destruction. The patina on bronze objects protects these objects well from further destruction. The patina covering iron objects does not have the protective properties just mentioned. It contains numerous pores and cracks through which gases and liquids penetrate relatively easily, causing further corrosion.
There are cases of pitting corrosion, when not the entire surface of a metal object is destroyed, but only individual small areas. In this case, as a rule, destruction goes deep into the metal, forming deep ulcers that lead to the formation of lunges with sharply defined edges.
With intercrystalline corrosion, the destruction of the metal occurs due to the disruption of the bond between the metal crystals and spreads deep inside. Objects affected by such corrosion become brittle and crumble into pieces upon impact. This type of corrosion is undoubtedly one of the most dangerous.
Very often, the effects of several types of corrosion can be observed simultaneously on one object.
Iron objects discovered during archaeological excavations are in most cases in a dilapidated state. The removal of such items from the ground must be approached with great care. If the metal is so damaged that it crumbles, then first of all it must be cleared as carefully as possible with a knife, soft brush or brush and secured. Only after fixing (impregnation and complete evaporation of the solvent) can the object be removed to the surface. For fixing, use a 2-3% solution of polyvinyl butyral. The butyral solution is prepared as follows: 2 g of polyvinyl butyral powder are dissolved in 100 cubic meters. cm mixture of equal quantities of alcohol and benzene. The method was proposed by Hermitage researcher E. A. Rumyantsev and tested in laboratory and field conditions during excavations in the Karmir-Blur expedition. Fixing with butyral is carried out repeatedly, using a soft brush or spraying from a spray bottle.
If the objects are in fairly good condition, then they must be cleaned on site from foreign substances and all kinds of growths that distort the object, and then fixed with the same butyral solution. Previously used methods for archaeological work of filling heavily destroyed iron objects with paraffin, gypsum, etc. should be considered of little use, because a thin layer of paraffin, due to its fragility, cannot firmly fix a destroyed object and, in addition, paraffin interferes with further processing of the object during restoration .
All iron objects received by the museum must be subjected to restoration and conservation. As already mentioned above, the process of formation of compounds of chlorine ion with iron, causing the destruction of the metal, which began in the soil, continues in museum conditions. To stop this process, it is necessary to remove the chlorine ion, which is achieved by repeated washing and boiling in distilled water. The presence of chlorine compounds in objects can be easily detected by placing the objects in a humid chamber. After 10-12 hours, such objects are covered with small droplets of water, then these droplets increase in size. By chemical analysis of these droplets it is easy to detect the presence of chlorine ion in them.
Before proceeding with the restoration of a particular iron object, it is necessary to take into account the safety, the presence of a metal core, and then use one or another cleaning method. The following methods are recommended based on experienced practical work, tested on numerous and varied materials in the restoration workshops of the Hermitage. According to the degree of preservation, all iron objects coming into restoration can mainly be divided into three groups:
1. Objects destroyed by corrosion, without a metal base, with a distorted shape and an increased original volume.
2. Objects whose surface has been severely damaged by a thick layer of so-called “rust,” but the metal core has been preserved. This surface corrosion distorts the original shape and volume of objects.
3. Objects in which the metal and shape are almost completely preserved, but the surface is covered with a thin layer of “rust”.
To clean items of the first group, repeated washing in hot distilled or rain water is required, as well as mechanical cleaning with a scalpel to remove dense growths, followed by thorough drying. To check the presence of chlorine ion, after these operations it is necessary, as mentioned above, to place the objects in a damp chamber. If after 10-12 hours blurry drops of water appear on objects, then washing must be repeated several more times. Only after complete removal of the chlorine ion can you begin to preserve and mount objects. Chemical cleaning should not be used in such cases, because under the influence of chemical reagents the salt-like compounds formed during corrosion dissolve, the connection between individual fragments becomes weak and the object can crumble into small pieces. This may lead to the final destruction of the item. When washing large objects and in the absence of distilled water, washing can be carried out in ordinary boiled water.
Preservation (surface fixation) can be done with a 3% butyral solution. If the object consists of several fragments, then individual parts are first coated with butyral solution, and then these parts are glued together. To glue objects made of iron, you can use BF-2 glue or glue prepared from the same butyral (8-9 g of resin per 100 g of solvent [alcohol-benzene]).
Items of the second group, as experiments have confirmed, are recommended to be cleaned with chemical reagents. Items are washed before cleaning hot water to remove soil and other contaminants, after which they are placed in a 5-10% solution of caustic soda for 10-12 hours to soften the corroded layer, remove fats and other contaminants. After treatment with caustic soda, objects must be washed under running water, and then, using a scalpel, they are partially cleaned of “rust” growths. After this operation, the objects are placed in a 5% solution of sulfuric acid, to which 1-2% glycerin is added. An object placed in acid must be removed from the acid every 10-15 minutes, washed in running water and cleaned with a soft brush and scalpel. These operations make it possible to control the action of the acid and speed up cleaning, which depends on the thickness of the layer and the nature of the “rust”. After cleaning in acid, the object is washed again with water and placed again in a 5-10% solution of caustic soda, where it is left for 10-12 hours. Cleaning is carried out until brown iron oxides are removed. Dark oxides (ferrous oxide and ferrous oxide) often form the bulk of the item and are therefore best not removed.
When cleaning objects made of iron of the third group, the best results are obtained by using a 10% solution of citric acid. In this case, before cleaning, the item is also washed with hot water and placed in a 5-10% solution of caustic soda for 10-12 hours. After this, the object, washed in running water, is placed in a 10% citric acid solution. After 5-10 minutes, the object is removed from the acid, washed with water using a soft brush and again immersed in the acid. The operation is repeated until the rust stains are completely removed. If the “rust” lies in a thin layer, then instead of citric acid it is better to use ammonium citrate. To do this, ammonia is added to a 10% citric acid solution until a drop of phenolphthalein gives a slightly pink color. The object to be cleaned is dipped into the solution prepared in this way. The cleaning technique is the same as with citric acid.
Instead of citric and sulfuric acids, you can use a 0.5-2% solution of phosphoric acid, but it should be borne in mind that phosphoric acid has a more active effect on iron, so leaving an object in acid for a long time is unacceptable. In this case, it is necessary to monitor the progress of the cleaning process at all times. The working method is the same as with the above acids.
To neutralize acids, cleaning in all cases must be completed by placing items in a 5% solution of caustic soda, followed by rinsing in hot distilled water and appropriate drying in a thermostat. After all these operations, the object must be processed on a rotating iron (steel) brush.
As a preservative that protects objects from further destruction, a 3-5% solution of butyral or a 3-5% solution of polybutyl methacrylate is used.
To preserve iron objects in the museum, it is necessary to eliminate the causes that contribute to the rapid formation of corrosion.
1. The relative humidity in the rooms in which these items are located should not exceed 55%.
2. The room must be clean, since dust settling on objects retains moisture and thereby contributes to the formation of “rust”.
3. When moving objects, your hands should always be wearing gloves, since the acids present on the skin of the hands, when in contact with iron, act on the metal and contribute to the formation of “rust”.
A big problem in restoration is the preservation of found ancient iron objects. Everyone knows that iron oxidizes quite quickly, becomes covered with rust and is destroyed in layers. How to save an ancient item found?
Alternative method for cleaning iron
Today we will look at an alternative method that does not yet have experimental, time-tested results. The fact of restoration and conservation of an iron object is obvious, but it is not known what will happen to the object in 5-10 years. It must be said: the dynamics and quality of recovery and conservation of iron are quite large and promising.
The main phases of restoration of ancient metal objects
It must be said that the main idea this method restoration consists of using the polymer Anacrol or Anatherma. That is, we impregnate the object in a vacuum chamber.
- Initially, the iron object should be desalted. How do we do this? Place the item in a container with distilled water for several days to desalt and loosen rust flakes.
- Next, the item is dried at a temperature of 100 degrees. The author of the technology suggests drying items in ovens with the door ajar.
- Polymer impregnation in vacuum. How does this happen? We take a rusty ancient object found in the ground and completely place it in a chamber filled with polymer. Next, we begin to suck the air out of the chamber, during this process, as if a process of boiling and seething occurs. After the air is pumped out, the polymer fills all the cavities in the body of the rusty iron.
- Afterwards, the item is again placed in the oven for 1 hour at a temperature of 120 degrees for drying (at 90-100 degrees the polymer hardens into a glass-like consistency).
- The final point is mechanical cleaning.
More detailed technologies and ideas for this type of restoration can be viewed in the attached video.
Interesting site materials
No metal is subject to such severe destruction in the soil as iron and its alloys. The density of rust is approximately half the density of metal, so the shape of the object is distorted. Sometimes it is impossible to determine not only the shape of objects, but also the number of objects. When rust forms in the soil, particles of earth and organic substances get inside it, which are gradually overgrown with corrosion products. All this distorts the shape of the object and increases its volume. Once removed from the soil, iron objects must be immediately restored.
Clearing the soil. The object is soaked in water or cleaned in a 10% solution of sulfamic acid, which dissolves the silicate components of the soil, but does not interact with iron and its oxides. When cleaning in acid, an object may disintegrate into fragments that were previously cemented by the earth. Areas of the object that have not been cleared of soil after the first treatment are sprinkled with dry crystalline acid (without removing the object from the prepared solution). Soil layers are removed with a hot solution of sodium hexametaphosphate. After cleaning, it is enough to rinse in tap water and then in distilled water.
Having cleared the object from the earth, it is determined what state the metal is in - active or stable.
Stabilization. Iron objects, after being removed from the soil during storage, quickly deteriorate. In the soil with the metal, almost all the changes that could occur under the given conditions occurred, and some thermodynamic equilibrium was established between the metal and the environment. After being removed from the soil, the object begins to be affected by higher oxygen content in the air, different humidity, and temperature changes. One of the main reasons for the unstable state of iron archaeological objects during storage is the presence of active chloride salts in corrosion products. Chlorides enter the soil from the soil, and their concentration in the object may be higher than in the surrounding soil due to specific reactions that occur during electrochemical corrosion. A sign of chloride salts is the formation at humidity levels above 55% of dark-rust-colored droplets of moisture at the site of increased chloride content due to its high hygroscopicity. When dried, a kind of fragile shell with a shiny surface is formed. The presence of such dried rust does not mean that the chloride stimulant has ceased to be active. The reaction began elsewhere, and the destruction of the object continues.
To identify chlorides in corrosion products, the object is placed in a humid chamber for 12 hours. If chlorides are detected, the metal must be stabilized. Without stabilization, an object may actually cease to exist (disintegrate into many shapeless pieces) within one or more years.
Then the presence of a metal core or its residues is determined, since an active destruction process occurs in objects with preserved metal, which reacts with chlorine ion. To determine the metal in an object, use:
1) magnet;
2) radiographic method (interpretation of radiograms is not always unambiguous);
3) measuring the density of an archaeological object. If the specific gravity of an object is less than 2.9 g/cm3, then the object is completely mineralized, if the specific gravity exceeds 3.1 g/cm3, then the object contains metal.
Stabilization by complete removal of corrosion products. Complete removal of all corrosion products also leads to the removal of active chlorides. If the metal core is massive enough and reproduces the shape of the object, then complete cleaning of the iron object is possible by electrolytic, electrochemical and chemical methods.
Stabilization while preserving corrosion products. The shape of an object that has a small iron core should be preserved even by oxides, bringing them to a stable state. Therefore, the most important operation, on the thoroughness of which the future safety of an object depends, is its desalting, removing chlorine-containing soluble compounds or transferring them to an inactive state.
We present almost all the methods used for stabilizing archaeological, oxidized iron, since only experimentally can one select the optimal option for the most complete desalting for the group of objects being restored.
Rust converter treatment. To stabilize the rust of an archaeological iron object, a tannin solution is used (as in the restoration of museum iron), the pH of which is lowered to 2 with phosphoric acid (approximately 100 ml of 80% acid is added to 1 liter of solution). This pH ensures the complete interaction of various iron oxides with tannic acid. A wet object is wetted with acidic solutions six times; after each wetting, the object must be air-dried. Then the surface is treated with a tannin solution without acid four times with intermediate drying, rubbing the solution with a brush.
Removal of chlorides by washing in water. The most common, but not the most effective way removal of chlorides is leaching in distilled water with periodic heating (Organ method). The water is changed every week. Washing in water takes a long time; for example, massive objects with a thick layer of corrosion products can be washed for several months. To control the process, it is important to periodically determine the chloride content by testing with silver nitrate.
Cathodic reduction treatment in water. Desalting by reductive electrolysis using current is more effective than washing in water. Under the influence electric field the negatively charged chloride ion moves to the positively charged electrode. Thus, if the negative pole of the power source is connected to the object, and the positive pole is connected to the auxiliary electrode, the desalting process will begin. First, ordinary tap water that has the necessary conductivity is poured into the bath. Objects are placed in an iron mesh, which is wrapped in filter paper, which is a semi-permeable partition for chlorides. A lead plate is used as an anode. The anode area should be as large as possible to speed up the process. Current density 0.1 A/dm2. When the installation is connected to the network, a significant amount of turbid substance is initially formed, consisting of sulfates and carbon dioxide salts found in the water. Gradually the formation of these salts stops. As it evaporates, distilled water is added to the bath.
Alkaline washing. The use of a 2% caustic soda solution for washing reduces the desalting time, which is caused by the higher mobility of the OH- ion, which allows it to penetrate into corrosion products. The solution is heated to 80-90°C at the beginning of washing; periodic stirring speeds up washing"; The solution is replaced with fresh one every week.
Alkali-sulfite treatment. The treatment is carried out in a solution containing 65 g/l sodium sulfite with 25 g/l sodium hydroxide at a temperature of 60°C.
Reductive processing leads to the fact that dense compounds of ferric iron are reduced into less dense compounds of ferrous iron, i.e. to an increase in the porosity of corrosion products and, accordingly, an increase in the rate of chloride removal.
The treatment ends with boiling in several changes of distilled water.
Heating to red heat. The method of heating to red heat is used for objects in which almost all the metal has turned into corrosion products. This method was first used in metal restoration by Rosenberg in 1898. However, it is still used by some restorers. The sequence of operations is as follows: the object is dipped in alcohol and dried in a vacuum oven. Then they wrap it in asbestos and entwine it with thin pure iron wire, the asbestos is moistened with alcohol. The object is heated in a conventional oven at a speed of 800° per hour. During heating, corrosion products are dehydrated, turning into iron oxides, and chlorides decompose. Then the object is transferred from the oven into a vessel with a saturated aqueous solution of potassium carbonate and kept in it for 24 hours at 100°C. Then washed in distilled water with periodic heating. The water is changed every day. The duration of such washing is selected empirically.
After restorative treatment and washing, it is recommended to treat the item with tannin according to the method already described.
Mechanical processing of an archaeological iron object. The next stage in the restoration of oxidized archaeological iron objects or objects in which the metal core in relation to the mass is small is mechanical processing - removal of irregularities, swellings, etc. to give integrity to the form. In some cases, the fragility of oxidized iron is so great that it is impossible to process it mechanically without prior strengthening. To strengthen it, you need to treat it with tannin, as described above, and soak it in wax or resins. When properly treated with tannin, an object acquires strength sufficient for mechanical processing. It is more reliable to carry out impregnation in a vacuum with heating.
For mechanical processing, files, sandpaper, burs, etc. are used. If the object contains iron oxides in the form of magnetite, which is very hard, then diamond or corundum tools are used for processing. When machining, it is unacceptable to saw out an object whose shape can only be guessed from a piece of oxide. It is better to stabilize the archaeological find.
If an archaeological iron object has a metal core preserved, the corrosion products must be completely removed, even if the surface texture is damaged by corrosion. Such an item can be cleaned after preliminary examination with any chemically or restoration with or without the application of current.
Owners of patent RU 2487194:
The invention relates to the field of conservation of metal products, in particular archaeological finds made of iron and its alloys, and can be used in archeology and museums. The method includes cleaning the archaeological object, its hydrothermal treatment in a dilute alkaline solution at a temperature of 100-250°C and a pressure of 10-30 atm for at least 1 hour, washing it until it is completely free of chlorine ions and drying, followed by applying a protective coating. In this method, after washing, the presence of chlorine ions in the prepared archaeological object is monitored. The invention makes it possible to increase the safety of archaeological finds made of iron and its alloys and the information contained in them while simultaneously simplifying and reducing the cost of the method. 1 salary f-ly, 2 ave.
The invention relates to the field of conservation of metal products, in particular archaeological finds made of iron and its alloys, and can be used in archeology and museums.
Almost all metals that one has to deal with in archeology are subject to corrosion; as a result of prolonged exposure to the ground, they are subject to varying degrees of mineralization. Special attention require archaeological finds from iron and its alloys, since compared to other metals, archaeological iron is more susceptible to destruction and has a complex mechanism of destruction. The most common destroyer is sodium chloride, usually found in soil large quantities. A metal archaeological object accumulates a high content of Cl - ions in the pores and channels of the metal and corrosion layers. In this case, the concentration of chlorides in the pores of an object may be higher than in the surrounding soil, due to their movement to the metal during the process of electrochemical corrosion.
The difficulty of working with archaeological finds made of metal is due to to varying degrees the safety of finds, the complexity of the corrosion system that archaeological metal represents, as well as the high responsibility for working with unique exhibits and the need to preserve as much as possible the information contained in an ancient object.
In addition to the need to preserve archaeological finds at the time of their direct extraction from the ground during excavations, there is the problem of reconstruction of museum exhibits or objects stored in archives.
The work currently being done in the field of preserving archaeological finds in the form of ancient metal products is predominantly of an applied nature, and existing conservation technologies are based on a variety of empirically developed techniques, often quite risky, therefore none of the known and currently used methods can be recommended definitely. Currently used passive conservation measures (protective coatings, impregnation) do not ensure long-term preservation of the object. The variety of archaeological sites requires study individual characteristics each item in combination with the development of scientifically based approaches to its conservation.
The difficulty in carrying out preservative treatment also lies in the fact that, simultaneously with imparting resistance against corrosion, it is necessary to preserve the integrity and shape of the archaeological object, individual details of its surface, the features of the find; if necessary, a specific corrosion layer must be preserved on the surface.
Currently, a number of methods are known for preserving metal products, in particular archaeological finds.
There is a known method for long-term protection of the metal surface of monuments from atmospheric corrosion (RU 2201473, published on March 27, 2003), which consists of spraying metal powder in the form of a porous layer onto the protected metal surface and impregnating this layer with a corrosion inhibitor. The known method is ineffective for archaeological finds made of metal, in particular iron, since it does not stop the destructive corrosion processes in the internal layers of the object. In addition, applying a protective layer of another metal to an archaeological find (for example, zinc to protect objects made of steel and cast iron) changes the properties of the conservation object and its appearance; after such processing, the find cannot be a historical document carrying the information contained in it, while the known method is irreversible.
There is a method for processing iron archaeological objects (RU 2213161, published on September 27, 2003), which consists in the fact that the objects, after preliminary cleaning, are subjected to copper plating, followed by etching with acid solutions. Disadvantage known method is the likelihood of destruction of the metal of the archaeological object, a change in its color during etching with nitric acid, as well as the need to first remove corrosion layers that repeat the relief of the find. In addition, the known method is not applicable for archaeological sites with a high degree of mineralization.
There is a known method of preserving metal products, in particular archaeological finds, for long-term storage (RU 2280512, published July 27, 2006), which includes preliminary preparation of the product by vacuum degassing and subsequent application of a protective coating with a solution or melt of an organic polymer. The known method does not provide sufficiently effective protection due to the low penetration ability of solutions or polymer melts into pores and surface defects, as well as due to the difficulty of removing the solvent used from the pores, which can initiate corrosion of the product.
The closest to the claimed technical solution is a method for obtaining protective coatings on the surface, in hard-to-reach pores and defects of metal products, providing the ability to process archaeological metal with varying degrees of mineralization (RU 2348737, published 03/10/2009), which includes pre-treatment by vacuum degassing of the surface products at temperatures from 200 to 600°C, saturation of the surface with gaseous substances, their polymerization in the plasma of a glow discharge constant or alternating current without air access, followed by application of a protective coating from a solution or melt of an organic polymer.
However, the known method does not provide a sufficiently high degree of preservation of archaeological objects, since the uncontrollability of the processes of vacuum degassing and polymerization in glow discharge plasma, as well as exposure to high (up to 600°C) temperatures (even short-term) can lead to metallographic changes in the structure of archaeological metal, with In this case, the archaeological find loses the information contained in it, for example, about the manufacturing method, the technology of its processing, and can no longer be a historical document. In addition, the technology of the known method is quite complex and requires expensive hardware.
The objective of the invention is to create a method for the conservation of archaeological finds made of iron and its alloys with varying degrees of mineralization, ensuring their maximum safety during processing and effective protection from further destruction.
The technical result of the method is to increase the safety of archaeological finds and the information contained in them during their processing while simultaneously simplifying and reducing the cost of the method.
The specified technical result is achieved by a method of conservation of archaeological finds made of iron and its alloys, including cleaning and preparation of the archaeological object with subsequent application of a protective coating, in which, unlike the known, the preparation of the archaeological object is carried out by hydrothermal treatment in a dilute alkaline solution at a temperature of 100-250°C and a pressure of 10-30 atm, followed by washing and drying, while after washing the presence of chlorine ions in the prepared archaeological object is monitored.
Mostly, a 0.01-0.1 M solution of sodium hydroxide NaOH is used as an alkaline solution, which, given the stated parameters of hydrothermal treatment, makes it possible to preserve the structure of the archaeological object and the information contained in it with minimal losses.
As is known, one of the main factors that complicates the conservation treatment of archaeological finds from iron and its alloys is the presence of iron oxohydroxide β-FeOOH (akagenite), which binds chlorine ions in its crystal structure (L.S.Selwyn, P.J.Sirois, V.Argyropoulos. The corrosion of excavated archaeological iron with details on weeping and akaganeite // "Studies in Conservation" No. 44, 1999. P.217-232).
Thus, in order to impart chemical stability and mechanical strength to archaeological finds (archaeological objects) made of iron and its alloys for a period of long-term storage, it is necessary to destroy the structure of the oxohydroxide β-FeOOH and the subsequent complete liberation of the archaeological object from chlorine-containing salts, without which the processing is insufficient. Otherwise, after applying a protective coating under the influence of Cl ions, the destruction of the object may continue at a higher rate.
In the proposed method, the stabilization of an archaeological find made of iron or its alloy is carried out during a preparatory operation by hydrothermal treatment of the object in an alkaline solution, which ensures the implementation of phase transformations in the corrosion products of archaeological iron (destruction of the β-FeOOH structure) and at the same time the complete removal of chlorine ions Cl - from pores and channels of the metal and corrosion layers of the specified object.
The method is implemented as follows.
First, the archaeological find is cleaned and washed. Cleaning includes mechanical cleaning to remove foreign substances, sand, earth, accumulations from the soil from the object and, if necessary, subsequent chemical or electrochemical cleaning, which are selected depending on the condition and material of the find, taking into account the requirements for its appearance. The cleaned object is washed in distilled water.
The archaeological find is then placed in a reactor for hydrothermal treatment. The reactor is a device operating on the principle of an autoclave, with a working medium in the form of a dilute alkaline solution, preferably 0.01-0.1 M aqueous solution sodium hydroxide NaOH. Heating is carried out to a temperature of 100-250°C at a pressure of 10-30 atm and maintained at the specified parameters for at least 1 hour, followed by cooling along with the reactor. A necessary condition processing is the presence of pressure created by the expansion of the working solution when heated. The hydrothermal treatment mode at a temperature of 100-250°C and elevated pressure ensures the stabilization of archaeological iron and its alloys due to phase transformations in corrosion products, as a result of which the structure of the oxohydroxide β-FeOOH is destroyed, which is accompanied by the release of chlorine ions Cl - from its crystal lattice and their subsequent removal into a working solution of sodium hydroxide.
After hydrothermal treatment and cooling of the archaeological object, it is washed in distilled water at room temperature until it is completely free of chlorine ions to prevent further possible corrosion processes. Monitoring the presence of chlorine ions in an archaeological object is carried out by determining their concentration in the washing waters by titration or chromatography.
After the archaeological find is completely freed from chlorine ions, it is dried at a temperature not exceeding 100°C, and then a protective coating is applied to its surface using one of the possible methods: impregnation with solutions, impregnation with a molten substance, adsorption of hydrocarbon compounds from the gas phase, it is also possible to use combined methods.
Thus, the proposed method makes it possible to preserve metal products from iron alloys of varying degrees of mineralization for long-term storage, while preserving their original structure as much as possible, as well as the information contained in them, with minimal losses, which is very important for archaeology.
Below are specific examples of implementation of the method.
Conservation of the archaeological find “Arrowhead”, recovered during excavations of the Gorbatka settlement in the Primorsky Territory, the estimated age of the find is 800-900 years. The object had a metal core and heterogeneous corrosion layers on the surface with a large number of pores and defects.
Previously, the object was subjected to mechanical cleaning and washing in distilled water in order to remove foreign contaminants and accumulations from the soil. After which it was immersed in a reactor for stabilizing hydrothermal treatment with a working medium in the form of a 0.1 M NaOH solution. The reactor was heated at a rate of 10°C/min to an operating temperature of 250°C, and a pressure of about 30 atm was created in the reactor. They were kept in operating mode for 1 hour, after which they were cooled.
After treatment in a hydrothermal reactor and cooling, the archaeological object was washed in distilled water under normal conditions until chlorine ions were completely removed. The presence of chlorine ions in the wash waters was monitored by gas-liquid chromatography.
Then the archaeological object was dried at a temperature of 85°C for 1 hour.
Phase analysis of the sample obtained from the surface of the sample was carried out on an automatic X-ray diffractometer D8 Advance (Cu K α radiation) before and after hydrothermal treatment. Before processing the archaeological find, the corrosion products were found to contain α-FeOOH (goethite) and β-FeOOH (akagenite) as the main phases. After the treatment, the β-FeOOH phase was completely absent; the main phase in the corrosion products was goethite.
The coating was applied on the basis of Paraloid B-72 acrylic resin by impregnation using a 5% solution of the said acrylic resin in acetone.
Conservation of a fragment of the archaeological find “Metal Plate”, recovered during excavations of the Lazovsky settlement in the Primorsky Territory, the estimated age of the find is 800 years. The object is highly mineralized, but the metal core has been preserved; the corrosion layers are very significant, loose, with a large number of pores and defects. After appropriate cleaning, the find was immersed in a reactor for stabilizing hydrothermal treatment; the working medium in the reactor was a 0.01 M NaOH solution. The reactor was heated at a rate of 10°C/min to an operating temperature of 100°C, while a pressure of ~10 atm was created in the reactor, maintained at the operating mode for 1 hour, after which it was cooled. After treatment in the reactor, the loose layer of corrosion products became significantly denser. Phase analysis of a sample obtained from the surface of an archaeological object after its processing in a hydrothermal reactor and washing in distilled water showed the absence of β-FeOOH oxohydroxide in the corrosion products, while the main phase in the sample was goethite α-FeOOH. Next, the archaeological find was processed in accordance with example 1.
1. A method of preserving products made of iron and its alloys in the form of archaeological objects, including cleaning and preparing the archaeological object with subsequent application of a protective coating, characterized in that the preparation of the archaeological object is carried out by hydrothermal treatment in a dilute alkaline solution at a temperature of 100-250°C and pressure of 10-30 atm for at least 1 hour, followed by washing until completely free of chlorine ions and drying, and after washing, the presence of chlorine ions in the prepared archaeological object is monitored.
2. The method according to claim 1, characterized in that a 0.01-0.1 M sodium hydroxide solution is used as an alkaline solution.
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