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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EXPERIMENTAL RESULTS


Artifacts from Group 1 (Renews) were extensively mineralized; the outer material consisted of a mixture of


Table 1 Experimental details of the chemical treatmentsa





Notes

a. The times are listed for each treatment step, including the initiai storage in water at CCI the alkaline treatment, rinsing, and dewatering in acetone.

b. A single value refers to both treatment solutions, and two values refer to solution 1. then solution 2

c. After the treatment in the acetone bath, additional treatment comprised one day in 5% v/v EN followed by three weeks hot washing (water, 50°C) and eight days in acetone.


small rocks, sand (quartz), soil minerals and iron corrosion products. For Group 1 objects, it was import­ant to retain the outer corrosion layers during the chemical treatment and then rely on the later use of controlled mechanical cleaning to expose the shape expected to lie beneath the outer material in a dense zone of magnetite (and not retained by what little metal remained). Artifacts from Group 2 (Ferryland) and

Group 3 (Renews) contained substantial iron cores. The outer material, thinner than that of Group 1, also consisted ot a mixture of small rocks, sand, soil minerals and iron corrosion products. For these Group 2 and 3 objects, it was not so important to retain the outer surface material because the shape of the object was expected to be well outlined by the remaining metal core.


Table 2 summarizes the experimental observations and results from Cl- ion analysis. The second and third columns list information about how well an object survived immersion in NaOH and/or E.V solutions, determined by noting whether or not the object re­mained in one piece. The core refers to the main object, describing the number of pieces of remaining metal with attached inner and outer layers. The clumps refer to pieces detached from the original. When the number of pieces was greater than four, the number is listed simply as 'many'. The behaviour of the objects in NaOH and in EN solutions is discussed in more detail below.


Effect of NaOH solutions on objects


During immersion ot the extensively mineralized Group 1 objects in NaOH first (treatment 1), the outer corro­sion and soil layer softened, sometimes in as little as two weeks. In four of the five pieces, large clumps broke from the main piece during their 10-13 weeks in NaOH. In two of these, large clumps of outer material fell otf to reveal evidence ot a hollow interior and a thin strip ot metal at the centre. The detached clumps usually maintained their shape in NaOH solutions. In object 6554, a crack was noted in the outer layer after eight weeks in NaOH and so it was removed trom NaOH to prevent further damage. During immersion of Group 2 and Group 3 objects with substantial metal cores in NaOH (treatment 1), the outer layer ot corrosion and soil usually softened and tell off as many small pieces. The inner corrosion layer and the metal core appeared to remain unchanged. For these objects, the removal of the outer layer was considered beneficial because it uncovered the layer that approximated to the shape ot the object.

During immersion of Group 1 objects in NaOH second (treatment 2), an interesting result was observed. Five of the six objects remained unbroken during immersion in NaOH for 10-23 weeks. These objects remained unchanged in shape even though the hard outer layer ot corrosion and soil softened during immersion in NaOH. Object 6749 broke in half, but the outer layers on each half remained intact. During immersion of Group 2 and Group 3 objects with substantial metal cores in NaOH second (treatment 2), the objects usually remained unchanged.


Effect of EN solutions on objects


During immersion in EN solutions, part of the outer corrosion fell off some objects to reveal an underlying

black layer which was easily removed during handling; sometimes black material settled at the bottom of the container. Table 2 lists those objects where the loss of the black layer in E.V was observed. For object 94991, so much ot this inner layer came off that the metal surf­ace was revealed; the black sludge that formed at the bottom ot the container was analysed and identified as magnetite [37].

For some iron objects immersed in EN, the solutions became murky, with what looked like an orange or red-brown suspension of colloidal or particulate material. Those objects tor which the E.V solutions appeared murky are also listed in Table 2. Sometimes, an orange sediment formed at the bottom of the container. For objects 94549 and 97018, samples of the cloudy EN solu­tions were collected, centnfuged for 15 minutes, and the solid was analysed by x-ray diffraction (XRD). Lepid-ocrocite was identified from 94549 and lepidocrocite, goethite and magnetite were identified from 97018 [41].

During immersion of Group 1 objects in EN in treat­ment 2, the outer layer of corrosion and soil did not soften as was noted for similar material when placed in a NaOH solution. As listed in Table 2, five out of the six Group 1 pieces remained complete and no clumps of corrosion fell off. even after immersion in EN for up to 29 weeks. This was desirable because the shape of the object was expected to be preserved within the layers of corrosion and soil. None of the solutions were murky. During immersion of Group 2 objects in EN in treat­ment 2, object 99441 remained unchanged but the other three suffered varying degrees of loss of their corrosion layers. Most of the solutions became murky. For object 99440, which remained in an EN solution for six months, most of the outer layer remained in place with only a small loss of the black inner layer. For object 94191, some bare metal was exposed after five weeks in EN: when this object was transferred to NaOH, the black layer stopped coming off and a brown layer started to form. Finally, for object 94991, after 13 weeks in an E.V solution, it was decided to stop treatment because bare metal was exposed. During immersion of Group 3 objects in E.V in treatment 2, typically for 17-25 weeks, these pieces remained essentially unchanged. (Group 3 objects had been pre-treated for two months in 1% w/v NaOH in Newfoundland.) None of them fell apart, none of the solutions became murky, and no inner black layer was exposed; only one clump of corrosion fell off object 2995.

During immersion of Group 1 objects in EN second in treatment 1, further disintegration of the objects was observed. Many of the clumps of corrosion and soil


Table 2 Experimental observations and results from chloride ion analysis.




Notes

a. The number of pieces after each treatment solution. The first figure is the number of core pieces and the second the number of outer clumps.

b. Entry indicates whether the inner black layer wiped off easily in EN.

c. Entry identifies murky EN solutions.

d. Total weight of Cl- removed in either NaOH or EN. The '=' symbol is used where the chloride content is approximate because one NaOH solution was not analyzed.

e. Percentage of chloride ions removed in NaOH

f. Initially black material came off. but this eventually stopped

g. Broke in half during hot wash h. Disintegrated during hot wash

i. Active corrosion noted after treatment


broke apart into smaller pieces. Object 6554 (which cracked m NaOH; lost one clump from its shell but otherwise remained intact. Most of the EN solutions turned murky, and the inner black corrosion could be easily wiped off. During immersion of Group 2 objects in EN in treatment 1, five of the six EN solutions became murky and the inner black layer (exposed during earlier immersion m NaOH) started to come off. The EN solutions remained murky and the black continued to come off for objects 94549 and 97018 (six-weeks in EN) and for object 94546 (18 weeks in EN). For objects 94419 and 99429, the black material stopped coming off the objects during their eight weeks in EN, but it was difficult to tell if the solutions were murky or just highly coloured. After object 94549 had finished treatment, including hot washing, rinsing m acetone and air-drying for eight days, it was returned to an EN solution for 24 hours to find out how the object reacted to EN after it had been rinsed and dried in air. For the first few hours, the surface remained unchanged but then the black material started to come off again.


Dissolved chloride ions


Table 2 summarizes the total weight of Cl- ions released into either the NaOH or the EN solutions for each artifact. The final column lists the percentage of Cl- ions released in NaOH relative to the total released. Figures 1, 2 and 3 show (for objects in Groups 1, 2 and 3, respectively), the weight of Cl- ions (in milligrams) in the treatment solutions as a function of time. Data points in the graphs are the sum of the weights of Cl- ions in each solution. The error in each data point was calculated by summing the squares of the error for each measure­ment and then taking the square root [42]. The concen­tration of Cl- ions (in ppm) was determined by titration of samples collected prior to each solution change. Because individual objects were not all treated in the same volume of treatment solution, it was necessary to convert from Cl- ion concentration (ppm) to Cl- ion weight (mg) to be able to compare Cl- ion results be­tween objects. The conversion was made by multiplying the concentration in ppm by the volume of the treat­ment solution (in litres). The errors in weight of Cl- ions were calculated by multiplying the error in ppm (±3 ppm [10]) by the appropriate sample volume (in litres).

In general, when objects were immersed in NaOH first, it took about three months before the Cl- ion concentration dropped to low levels. When the objects were then transferred to an EN solution for another two months, only a small additional weight of Cl- ions was

removed. In contrast, when objects were immersed in EN first, many of the objects needed about seven months before the Cl- ion level dropped to low values. Furthermore, when these objects were transferred to a NaOH solution, it was often observed that roughly the same weight of Cl- ions was released into NaOH during immersion for three to six months as had already been released in EN.

Figure 1 graphs the Cl- ion results for the 11 extens­ively mineralized Group 1 objects; Figure 1a shows the results for objects treated first in NaOH and Figure 1b shows the results for objects treated first in EN. Comparing the overall slopes for objects in NaOH with objects in EN shows that, in general, the Cl- ions are released faster into NaOH than into EN solutions. For the five objects treated first in NaOH, over 87% of the total Cl- ions were released into the NaOH solution. For the six objects treated first in EN, only part of the total Cl- ions was released into the EN solution, with





Figure 1 Weight of chloride ions removed from the objects as a function of time for Group 1 objects from Renews: (a) objects treated first in NaOH; (b) objects treated first in EN. Error bars are approximately the same height as or smaller than the symbol size. The numbers refer to the accession number of the object.


more released into NaOH. For object 4824. 8% of the total Cl- ions were released in EX, with the remaining 92% released in NaOH. For objects 6737, 6741 and 6749, approximately equal weights of the total Cl- ions were released in EN and NaOH. For objects 6566 and 6569, the results are not considered meaningful because such a small total weight of Cl- ions was removed.

Figure 2 graphs the Cl- ion results for the 10 Group 2 objects with substantial iron cores; Figure 2a shows data for objects treated first in NaOH and Figure 2b shows data for objects treated first in EN. In general, the Cl-ions appear to be released into NaOH faster than into EN solutions. For the six objects treated first in NaOH. over 89% of the total Cl- ions were released into the NaOH solutions. For the three objects treated first in EN, then NaOH, roughly equal weights of Cl- ions were released into each solution type.

Figure 3 contains the Cl- ion results for Group 3 objects with substantial iron cores; Figure 3a shows the results for object 4859 which is representative ot the other objects in the group. All the graphs have similar





Figure 2 Weight of chloride ions removed from the objects as a function of time for Group 2 objects from Ferryland: (a) objects treated first in NaOH; (b) objects treated first in EN. Error bars as Figure 1.


shapes, with the weight of Cl- ions removed generally increasing with time; only rarely did a curve flatten (as often seen in Figures 1 and 2). For all 10 objects, the total weight of Cl- ions removed was always less than 40 mg: these objects had been pre-treated tor two months in 1% w/v NaOH in Newfoundland. Figure 3b graphs the Cl- ion data for Group 3 objects 3004a and 3004b, each ot which contained a substantial amount of metal, was covered with similar outer crusts and, in common with other Group 3 objects, had been pre-treated in NaOH. Pieces 3004a and 3004b were treated differently from other Group 3 objects and differently from each other. Object 3004a was treated first in NaOH, then EN while object 3004b was treated first in EN, then NaOH. Almost six times more Cl- ions were removed from 3004a than 3004b. suggesting that, over the same penod of time, the 2% w/v NaOH solution was more effective than the 5% v/v EN solution in promoting the release of Cl- ions.





Figure 3 Weight of chloride ions removed from the objects as a function of time for Group 3 objects from Renews: (aj results for one object fthe other nine objects nave similar curves) treated only in EN solutions at CCI; Ib) results for object 3004a and 3004b, Where error bars are not shown, they are approximately the same height as or smaller than the symbol size.


The decision to stop treatment was usually made when the Cl- ion concentration reached low levels in successive changes of the treatment solutions, typically <20 ppm. This approach should be used with caution because the concentration of Cl- ions depends on the volume of the treatment solution used. Compare, for example, two treatment volumes, one 500 mL and one 200 mL. each containing 20 ppm Cl-. More Cl- ions (10 rag) will have been removed from the object im­mersed in 500 mL than from the object immersed in 200 mL (4 mg). The final Cl- ion data were not analysed and graphed until after the treatments had been stopped. In hindsight, the data should have been analysed and plotted before making the decision to stop treatment. It is possible that some of these objects (particularly Group 3 objects) still contain residual Cl- ions and may suffer corrosion problems in the future.1


Dissolved metal ions


The concentrations (in micrograms per litre, µg.L-1) of 26 elements were analysed by ICP-AES and reported by Seprotech Laboratories. For each solution, the weight of each element analysed was calculated by multiplying the concentration by the appropnate solution volume. The errors in weight for each element were calculated from the appropriate percentage error provided by Seprotech. No correction was made for background level of the elements in treatment solutions because they were assumed to remain constant, given that all solutions were prepared from stock solutions of NaOH or EN. The weight of each element removed by each solution change of NaOH or EN was added together to calculate a total weight removed by either solution. For each element, the results were then plotted against time. For many of the elements analysed, there was an initial increase followed by a flattening of the curve. The ICP-AES results for the five elements detected in significant amounts are listed in Table 3. For each object, the results are reported as the total weight and percentage of an element removed into NaOH or into EX. Except for iron, the general trend for these elements was for more of an element to be dissolved into the first treatment solution in which the object was placed.

The results for iron are particularly interesting (Figure 4); remarkably little iron was detected in the NaOH


1 In 2003, Cathy Mathias surveyed the condition of these artifacts and found that they were stable except for two from Group 1 13346, 6563), one from Group 2 (9944V), and two from Group 3 13007, 4859).


solutions while more was detected in EN solutions. In Figure 4a (NaOH first), there was relatively little iron detected in the NaOH solutions. But when the objects were transferred to EN, the weight of dissolved iron increased. For objects 94419 and 99429, more iron was dissolved in the first EN solution than in the second. In Figure 4b (EN first), the iron level increased and then levelled oft in subsequent EN baths as less iron dissolved in later solution changes. Once the objects were transferred to a NaOH solution, very little additional iron dissolved.


DISCUSSION


Diffusion of chloride ions


In this study, the approach was to treat each of the 32 iron objects in separate solutions, with regular changes in the treatment solution, and quantitatively monitor the Cl- ion concentration over time. The overall observa­tion was that more Cl- ions were removed more quickly from iron treated in a NaOH solution than from iron treated in an EN solution. When NaOH was used first to treat Group 1 and 2 objects, immersion for about three months was needed before low Cl- ion levels were achieved, with relatively few additional Cl- ions re­moved in the following two or three months of immer­sion in an EN solution. In contrast, when EN was used first to treat Group 1 and 2 objects, an immersion of up to seven months was often needed to achieve low Cl-ion levels in solution and, for many of these objects, this only accounted for about half of the total chloride ions removed. The remainder were removed when the objects were transferred to NaOH solutions.

The authors wanted to use the diffusion model developed in 1978 by North and Pearson to assess the Cl- ion data [28], For their diffusion model, North and Pearson assumed the Cl- ions were initially evenly distributed within the corrosion layer and no Cl- ions were present outside the corrosion layer (i.e., in the treatment solution). They also assumed that the Cl- ions were diffusing through a constant distance (i.e., the corrosion layer) and that the solid matrix remained physically unchanged. Their model provided a general expression for the diffusion of Cl- ions from objects of any shape over a short time. The expression predicted that when the amount of Cl- ions in solution was plotted against the square root of time (t1/2), the resulting graph would comprise a straight line (passing through the origin) with a slope proportional to the Cl- ion diffusion constant. For Cl- ions diffusing through the solution in


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