The Influence of Gaseous Pollutants on Silver Artifacts Tarnishing

The present work investigated the effect of the common gaseous pollutants on silver artifacts corrosion. The study will be carried out on manufactured coupons of silver alloy (91 silver, 9 copper) which have chemical composition similar to ancient Egyptian silver artifacts. These coupons will be exposed to gaseous pollutants of each individual gas; such as Sulfur dioxide, Nitrogen dioxide, Carbon dioxide, Hydrogen sulfide and Chlorine. The exposure period will be four weeks in a climate chamber with gas concentration 10 PPM. After the test Examinations by SEM and PM were used to evaluate the effect of each gas and description the morphology of the corrosion layers. The results revealed that all gases reacted with the surface except carbon dioxide. The formed tarnishing layers varied in coverage and density rate. Corrosion products are analyzed by XRD and the results revealed Ag2S, AgCl, Ag2SO4 and Ag2O as corrosion products.

Among the family of ancient metals, the fewest studies on the laboratory and field exposure of the gaseous pollutants were presented to silver. Previous lab exposure tests often focused on silver tarnish due to Sulphur containing pollutants [2]- [7]. These studies have been agreed in their results which can be summarized in the formation of silver tarnish as black layer consisting of Ag 2 S as major corrosion product and Ag 2 SO 4 in a negligible quantity. Few lab exposure studies have been presented to impact other gaseous pollutants such as Cl, NO 2 , and CO 2 , but the results were different and varied. The differences were as follow.
Results of Previous studies indicated that nitrogen dioxide is not considered a corrosion factor of silver because silver does not react with it, but it only acts as an accelerated factor of silver tarnish with other gases [8] [9] [10] [11] [12] even though Previous study found silver nitrate Ag 2 NO 3 as corrosion products on exposed silver to 1.2 ppm NO 2 for 40 hours [2].
Also Ag 2 SO 4 as corrosion product was observed on silver coupons in field exposure [13] although the studies indicated that Silver sulfate (Ag 2 SO 4 ) forms only in artificially high levels of sulfur dioxide [14] [15].
Silver is sensitive to chloride (Cl − ) and silver chloride will be formed as a result of the reaction [10] [11] [12] [15] [16] [17]. Also this does not agree with results revealed that silver chloride compound was not identified on surface film of silver coupons after the exposure in an ASTM B117 salt spray chamber [18] and this compatibility with previous studies mentioned that silver does not react directly with chlorine gas and the presence of silver chloride as corrosion product due to burial in a chloride rich environment [19] [20].
The laboratory-exposure studies of the effect of CO 2 gas on silver are very few, although Ag 2 CO 3 is distinguished as corrosion product of silver as result of the reaction with CO 2 and although CO 2 is abundant in the ambient environment of silver artifacts. So very little is known about the formation mechanism of Ag 2 CO 3 Such as it is expected only in strong alkaline solutions [21]. Ag 2 CO 3 product corrosion has not been detected as corrosion product on the silver artifacts. Also Ag 2 CO 3 was identified in a recent study of field-exposure study on the silver coupons exposed to various outdoor environments and the product was detected only in one site which distinguished above the temperature inversion layer [1]. Therefore, in this study, the effects of gases (NO 2 , Cl, CO 2 ) on silver will be presented as a laboratory exposure and the results will be compared with previous studies. Also they did not take sufficient laboratory studies although they were common gaseous pollutants in outdoor and indoor atmospheric environment, also hydrogen sulfide and sulfur dioxide were chosen as main gases in silver tarnish and lead to silver sulfide which was often observed as corrosion product of silver.

Coupons Preparation
Silver coupons should be similar for archaeological silver of ancient Egypt civili-  [23]. Also analyses of Babylonian coins from silver copper alloy were about 87% -90% purity [21]. So that a composition of coupons was Ag 89.4% and Cu 10.5% .This concentration was not artificial available and was manufactured by alloying a mixture of silver (pieces from the pure silver) with pure copper pieces.
The coupons were made by direct addition of silver and copper in a crucible Figure 1 [24]. The dimensions were (3 cm × 5 cm × 0.08 cm) Figure 2. XRF analysis used to determine the Composition.

Design and Preparation of Climate Chamber Test
Climate chamber was designed according to ASTM [25]. It is made of 1000 cm

The Test Gases
The gases for the test were as follows: Sulfur dioxide SO 2 , Nitrogen dioxide NO 2 , Carbon dioxide CO 2 , Chlorine gas Cl and Hydrogen Sulphide H 2 S. The gas is mixed with the present air in inside chamber. Those types were the most influence in deterioration of silver artefacts.

Test Procedures
Each five coupons were exposed to humidified air containing concentration 10 ppm of one gas only. Accelerated conditions parameters were as shown in Table   1. The values of conditions parameters were chosen after survey of 29 previous studies which included lab exposure of ancient metals family (copper, bronze, silver, steel and lead) as shown in Table 2 Gas syringe was used to get the required volume and injected it into inside chamber Figure 3, the fan was used for mixing gases and its distribution inside the chamber.

Examination of the Samples after the Test
All the coupons surfaces interacted with the gases from the first week except the exposed coupons to carbon dioxide. The reaction behavior inside the chamber and the growth rate of the tarnishing layer were similar among Cl, H 2 S, and NO 2 . The interaction began as a very thin layer on the surface and the growth of tarnishing was generally rapid then became slow. The surface appearance turned from light interference tones to a grey and, finally, black film. Also the tarnishing rate was increased with H 2 S, Cl, decreased with SO 2 , NO 2 as shown in Figure   4.
The showed the least tarnishing layer and the reaction was very slow although humidity existed, the slow reaction of SO 2 with silver was mention in previous studies [35].

The Analysis of Corrosion Products by X-Ray Diffraction and Raman Spectroscopy
The coupons were exposed to X-ray diffraction to analyze the formed patina.  (c) The coupons exposed of SO 2 ; (d) The coupons exposed of NO 2 ; (e) The coupons exposed of CO 2 .
This was showed many of corrosion products as in Figure 6 and Table 3.   The Raman spectrum of Cl gas coupon presented sharp and highly intensive band at 236 cm −1 , two weak bands at 145 and 349 cm −1 , these beaks agreement with the main beaks of AgCl bands as in reference [54] Figure 7(b), the layer tarnishing of other gases was so slight that it was not identified by Raman.
Silver sulfide (Ag 2 S Acanthite): they were identified as abundant on silver artifacts. The reaction behavior between gas H 2 S and the silver surface to form these products explains Equations (6)   Silver chloride (AgCl Cerargyrite, chlorargyrite): In this case, the dominant theory in the interpretation of formation mechanism of chloroargyrite AgCl is the transformation of Ag 2 O to AgCl as Equation (8)  Silver sulfite (Ag 2 SO 4 ): This product was identified in a previous study as corrosion product of silver [1] and the Equations (9)-(12) were suggested for the mechanism of its formation on silver coupons surface.  on silver artifacts surfaces.
Ag Ag e Silver ammine nitrate (Ag(NO 3 ) 3 (NO) 3 ): previous studies of filed and lab. exposure were identified only one product (AgNO 3 ) from nitrates anions as corrosion products on silver surface, Therefore this compound was expected of NO 2 gas, but analysis showed Silver ammine nitrate (Ag(NO 3 ) 3 (NO) 3 ).
Copper Nitrate Hydroxide (Rouaite, Cu 2 (NO 3 )(OH) 3 ): Silver and copper are the coupons alloy elements, the formation of Rouaite corrosion product is contributed to selective corrosion of copper by the interaction of cu (as the main alloying element) with NO 2 gas.

Conclusions
Silver in the ambient atmosphere and presence humidity is susceptible to the reac-Y. Salem Open Journal of Air Pollution tion with air pollutants. The tarnishing is formed not only in the presence of Sulphur containing pollutants but also there are other gaseous such as Cl and NO 2 .
Except the coupons of CO 2 , all the test coupons were exposed to tarnishing layer which was formed on the surface as a blackish thin film caused by the reaction of the metal surface with test pollutants. But highest layer in density and tarnishing was to H 2 S gas.

Found
This research was founded by south valley university (Egypt).