The aim of this document is to offer guidance as to the best practice to be followed when using pre-fabricated sheet and wire products in Argentium® silver.
Typically the Argentium silver sheet will be supplied in a fully annealed (soft) condition and at the thickness you require. If it is necessary to further reduce the thickness of the sheet, this can be done using conventional rolling machines. The Argentium sheet can be reduced using a series of rolling passes by up to 70% of its total thickness before an annealing operation is required.
Wire Drawing Operations
Typically the Argentium silver wire will be supplied in a fully annealed (soft) condition and at the diameter or cross-section that you require. If it is necessary to further reduce the size of the wire this can be done using conventional wire drawing equipment. The wire can be reduced by a series of drawing operations, no single stage of which should exceed a 25% reduction in cross-sectional area, until a total maximum reduction of 70% of the cross-sectional area has been achieved. The wire will then require annealing if further reductions are required.
When carrying out a conventional annealing operation in a furnace using a protective atmosphere it is very important that the furnace gas, although protective, does not deplete the surface layer of germanium oxide, as this will reduce the tarnish resistance of the alloy and its resistance to ‘fire stain’. It is the ability of the surface layer of germanium to form a germanium oxide which then acts as a barrier preventing any further penetration of the oxide layer or build up of tarnishing products.
For this reason a furnace atmospheres that are inert (i.e. nitrogen or argon) or which contain up to 10% hydrogen in nitrogen are to be preferred. The annealing temperature should be within the range 620 – 650ºC. It is important not to exceed a maximum temperature of 650ºC. The annealing time for this temperature range is 30 to 45 minutes at temperature.
Alternatively the sheet, strip or wire can be torch annealed in the traditional silversmithing manner; however care must be taken not to overheat the Argentium silver as when at its annealing temperature it has an orange colour rather than the red colour associated with traditional sterling silver alloys. Once the Argentium silver has reached the correct temperature, allow it to cool slightly, the orange colour should disappear and then it can be quenched into water.
Argentium silvers have the capability to be hardened without the need of a high temperature heat treatment. The cleaned and dry items to be hardened should be placed in an oven with an air atmosphere. They should be heated to 300ºC and held at temperature for 2 hours. This heat treatment should increase the hardness of an Argentium silver item by up to 20HV.
Polishing and Cleaning Operations
The polishing and cleaning operations are of critical importance in producing a quality product. It is important to emphasis that Argentium silver alloys, because of their high degree of tarnish resistance, will in most cases be sold without having undergone a plating operation (either silver or rhodium). Where it is not the intention to subsequently plate a more rigorous polishing operation will be required to achieve the level of finish suitable for release to the customer.
The first stage of the polishing process is the surface stripping and burnishing operation which creates a gloss finish on areas of the casting which are hard to reach by traditional polishing methods. This involves the use of magnetic pin type media in accordance with the media manufacturer’s recommendations. All pieces should be removed from the cast tree, have the remains of the sprue which connected them to the tree hand-filed flush with the main body of the piece and then receive a minimum of 20 minutes processing in this media before then proceeding to the grinding or cutting polishing operation. Ensure that the water and soap mixture used in this operation has a pH of between 6 to 9.
The second stage of the polishing process is the cutting operation which dresses the surface of the piece and smoothes the surface, while removing the cast skin present on the surface of all investment castings. This can either be a mechanical polishing operation involving the use of ceramic or plastic media and utilising water as a vehicle to enable the optimal flow of the polishing media over the pieces in accordance with the media manufacturer’s recommendations or it can be the more traditional hand polishing process involving the used of abrasive wheels and emery paper.
The final stage of the polishing process is the finishing operation which gives the piece its deep lustrous surface finish. Where this final polishing stage is a mechanical operation it should be a ‘dry’ process involving the use of a media such as crushed walnut shells in accordance with the media manufacturer’s recommendations. Where manual polishing involving the use of polishing compounds and polishing mops is very operator dependent and strict quality control is required.
Between each polishing stage the pieces should be cleaned using an ultrasonic tank using an alkaline, aqueous cleaner at a temperature of between 40-60ºC and a pH in the range 7 to 9. The pieces should be immersed for 2-3 minutes to remove oils, greases and loose particles before rinsing in tap water, drying and progressing to the next polishing stage.
It is important that deionised water is not used to rinse the pieces at any stage as its reactivity can affect the final tarnish resistance of the Argentium alloy. In addition electrolytic cleaning systems should not be used to clean this alloy as they can strip the germanium from the surface of the Argentium.
Argentium silvers work by developing a protective germanium oxide layer on their surface. To assist the formation of this layer on to promote this reaction in more aggressive environments this passivation treatment has been developed. After the pieces have received their final polish and cleaning operations they are heated in an oven with an air atmosphere at 100oC for 1 hour. No further polishing should be required after this process provided the oven is clean.