Oxidised Finish on Silver Alloys

When silversmiths talk of an oxidised finish on silver they typically mean the deep blue-black sulphide layer on the surface of a silver alloy created by treating it with a chemical compound such as Liver of Sulphur. This is different to a true silver oxide; that is oxygen bonded with silver, which is a difficult chemical compound to form.

Liver of Sulphur is a mixture of potassium polysulphide, potassium bisulphide, potassium sulphide and potassium thiosulphate. It is produced by reacting potassium carbonate with sulphur and immersing a silver item in it allows the formation of a thick sulphide film on the surface of silver items, typically a deep blue-black colour although other finishes are possible by removing the pieces after shorter times or by mixing the chemical with substances other than water to obtain a solution with a different pH (acid content) which would give a different rate of reaction.

Liver of Sulphur is available in different forms; the solid chemical (lump or flake) is affected by moisture and light and needs to be stored in dark dry conditions. The ready made solution has a limited shelf life as it is affected by bright light and needs to be stored in the dark.

So, you may ask yourself, how can a sulphide coating on the surface of a silver alloy be called an oxidised finish? Well this goes back to some basic chemistry and the definition of what an oxidative reaction is and what a reducing reaction is.

In chemistry oxidation is a word which originally meant combination with oxygen gas. However so many other chemical reactions were seen to resemble reactions with oxygen that the definition was broadened to refer to any reaction in which a substance or species loses electrons. This is remembered by use of the mnemonic:

OILRIG – Oxidation is Loss (of electrons), Reduction is Gain (of electrons).

If we think of the tarnishing reaction where silver combines with a ‘free’ sulphur (from either hydrogen sulphide (H2S) and sulphur dioxide (SO2) in the atmosphere) to form a silver sulphide we are looking at a reaction where each silver atom gives up one electron to bond with the ‘free’ sulphur atom.

2Ag + [S] → Ag2S


As the silver atoms are losing electrons they are undergoing an oxidative reaction in forming the silver sulphide at the surface. This is why use of Liver of Sulphur to get a black surface finish can correctly be called an ‘Oxidised Finish’ on silver.

To quote some meerkats, “Simples”!

Definitions - Soldering and Brazing

Nothing has the potential to cause more confusion than the use of the terms soldering and brazing when talking with a silversmith. The aim of this post is to define what the difference is between a brazing alloy and a solder according to International Standards, explain how a silversmith uses the term solder in a different way to the standard definition and what the basic principles are of how to get a good brazed or soldered joint.

Both soldering and brazing use a filler metal which melts below the melting point of the parent metals being joined. This filler metal ‘wets’ onto both parent metals and is drawn into the joint gap by capillary attraction where it solidifies to give a strong, ductile bond. Different filler metal alloys melt at different temperatures, meaning it is possible to ‘step’ joints that are close together starting with a higher melting point filler metals and then progressively using lower melting point filler metals. In addition some filler metals flow better than others. The more free-flowing filler metals are better for very tight narrow joint gaps, the more stodgy filler metals are better at filling wider joint gaps.

Brazing operations are defined as taking place at 450C (840F) or above and soldering takes place below 450C (otherwise the processes are the same).

Silversmiths however frequently refer to the higher temperature brazing process as soldering and this can be a cause of some confusion. Therefore in this post when I am referring to the higher temperature joining process I will call it soldering (brazing) to differentiate it from the low temperature soldering process.

Although the staring point for soldering (brazing) alloys is 450C the silver containing solder (brazing) alloy family generally have a much higher melting ranges, starting at about 640C (1184F). The correct solder (brazing) alloy for silversmithing use depends greatly on the hallmarking requirements of each different territory. For example the United Kingdom Hallmarking Act specifies that any soldering (brazing) alloy used on a silver article must have a minimum silver content of 65%. However the different hallmarking criteria in other European countries allow the use of lower silver content soldering (brazing) alloys, typically containing 55% silver which melt at lower temperatures and are generally much easier to use than those with higher silver contents.

To get a good joint it is important that the soldering (brazing) alloy wets and flows well on the parent metals being joined. This is achieved by having very clean surfaces on the parent metals being joined, correct joint gaps and developing a heat pattern to promote the flow of the soldering (brazing) alloy.

To obtain a clean surface a flux is generally used. The flux reacts on the surface of the parent metals to give a chemically clean surface, free from oxides, to enable a good sound joint to form. When choosing a flux it is important to look at the temperature range over which it works. You want it to start working at least 50C (122F) before your soldering (brazing) alloy starts to flow and to remain active at least 50C above the highest temperature that you will reach when heating the parent metals to carry out the joining process.

The correct joint gap is also important to ensure that you get good fill along the length of the capillary joint length with no voids present. Remember these are joint gaps at the joining temperature, as heated metals expand your joint gaps will alter from those at room temperature. The joint gap at temperature for the more free-flowing soldering (brazing) alloys should be between 0.05 - 0.15mm (0.002 - 0.006'') and for the stodgier soldering (brazing) alloys this should be between 0.05 - 0.2mm (0.002 - 0.008'').

The part of the soldering (brazing) process which requires the most skill is the control of the heat pattern in the pieces being joined to encourage the flow of the soldering (brazing) alloy into the joint gap. The simplest way to visualise this is that the molten soldering (brazing) alloy is drawn towards the heat source. You should therefore aim to heat the entire joint area to close to the joining temperature for your soldering (brazing) alloy and then when you apply your filler metal allow the conduction of the heat to draw the alloy into the joint and encourage the filler metal to flow towards the heat source.

The soldering (brazing) process is one where the best way to get good joints is to practice as often as you can using small scrap pieces of metal so that you can control the heat pattern that you are creating with your torch.

Coatings Used to Protect Silver Alloys

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.