Removal of Chloride and Iron Ions from Archaeological Wrought Iron with Sodium Hydroxide and Ethylenediamine Solutions




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НазваниеRemoval of Chloride and Iron Ions from Archaeological Wrought Iron with Sodium Hydroxide and Ethylenediamine Solutions
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Removal of Chloride and Iron Ions from Archaeological Wrought Iron with Sodium Hydroxide and Ethylenediamine Solutions


L.S. Selwyn and V. Argyropoulos


Results are presented on the effectiveness of treating archaeological iron by immersion in an aqueous sodium hydroxide (NaOH) solution (2% w/v, pH 13.5, room temperature) followed by immersion in an aqueous 1,2-diaminoethane (ethylenediamine, EN) solution (5% v/v, pH 11.5, 50°C). This study was undertaken to determine the effectiveness of these solutions in removing dissolved chloride ions and to explain the occasional observation of rapid iron corrosion. Thirty-two archaeological wrought iron pieces were treated. Some were immersed in NaOH followed by EN, and, for comparison, others were treated first in EN, then NaOH. Each artifact was treated separately and solutions were changed on a regular basis. The chloride ion concentration was determined by potentiometric titration with a silver nitrate solution. For nine artifacts, solutions were analysed quantitatively for 26 different dissolved elements using inductively coupled plasma atomic emission spectrometry. The results demonstrate that chloride ions are more effectively removed from archaeological iron by immersion in NaOH than by immersion in EN. The results also demonstrate that heavily mineralized iron is more likely to remain unbroken if immersed in EN before immersing in NaOH. Unfortunately, the corrosion of iron can be stimulated by EN because it forms soluble complexes with iron(II) ions.


INTRODUCTION


In the early 1980s, James Argo developed a new treat­ment for archaeological iron at the Canadian Conserva­tion Institute (CCI) in response to the need to treat iron recovered from a wet, saline, land site in Red Bay, Labrador [1-4]. This treatment is based on the immer­sion of iron in a 2% w/'v aqueous solution of sodium hydroxide (NaOH) at room temperature followed by immersion in a 5% v/v 1,2-diaminoethane (ethylene­diamine, EN) solution heated to 50°C [5]. It was developed to preserve the shape of the object and to promote long-term stability by removing soluble salts. This treatment has been applied successfully at CCI. as determined by collection surveys after treatment [6, 7], Elsewhere, however, there has been only limited use ot EN solutions to treat archaeological iron [8. 9], In general, this treatment has not found favour because EN is toxic [10] and because some objects lost their corro­sion layer and corroded rapidly [8. 9, 11]. The authors felt that there was still a need for further study of this treatment approach because several questions remained unanswered. Was the success of the treatment due to the primary use of aqueous NaOH or was it due to the combined effect ot using both NaOH and EN solutions? How effective was each of these solutions in removing chloride ions (Cl-) and in preserving the corrosion layer? Why were there occasional examples of rapid corrosion problems?

We wanted to evaluate the CCI treatment approach using archaeological iron objects without using any destructive methods such as exposing the iron to high relative humidity after treatment [11-14], or dissolving the piece to detect residual chlorides after treatment [12, 15, 16], It was decided to treat objects separately and analyse individual NaOH or EN treatment solutions quantitatively for dissolved Cl- ions. One drawback of this approach is that it does not provide any information about the quantity of Cl- ions remaining in the object. It was also decided to monitor a limited number of treatment solutions for trace elements using inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis to find out whether ions other than Cl- were being removed bv the alkaline treatment solutions.


ICP-AES is a relatively inexpensive technique that is routinely used to analyse quantitatively for many elements in a solid sample simultaneously. Applications include the analysis of archaeological material such as ceramics [17], glass [18] and metals [19]. The technique is popular because it can analyse simultaneously for major, minor and trace elements.

Thirty-two objects were obtained from Ferryland and Renews, two terrestrial sites in Newfoundland with wet, saline environments similar to the Red Bay site [10]. To find out if the immersion order made a difference, some objects were immersed first in a 2% w/v NaOH solution followed by immersion in a 5% v/v EN solution; for other objects, the order was reversed. Treatment was continued until low Cl- ion levels were measured in successive changes of the treatment solution. For all 32 objects, the Cl- ion content was determined at each solution change using a potentiometric titranon with a silver nitrate solution [20, 21]. In addition, tor nine artifacts, each solution change was analysed quantita­tively for 26 different dissolved elements using ICP-AES. This paper reports the results from the assessment of the effectiveness of these two alkaline treatment solutions in stabilizing archaeological iron.


BACKGROUND INFORMATION


As iron corrodes during burial, the iron dissolution reaction (Fe → Fe2+ + 2e-) takes place at the interface between the metal and its corrosion products. In aqueous solutions with pH greater than about 4. this half-reaction is counterbalanced by the reduction of dissolved oxygen (O2 + 2H2O + 4e- → 4OH-) [22]. At the iron surface. iron(II) ions dissolve, accumulate, and undergo hydrolysis, Fe2+ + H2O ↔ Fe(OH)+ + H+. which causes local acidification [23, 24]. Electrical neutrality must be maintained and this is achieved by anions (e.g., Cl-) diffusing in from the surrounding environment to balance the charge of the Fe2+ cations. Chloride ions, in particular, tend to concentrate at the interface because of their high mobility and because they are often the predominant environmental anions, especially in a manne environment. The net result is that the cracks, pores and open spaces within the corrosion layer on archaeological iron are filled with an acidic iron(II) chloride (FeCl2) solution, with the Cl- ions strongly concentrated inside the corrosion layer at the surface of the corroding iron [22, 25, 26].

Immersion treatments for archaeological iron are designed to remove the acidic FeCl2 solution, making it less likely to corrode [25], Such immersion treatments

involve placing excavated iron in a liquid, which is usually near-neutral or alkaline, and waiting for the Cl-ions to diffuse out [27, 28]. Immersion in sodium hydroxide (0.1 to 0.5 M) is recommended [27, 29-32], and has been noted as more effective than other treatment methods [6, 7, 11, 16, 28, 33, 34], The main driving force for Cl- ion removal is diffusion, with the Cl- ions diffusing from a region of higher concentration (at the metal/corrosion interface) to one of lower concentration (the treatment solution).

The corrosion layer and its porosity play an important role in the ability of Cl- ions to diffuse away from the metal surface into a treatment solution. Inside the corro­sion layers, the Cl- ions are dissolved in the solution filling the interconnecting pores and channels [26]. When an object is immersed in a treatment solution, it is the porosity within the solid that allows the treatment solution to diffuse in and the Cl- ions to diffuse out. The rate at which the Cl- ions diffuse out of the solid depends on the size of the open spaces within the solid, how well they are linked together, and if continuous pathways exist from the metal/corrosion interface to the outer surface of the object. If the treatment solution can diffuse in, then the Cl- ions can diffuse out. If the Cl-ions are isolated in discrete pores, they will be trapped and unable to diffuse out. If Cl- ions are still trapped after treatment, they may cause problems in the future, especially if a channel opens up, allowing water and oxygen to enter [25].


ETHYLENEDIAMINE (EN)


Ethylenediamine (H2N-CH2-CH2-NH2) contains two amino groups (-NH2) separated by a two-carbon chain. It dissolves readily in water to form an alkaline solution; the lone pair of electrons on each nitrogen atom in EN interacts with protons (H+) in water to form either ENH+ (singly protonated H2N-CH2-CH2-NH3+) or ENH22+ (doubly protonated +H3N-CH2-CH2-NH3+), as described by:




Whether the predominant species in solution is neutral or protonated depends on pH, as determined by:





At 25°C, the formation constants K1,H+ for equation (3) and K2,H+ for equation (4) are [35, 36]:





The predominant species is ENH22+ below pH 6.9. ENH- between pH 6.9 and 9.9, and neutral EN above pH 9.9. In the 5% v/v EN solution (pH 11.5) used in this study, the predominant species is the neutral EN molecule.


EXPERIMENTAL PROCEDURE


Samples


Thirty-two freshly excavated wrought iron artifacts from the Renews and Ferryland sites in Newfoundland) were selected by Cathy Mathias, Memorial University, for use by CCI. The 32 objects (identified by their accession numbers) consisted of nails, handles, hooks, knife blades and other tools. They were all relatively light (10-120 g), except for a hinge (400 g) and a large axe-head (2000 g), and had been stored wet after excavation. Many were stored in water at room temp­erature and the others in 1% w/v NaOH; prior to shipping to CCI, those pieces stored in NaOH were rinsed with hot tap-water for two days. Additional details about these 32 pieces are available elsewhere [10]. All the artifacts were shipped wet to CCI and, on arrival, they were stored in a refrigerator at about 7°C and 70% relative humidity for either seven or 56 weeks, depending on when they underwent treatment at CCI. (Wet artifacts tend to dry out if stored in a refrigerator with a freezer that lowers the relative humidity.) The artifacts were photographed and X-radiographed.

The iron pieces were divided into three groups, based on site and storage history. Group 1 contained 12 Renews nails, previously stored in water; in the radio­graph, these pieces appeared to be extensively mineral­ized. Group 2 contained 10 Ferryland iron tools, previously stored in water; in the radiograph, these pieces appeared to have a substantial metal core, except possibly artifact number 94191. Group 3 contained 10 Renews iron tools and scrap iron that had been stored first in water, then in 1% w/v NaOH; in the radiograph,

these pieces showed substantial metal cores, except for No. 3013 and possibly No. 2995.

Samples from the surface of six artifacts (two from each group) were analysed by X-ray diffractometry using iron-filtered cobalt radiation, 45 kV, 160 mA [37]. Their colour ranged from orange to brown to black with one instance of red. Group 1 pieces (Renews 6554 and 6566) were covered with lepidocrocite [(γ-FeO(OH)], quartz (SiO2). and muscovite [KAl2(Si3Al)Ol0(OH,F)2]. Group 2 pieces (Fern-land 94991 and 97018) contained goethite [α-FeO(OH)], akaganéite [ß-FeO(OH)], lepidocrocite, magnetite (Fe3O4), quartz, and albite (NaAlSi3O8). Group 3 pieces (Renews 3013 and 4859) were covered with lepidocrocite, goethite, quartz and muscovite. Quartz, albite and muscovite are typical soil minerals; albite is a type of feldspar, and muscovite a type of mica [38], The iron compounds goethite, aka­ganéite, lepidocrocite and magnetite have most likely formed from the corrosion of the iron artifacts, although goethite is also common in soil.


Treatment


Each iron artifact was treated by one of the following three chemical treatments:


Treatment 1, NaOH/EN. Artifacts were immersed first in aqueous 2% w/v NaOH (0.5 M, pH 13.5 ± 0.5) at room temperature for eight to 13 weeks. Next, they were immersed in aqueous 5% v/v EX (0.75 M, pH 11.5 ± 0.5) and heated to 50°C on working days. Immersion lasted until the Cl- ion concentration in the last solution change was less than 20 parts per million (ppm), typically between six and 18 weeks. The decision to transfer an object from NaOH to EN was based partly on the degree of mineralization and not necessarily on a low chloride ion concentration in the treatment solution. If, for example, the object had a substan­tial metal core and the shape of the object was retained by the inner corrosion layer (e.g., the magnetite layer), then it was left in NaOH until the outer corrosion layers fell away easily. If, on the other hand, the object was extensively min­eralized and the shape of the object was retained somewhere in the mineralized layer, then these objects were removed from NaOH after the outer corrosion had softened (as determined by feel) but before the corrosion started to fall off. There was concern that the shape of the object could be destroyed or irreparably damaged by continued immersion in NaOH. It lumps ot corrosion prod­ucts fell off the object that were required to pre­serve its shape, they were retained for consolidation.

Treatment 2, EN/NaOH. Artifacts were immersed first in aqueous 5% v/v E.V heated to 50°C on working days for five to 29 weeks. Next, they were placed in aqueous 2% w/v NaOH at room temperature, typically for 10 to 23 weeks. The objects were removed from E.V it continued immersion appeared not to be removing large numbers of Cl- ions or if prolonged immersion in E.V resulted in the dissolution ot the corrosion layers. Artifacts were removed from NaOH when the Cl- ion concentration was less than 20 ppm. or it the mineralized regions were becoming soft and the outer corrosion layers started to come off.

Treatment 3. EN. Artifacts were immersed in aqueous 5% v/v EN heated at 50°C. Immersion lasted until the Cl- ion concentration measured in the last solution change was less than 20 ppm, typically from 17 to 25 weeks.

Table 1 lists the chemical treatment used on each arti­fact. Treatment 1 was used on five iron pieces randomly selected trom Group 1, six pieces randomly selected from Group 2. and one piece (3004a) chosen trom Group 3. Treatment 2 was used on six pieces randomly selected trom Group 1, tour pieces randomly selected from Group 2, and one piece (3004b) chosen from Group 3. Treatment 3 was used on 10 pieces from Group 3.

Artitacts were immersed in the first treatment solu­tion, each in a separate container. All the treatment solutions were changed at the same time, usually weekly during the first month (because many of the objects became obscured as dirt, soil and other material dissolved and coloured the treatment solutions), then monthly thereafter. For Cl- ion analysis, a 30 mL sample was collected prior to each solution change: for ICP-AES analysis, 300 mL samples were collected prior to solution changes for nine artifacts. The Cl- ion content in the samples was determined using a potentiometric titration with silver nitrate [20, 21]. These were usually analysed over a one- or two-day period, after standards had been prepared and the titration checked, with up to 30 samples being analysed per day [10]. The samples and standards for ICP-AES analysis were prepared and analysed for 'total metals' by Seprotech Laboratories in Ottawa [10]. The decision to change treatment solutions from NaOH to E.V (or vice versa), or to stop treatment.

was usually based on the detection of low Cl- ion concentrations (e.g., below 20 ppm) from several consecutive treatment solutions.

The treatment solutions were made up with deion-ized water (pH 6.0 ± 0.2) and with NaOH pellets (Fisher Scientific, American Chemical Society (ACS) certified) or anhydrous ethylenediamine (Fisher Scien­tific, 98-100%). Artifacts were always completely immersed in the solutions. Table 1 lists the solution vol­umes and the ratios of solution volume to surface area used for each artifact. For practical reasons, a 5:1 mL:cm2 ratio (treatment solution volume : estimated surface area) was used where possible, although a ratio of 20:1 mL:cm2) is often recommended to avoid appreciable changes m solution composition during experiments [39]. The containers (glass for EN solutions, plastic for NaOH solutions) were covered with plastic lids or sealed with a polyethene-based plastic wrap secured with a rubber band. All the containers were placed in a large fume-hood. The EN containers were placed on hot plates and the temperature was adjusted to maintain 50 ± 10°C for about eight hours during working days. For safety reasons, the hot plates were turned off during the night or at weekends. The NaOH solutions were left at room temperature (about 22°C).

After chemical treatment, the objects were rinsed for two to nine weeks in hot (~50°C) deionized water to remove residual chemicals and to lower the pH of the surface of the artifact to 7. Whenever the water was changed (about once every two weeks), the surface pH was checked with pH indicator paper. After hot wash­ing, the artitacts were immersed in an acetone (propa-none) bath for three to 12 days to remove residual water. After air-drying, most of the artifacts were mechanically cleaned to reveal the shape of the object, using hand tools and/or an air-abrasive unit (abrasives: aluminium oxide 27 µm, silicon carbide 50 µm, glass beads 27 µm). Following cleaning, most of the artifacts were coated with tannic acid [40]. Artifacts that had broken into two or more pieces were glued together using either a cellulose nitrate adhesive (H.M.G. Heat and Waterproof Adhesive, H. Marcel Guest Ltd) or a mixture of approximately equal amounts of Acryloid B-72 (Rohm & Haas) and acetone. Treated artifacts were packed in Ethafoam, placed in perforated polyethene or polypro-pene bags, and returned to Newfoundland.

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