WO1999036161A1 - Method for preserving silver-plated objects and filtering device for carrying out said method - Google Patents

Abstract

The invention relates to a method for preserving and conserving silver-plated objects (1), according to which the objects (1) are placed in an atmosphere of desulfurized gas. The invention is characterized in that the gas is desulfurized by contact with cathodically activated silver metal.

Description

 Method for preserving silver objects and purification device for its implementation

The invention relates to the field of protection and conservation of silver metallic objects. The method according to the invention makes it possible to purify gases intended to constitute the environment of these objects. The method according to the invention is implemented, for example, to protect and conserve such objects while they are exposed to the public. It is known that sulfur compounds in contact with silver form silver sulphides responsible for tarnishing and corrosion of silver objects.

Table 1 shows the main air polluting species responsible for tarnishing silver, as well as their respective concentrations.

atmospheric sulfurous qases », Corrosion Science, Vol 25, n°2.

atmospheric sulfurous qases ”, Corrosion Science, Vol 25, n ° 2.

Carbonyl sulfide, OCS, is the main pollutant identified, it is also the most reactive, ahead of H ₂ S, SO ₂ then CS ₂ . Sulphurization of silver is moreover very sensitive to air humidity because the presence of a few monolayers of water on the surface of the metal is sufficient to hydrolyze OCS, to dissolve and dissociate H ₂ S allowing the reaction:

4Ag + 2HS- ™ 2Ag ₂ S + H ₂ + 2e ^'

This reaction is responsible for the blackening, tarnishing and corrosion of the silver.

This corrosion by sulfur pollutants alters the electrical properties of the silver contacts which are still numerous in connection technology. Similarly, the blackening of silver metal objects such as jewelry, musical instruments, daguerreotype, tableware, objects of art and archeology, etc., is a very embarrassing problem with regard to aesthetics and their presentation to the public, but also, in the long term, with regard to the integrity of these objects or these works themselves, especially when the thicknesses of metallic silver are very small (daguerreotype, silver film on wooden picture frames, electrolytic silvering of musical instruments, etc.).

To prevent the deterioration of silver surfaces, two main categories of displays are used: a - the total elimination of polluting chemical species in the environment of the silver metallic object; in this category fall often heavy and very expensive processes, such as inerting, which consists in putting the enclosure which contains the object under neutral gas (nitrogen for example) after having purged the volume; in this category between another parade which consists in having in the enclosures containing the objects to be protected, diffusers of products inhibiting the tarnishing of silver; but these inhibitors are volatile and have some drawbacks, such as the absence of exhaustion control and the absence of data on their harmlessness with regard to materials constituting the objects to be protected (wood, ivory, mother-of-pearl, cork, paint, etc.); b - a physical barrier between the object and its environment; this second category includes, for example, varnishes, encapsulation and bagging; the installation of a varnish consumes approximately 50% of the time of restoration of the objects and causes very unsightly differential corrosions, when there are defects or gaps in the varnish; encapsulation and bagging cannot be a universal solution either; they cannot, for example, be suitable for the exhibition to the public of objects thus protected.

The object of the present invention is to provide a method of preserving silver objects which does not have the drawbacks noted above, and which is nevertheless very effective.

More specifically, the invention is a process for the preservation and conservation of silver objects comprising the step of conditioning the objects in a desulfurized atmosphere characterized in that the atmosphere is desulfured on contact with metallic silver activated cathodically.

Other aspects, objects and advantages of the invention will appear on reading the detailed description which follows. The invention will also be better understood by means of the references to the appended drawings, in which:

- Figure 1 is a perspective view, partially broken away of a side wall and back, of a display case for silver objects, whose atmosphere is desulfurized by the method according to the invention;

- Figure 2 shows schematically, in longitudinal section, an example of enclosure compatible with the use of the method according to the invention;

- Figure 3 shows schematically, in longitudinal section another example of enclosure compatible with the use of the method according to the invention;

- Figure 4 schematically illustrates yet another example of implementation of the method according to the invention;

- Figure 5 shows in perspective an example of a purification device for implementing the method according to the invention;

- Figure 6 schematically shows a longitudinal section of the purification device shown in Figure 5;

- Figure 7 schematically shows two examples of possible positions for placing a purification device such as that shown in Figures 5 and 6; in FIG. 7a, flaps placed at the outlet of this device are oriented upwards; in FIG. 7b, these flaps are oriented downwards;

- Figure 8 shows schematically the structure and arrangement of a cassette for the implementation of the method according to the invention;

- Figure 9 schematically shows a device for cathodically activating the metallic silver necessary for the implementation of the method according to the invention; - Figure 10 is a photograph of two silver wool samples subjected to sulfurization, for 4 min; one of the samples was previously activated cathodically and the other not; and

- Figure 1 1 is a photograph of two samples of silver wool subjected to sulfurization for 65 hours; one of the samples has been cathodically activated beforehand, the other not.

In the process according to the invention, a material composed of metallic silver makes it possible to trap sulfur-containing compounds which may be present in a gas intended to form the atmosphere of silver objects 1 to be protected, before this gas reaches the contact with these objects 1.

To increase the efficiency of this process, metallic silver is preferably put in a form having a large specific surface. The more this surface is developed, the greater the trapping surface of the sulfur compounds. This makes it possible both to promote exchanges with the gas and to increase the quantity of molecules that can be trapped before reaching the saturation of this surface with respect to the sulfurization reactions.

According to a particular embodiment of the method according to the invention, the silver objects 1 to be protected are placed in an enclosure 300, in order to confine the atmosphere surrounding these silver objects 1 (FIG. 1). But we can also consider placing these objects 1 under a stream of gas. When the silver objects 1 are placed in an enclosure 300, the atmosphere of the latter is maintained in overpressure relative to the outside of the enclosure 300. In this way, enclosures 300 can be used for limited sealing, and therefore less difficult to produce and less expensive than waterproof 300 enclosures. The overpressure inside the enclosure 300 limits the penetration of gas that has not been desorbed through the interstices thereof.

Preferably, the gas is de-fumed before or at its entry into the enclosure.

According to a variant of the method according to the invention, it is possible to use a purification device 100 and then to route the gas desulfured by the device treatment plant 100 to enclosure 300 by a suitable conduit 150 (see FIG. 2).

According to another variant, the metallic silver for trapping sulfur compounds is placed in a cassette 200, at an inlet opening 310 for the gas in the enclosure 300, while a fan 50 is located in an outlet opening 320 of the gas from the enclosure 300. In this way, the gas is sucked into the enclosure 300 and de-fumed at its inlet while it is extracted from the enclosure 300 by the fan 50 (FIG. 3).

According to yet another variant of the process according to the invention, the metallic silver for trapping sulfur compounds is placed in a cassette

200 so as to be both swept by the gas entering the enclosure

300 and part of the gas from the enclosure 300 (FIG. 4). In this way, the gas from the enclosure 300 undergoes recycling.

The method according to the invention can also be implemented by placing metallic silver activated cathodically at several places in the enclosure 300. For example, metallic silver activated cathodically can be placed so as to desoother the gas entering the enclosure 300, while another quantity of cathodically activated metallic silver can be placed elsewhere in the enclosure 300 to desulfurize the recycled gas from this enclosure 300.

According to a preferred embodiment of the method according to the invention, the enclosure 300 is a showcase. As illustrated in FIG. 1, in this case, the enclosure 300 comprises a window 305, behind which the silver objects 1 are exposed, on a shelf 5. A purification device 100 is placed in the window 300, with a inlet grille 12 communicating with the outside of this display case 300. Thus, the gas to be purified enters the device 100, passes through an extractable cassette 200 containing the cathodically activated metallic silver and comes to constitute the environment for silver objects 1 to protect (arrow in broken lines). By crossing the cassette 200, the sulfur compounds are retained on the cathodically activated metallic silver. The environment of the silver objects exposed in the display case 300 is thus unraveled. The purification device 100 for implementing the method according to the invention can be adjustable in terms of extraction power and air circulation. So these dimensions are variable, depending on whether you want to purify an enclosure with a volume of 1 to 10 m ³ or an enclosed storage room of several tens of cubic meters.

According to a preferred embodiment of the purification device 100, it is a small module. It essentially comprises a sealed cover 30, an extractor fan 50 and extractable cassettes 200, 210, 220, 230 (FIGS. 5 and 6). The cover 30 is generally an elongated rectangular parallelepiped. The two faces of this parallelepiped, perpendicular to its longitudinal axis, respectively form the inlet face 10 and the outlet face 20 of the purification device 100. The inlet face 10 is provided with a grid of entry 12 of the stale gas. The outlet face 20 is provided with flaps 22. One 14 of the faces perpendicular to the inlet face 10 and to the outlet face 20 is provided with a handle 40. One 16 of the faces perpendicular to the face of inlet 10 and the outlet face 20, adjacent to the face 14 provided with the handle 40, comprises openings suitable for receiving the cassettes 200, 210, 220, 230. Optionally, an electrical power outlet 60 and a switch 70 are arranged on this same face 16. According to a particular, but not limiting, embodiment of the invention, the inlet face 10 and the outlet face 20 have the approximate dimensions 130 mm x 160 mm.

The fan 50 is electrically powered and ensures the extraction and circulation of air. As shown in FIG. 6, this fan 50 is located inside the cover 30, near the outlet face 20. This fan 50 puts all of the extractable cassettes 200, 210, 220, 230 placed in depression upstream of it. The fan 50 diffuses the purified gas towards the flaps 22.

Preferably, to treat a closed showcase of 25 m ³ , we will have a fan 50 whose approximate dimensions are

119 x 119 x 32 mm ³ . It will be powered by a direct current at a voltage of 12 volts (delivered by a battery for an autonomous device or by a transformer connected to 220 volts mains) for a power of 1.2 W (1,550 rpm). Its flow will then be around 95 m ³ / h. It will have a noise level of 30 dB.

According to a variant of the purification device 100, this may include a device with flaps 22 oriented upwards to ensure the circulation of air from the enclosure 300, when for example, the device 100 is arranged on the bottom wall of the enclosure 300 (FIG. 7a).

Conversely, the flaps 22 can be oriented downwards when the purification device 100 is placed at a height (FIG. 7b).

With an adapted orientation of the flaps 22, the preferential recycling of the air near the purification device 100 is minimized.

To improve the circulation of purified air in the enclosure 300, it may be advantageous to equip the purification device 100 with oscillating flaps 22 thanks to a motor.

According to another variant of the purification device 100, it is portable.

According to yet another variant, the purification device 100 may include rollers to facilitate its transport.

The purification device 100 comprises at least one cassette 200 containing cathodically activated metallic silver, with a large specific surface. This money is in the form of wool, felt or silver foam. This money fixes the sulfur compounds. It can be delivered as a consumable in sealed plastic bags.

As shown in FIG. 8, the silver is for example in the form of compacted wool 150. This wool 150 is inserted into a retaining frame 160. The assembly consisting of the frame 160 and the wool 150 is introduced into the 'purification device 100 through sliding rails 170 fixed horizontally therein.

In another variant, the cassette 200 loaded with wool, felt or cathodically activated silver foam, is itself delivered in the form of a consumable, in sealed plastic bags. Such a cassette can also be sent back to the manufacturer, who can judge the advisability of cathodically reactivating its content (under a maintenance contract, for example). The cathodic activation of a cassette 200 is therefore carried out on new cassettes or to be recycled.

The wool, felt or silver foam consists of 99.9% purified silver or an alloy where the concentration of silver is greater than or equal to 90%.

The specific surface of wool, felt or silver foam is approximately 100 cm ² / g.

A cassette 200 for the purification device 100 preferably contains 10 g of silver, which represents a total area of deployed silver of approximately 1000 cm ² . Such a cassette 200 can fix 0.1 g of sulfide.

The kinetics of formation of Ag ₂ S is rapid until approximately 100 nm thick has been formed, then this kinetics becomes slow until saturation. So if you want to maintain maximum cassette efficiency

200, you must keep fast kinetics and change or reactivate cassettes 200 before switching to slow kinetics. In the latter case, the longevity of a cassette 200 having the characteristics described in the preceding four paragraphs, is estimated at approximately one year, in an atmosphere having a level of H ₂ S of the order of 0.5 ppm.

The cathodic activation method, implemented to activate metallic silver intended for use in a purification apparatus 100 comprises the steps consisting in:

- dusting, wool, felt or raw silver foam for reception by blowing then soaking in an aqueous bath of surfactants with ultrasound;

- degrease in acetone;

- rinse in ethanol and water;

- immerse in an electrolytic cell 400 with three electrodes with a neutral electrolyte, this metallic silver or a cassette 200 containing this metallic silver;

- carry out a potential cathode scan with recording of. intensity as a function of potential, of solution electrolytic at room temperature, from a potential of immersion to the potential of reduction of pollutants present on the surface of silver;

- totally reduce, at room temperature, the species by maintaining the potentiostatic potential for reduction;

- rinse, dry quickly, condition under neutral gas and encapsulate under sheets of waterproof thermo-welded plastic, metallic silver or else the cassette 200 containing this metallic silver. The electrochemical circuit for implementing the cathodic activation process for silver is shown in FIG. 9. This circuit preferably includes an electrolytic cell 400 containing an electrolytic bath, a stabilized supply 410, a voltmeter 420, a grid in stainless steel 430, a reference electrode 440. The electrolytic bath is constantly mixed by agitators 450.

The positive terminal of the stabilized power supply 410 is connected to the grid 430, while its negative terminal is connected to the silver wool via, for example, the conductive holding frame 160 of a cassette 200. The potential is measured by the voltmeter 420 between the reference electrode 440 and the silver wool via the retaining frame 160.

During the electrochemical activation steps, the cassette 200 is completely immersed in the electrolytic bath. The nature of the electrolysis bath can be a solution of carbonate (Na ₂ C0 ₃ ), sodium or potassium sulfate (Na ₂ S04, K ₂ S04) of sodium chloride (NaCI), etc., with a concentration between 0.1 and 1 mol / l, in pH ranges between 6 and 8.

The electrolysis currents for cathodically activating a cassette 200 such as the one whose characteristics have been specified above are in a range from 100 mA to 1A. The amount of current (or the duration of electrolysis) is related to the amount of product to be treated.

To demonstrate the effectiveness of cathodic activation for the preferential fixation of silver sulfides, tests were carried out Accelerated sulphidation ^of new money wool. These tests were carried out in a closed, sealed enclosure (glass bell), in which an atmosphere saturated with sulphide prevails. This atmosphere consists of a mixture of 20 cm ³ of water and 4 cm ³ of ammonium sulphide (NH. _T ) ₂ S at 20% by weight.

The results of these tests are shown in Figures 10 and 11. In Figure 10 are shown a sample of cathodically activated wool 162 and a sample of non-activated wool 164, after 4 min of sulfurization. Cathodically activated wool 162 was much more blackened during sulfurization than non-activated wool 164. Cathodically activated wool 162 therefore fixed much more sulfur compounds than non-activated wool, which highlights the effectiveness of cathodically activated silver wool 162 and the importance of the cathodic activation process. In FIG. 11, a sample of cathodically activated wool 162 and a sample of non-activated wool 164 are also shown, after 65 hours of sulfurization, the result is similar to the previous one, which shows that the cathodic activation has a lasting effect.

According to a variant of these cassettes 200, these can be provided with saturation sensors. They can be optoelectronic sensors, color-changing chemical sensors, electrochemical sensors, sensors sensitive to conductivity, pH or depression.

Preferably, a saturation indicator 205, consisting of a transparent sight glass, is arranged on the face of the cassette 200, visible on the face 16 of the purification device 100. This sight sight makes it possible to observe the content of the metallic silver with large specific surface, contained in the cassette 200 to verify its state of saturation. However, such saturation indicators 205 are not always necessary, as is the case, when procedures for systematic change of cassettes 200 are drafted after prior study.

The fixing of the sulfur-containing compounds can be carried out by the circulation of the gas from the enclosure 300 in the purification device 100 introduced into it. this. This purification device 100 does not put the air in the enclosure 300 (display case for example) in depression and therefore does not cause the perpetual introduction of pollutants.

It is also conceivable to have the purification device 100 at the entrance to an enclosure that is not very tight, so as to supply gas with gas to the latter, while causing a slight overpressure preventing the entry of pollution. .

The purification device 100 can be designed in a modular fashion:

- in volume processing capacity; in this case, the dimensions of the treatment cassettes 200 and of the fan 50 are larger;

- in qualitative processing capacity; in this case other specific cassettes are installed using a “rack” type system which the purification device 100 can have.

It is then possible, in addition to the cassettes 200 of metallic silver, to have cassettes 210 for filtering the dust or cassettes 220, 230 containing various filters capable of specifically fixing other polluting species or even humidity. These other cassettes 210, 220, 230 may contain other filtering substances than cathodically activated metallic silver. These substances are for example activated carbon, zeolites, carbonate or alkali hydroxide tablets (with retention tank for deliquescent products), zinc oxide, molecular sieves, etc.

To be used, the cassettes 200 are extracted from their waterproof packaging and slid into slide rack supports. The purification device 100 is then powered up and an optional hour meter is triggered.

Regular monitoring ensures the proper functioning of the purification device 100 and identifies the saturation of the cassettes to change them if necessary. After a certain operating time (for example after 6 months), an in-depth appraisal can be carried out on the content of the 200 cassettes. This expertise determines the exact nature of the pollutants, for example by scanning electron microscopy, coupled with micro fluorescent X and X-ray diffraction. Specific solutions, with special cassettes 200, 210, 220, 230, can then be proposed to improve the purification of the atmosphere of enclosure 300.

Additional control and measurement devices (hygrometry and temperature telemetry) can also be installed to establish this in-depth diagnosis, in order to optimize the operation of the purification device 100 according to the invention, as a means for fixing sulfur compounds and more generally to prevent tarnishing and corrosion of silver metallic objects.

A purification device 100 is particularly suitable for removing polluting gaseous molecules from the gases supplying the atmosphere of display cases.

Claims

1. A method of preserving and preserving silver objects (1) comprising the step of conditioning the objects (1) in an atmosphere composed of a desulfurized gas characterized in that the gas is desulfurized in contact with silver metallic cathodically activated. 2. Method according to claim 1, characterized in that the metallic silver is in a form having a large specific surface. 3. Method according to any one of the preceding claims, characterized in that the silver objects (1) to be preserved and preserved are placed in an enclosure (300). 4. Method according to claim 3, characterized in that the atmosphere of the enclosure (300) is maintained in overpressure relative to the outside of this enclosure (300). 5. Method according to any one of claims 3 and 4, characterized in that the gas from the enclosure (300) is dewatered before or at its entry into the enclosure (300). 6. Method according to claim 3 to 6, characterized in that the gas from the enclosure (300) undergoes recycling. 7. Method according to any one of claims 3 to 6 characterized in that the enclosure (300) is a showcase. 8. Purification apparatus (100) for implementing the method according to any one of the preceding claims, by which an atmosphere intended to be brought into contact with silver objects (1) is characterized, characterized in that it includes cathodically activated metallic silver. 9. A purification device (100) according to claim 8, characterized in that it comprises at least one cassette (200) containing metallic silver activated cathodically. 10. A purification device (100) according to any one of claims 8 and 9, characterized in that the metallic silver activated cathodically is in the form of wool (150). 11 purification device (100) according to any one of claims 8 to 10, characterized in that it comprises cassettes (210, 220, 230) containing other filtering substances than I metallic silver cathodically active 12 device purification (100) according to any one of claims 8 to 11, characterized in that it is provided with an indicator of saturation (205) of metallic silver 13 purification device (100) according to claim 12, characterized in that the saturation indicator (205) is a transparent sight allowing to observe the color of the metallic silver contained in the cassette (200) 14 purification device (100) according to any one of claims 8 to 13, characterized in that it is portable 15 purification device (100) according to any one of claims 8 to 14, characterized in that it is equipped with a device with oscillating flaps (22). 16 A cathodic activation method, characterized in that it is applied to metallic silver intended for desulfurizing, by the method according to one of claims 1 to 7, the atmosphere for conditioning silver objects (1) 17. Metallic silver for implementing the method according to any one of claims 1 to 7, characterized in that it is activated cathodically 18. Metallic silver according to claim 17, characterized in that it has a specific surface of the order of 100 cm / g 19 Metallic silver according to any one of claims 17 and 18, characterized in that its silver concentration is greater than 90% 20 metallic silver according to any one of claims 17 to 19, characterized in that it is in the form of wool 150 21 metallic silver according to any one of claims 17 to 20, characterized in that it is inserted in a cassette 200 22. Cassette (200) for implementing the method according to one of claims 1 to 7, characterized in that it comprises metallic silver activated cathodically.