It is important to acknowledge one thing before we start our discussion of the different protective coatings available for use with silver alloys – all silver alloys eventually tarnish. The only way to prevent this is to place a physical barrier between the silver alloy and the atmosphere/sweat or chemicals which cause the tarnish reaction to take place.
Anti-tarnish coatings can be broadly split into five families:
a) Metal Coatings – i.e. Rhodium. This is a platinum group metal which is plated over the top of silver jewellery to give a hard, white, un-reactive surface layer. It is typically only a thin coating of 0.2-0.3 microns thick and it will wear off as a jewellery item is worn. The underlying silver alloy is usually a different colour to the rhodium so the worn areas are noticeable; more so as the underlying silver starts to react with the atmosphere.
b) Passivating Solutions – i.e. Self Assembled Monolayer based on Thiols. The description may seem scary but these are simply the ‘dip’ solutions which offer protection against tarnishing. They contain long chain molecules (thiols) which attach themselves to the surface of the silver alloy and create a chemical surface layer which repels water. For a tarnish reaction to take place there needs to be moisture present on the surface of the silver so the presence of this thiol coating prevents the reaction starting. These coatings are between 0.05 - 0.15 microns thick and although they offer good protection on display items they can be easily rubbed off as a piece is worn.
c) E-coatings – Acrylic or Polyurethane Electrophoretic Coatings. These are specialist coatings which are applied by electrodeposition and they form a thin clear lacquer on the surface of the piece after they have been cured. As with any plating process careful control of the coating solutions and filtration of the rinse waters is necessary to form a coating which is non-porous. On decorative products the coating thickness is usually between 2-5 microns and although these coatings have reasonable wear characteristics even at the 2-5 microns thickness they can be detected visually and by touch.
d) Lacquer Coatings - Cellulose Nitrate or Acrylic Coatings. These coatings require a high degree of skill to apply to get an even coating, particularly in the hidden or hard to reach areas. They are typically between 5 -50 microns thick and offer good resistance to tarnish but can degrade and yellow when exposed to ultraviolet light (in display cases the tungsten light bulbs typically used also emit ultraviolet light). They are easily detectable visually and have a ‘plastic’ feel when touched.
e) Oxide coatings – Atomic Layer Deposition. These coating are mixtures of metal oxides that are applied by either a physical vapour deposition or a chemical vapour deposition process. It is an expensive process to carry out as the machines used to apply these coatings need to be able to produce a very low vacuum and require regular maintenance. The coatings they produce are about 0.8-1 microns thick and have good tarnish resistance; however they are very easily removed when the coated piece is worn.
So how do the Argentium silver alloys compare to these different barrier coating protection techniques?
Argentium silvers also work by producing a protective oxide layer at their surface but in this case the layer is not applied but is self generated as the germanium content of the alloy oxidises naturally in air. The protective germanium oxide is slowly worn away as the piece is worn, but this only exposes fresh germanium at the surface of the piece. This germanium then oxidises in air to renew the protective oxide layer. This self-generation of the protective surface layer is a unique characteristic of Argentium silver alloys and while it does not offer complete protection from tarnish it does remarkably slow the rate at which Argentium silver alloys tarnish compared to other silver alloys that are commercially available.
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