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”!
Tuesday, 19 April 2011
Wednesday, 13 April 2011
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.
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.
Friday, 8 April 2011
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.
Monday, 28 March 2011
Definitions - Solidus and Liquidus
I was asked yesterday what solidus and liquidus meant. This made me think of all the terms we use in silversmithing or metallurgy which we assume people understand but may mean different things in different contexts. So I thought that it might be of interest if I defined and tried to explain some technical terms.
If we start with the basics; pure metals have a single melting point. This is the point at which they change from a solid to a liquid. For silver this is 960.8C (1761F), copper 1083.4C (1982F) and germanium 937C (1719F); so all the main constituents in Argentium silver alloys have very similar melting points.Similarly the boiling point of a pure metal is the temperature at which it changes from a liquid to a gas. For silver this is 2163C (3925F), copper 2560C (4640F) and germanium 2830C (5126F).
If we start with the basics; pure metals have a single melting point. This is the point at which they change from a solid to a liquid. For silver this is 960.8C (1761F), copper 1083.4C (1982F) and germanium 937C (1719F); so all the main constituents in Argentium silver alloys have very similar melting points.Similarly the boiling point of a pure metal is the temperature at which it changes from a liquid to a gas. For silver this is 2163C (3925F), copper 2560C (4640F) and germanium 2830C (5126F).
One metal which is commonly used in silver alloys which has a very low boiling point is zinc. Its melting point is 419.5C (787F) but its boiling point is only 911C (1672F). If you are making an alloy of silver and copper which also contains zinc you can see that at a temperature when you have the silver and copper molten you are already above the boiling point of zinc. This is why zinc is such a difficult constituent to control in silver alloys and why we refer to zinc loss on melting, the zinc does literally boil away!
If we now think about what happens when we add copper to silver we will be able to understand what we mean by solidus and liquidus. As we have said, pure silver melts at 960.8C (1761F) and you would think that by adding copper to it which has a higher melting point of 1083.4C (1982F) the overall melting point would increase, but that is not the case it actually falls.
To explain this we have to think of what is happening to the silver atoms when we add copper to them. The atoms of silver and copper have different sizes (silver is larger than copper) and in a crystal of pure silver the bonds between each atom are of equal length. As you add copper to silver the smaller copper atom replaces a silver atom in the crystal structure and as a consequence increases the length of the bonds between itself and the silver atoms because of its smaller size. This longer copper-silver bond is not as strong as a silver-silver bond and so it is more easily broken when we apply heat to the solid silver-copper alloy when we want to melt it.
This is why we have a melting range with alloys, the weaker bonds are broken first at the point the alloy starts to melt and all the bonds are broken when it is completely molten. So the point at which melting starts (or if you think of the molten metal cooling, the point at which the molten metal becomes completely solid) is called the solidus. The point at which the metal is fully molten (or again to think of the metal cooling, the point at which the completely molten metal stops being completely liquid) is the liquidus. So these two terms, the solidus and liquidus define the melting range of the alloy.
The best definition though is that of the temperature range between the solidus and liquidus when the alloy is not completely molten; that is called either the ‘mushy’ or ‘pasty’ range. You have to smile when you can legitimately call the state of a metal ‘mushy’!
Wednesday, 23 March 2011
Coming soon... Argentium Guild Forum
Following requests from some of our Members, we will be setting up an exclusive 'Argentium Guild Forum' (members only). Please watch this space for further details.
Tuesday, 22 March 2011
Different Silver Alloys
One thing I am always asked is, “how is Argentium silver different from other silver alloys”? With there being so many different alloys now available I thought I would try to categorise and summarise each alloy type’s particular quirks.
Traditional sterling silver – This is the simple 92,5% silver, 7.5% copper combination. A good basic alloy with good hardness. For silversmiths the problems of firescale and tarnish are well documented.
Spinning silvers – Historically these alloys had part of the copper content of the traditional sterling silver composition replaced with cadmium (usually about 2%). This gave an alloy which was about 10-15HV lower in hardness than traditional sterling silver which had excellent deep drawing and stamping characteristics. Cadmium containing silvers are now prohibited by worldwide legislation and attempts to create similar alloys by simply replacing the cadmium with tin or zinc failed because the oxides of the tin and zinc, formed when torch annealing, were very hard to remove. Some manufacturers now use Britannia silver (95.8% silver, 4.2% copper) as a spinning silver because of its lower hardness.
Deox silver alloys – These are alloys developed primarily for casting applications and their properties were recently reviewed in an excellent paper presented at the 2010 Santa Fe Symposium by Joerg Fischer Buhner (click here to download pdf). Some of the copper content of these alloys is replaced with zinc and/or silicon with the aim of giving bright, firestain free castings. While the silicon and zinc additions do limit the formation of firestain it is not always a complete success. The higher silicon content alloys can be more difficult to cast consistently and cannot be fabricated easily limiting their use; whereas zinc is well documented to fume at typical investment casting temperatures. These alloys are an improvement on the traditional silver composition for investment casters but have no significant benefits for the practicing silversmith working with sheet and wire.
Platinum group metal additions – these are additions of either gold, palladium or platinum which replace some of the copper content of the traditional sterling silver composition. Aside from the considerable cost implication of replacing copper with a precious metal it has yet to be demonstrated that an alloy which contains an addition of one of these elements has a significantly improved tarnish resistance compared to the deox alloys detailed earlier. Gold additions make the alloy more yellow; platinum creates difficulties on melting and increases the potential for hard spots to form and silver alloys containing palladium have shown sensitivity to ultraviolet light when tested under ‘showroom’ conditions.
Alloys containing germanium - the mechanism by which the germanium content of Argentium silver alloys protects against tarnish and firestain by forming a transparent germanium oxide is something that I will discuss in another blog. Our work has shown that the composition of the Argentium silver alloys with about a 1% germanium addition optimises the mechanical working and casting characteristics of the Argentium silver alloys while giving exceptional tarnish resistance and firestain resistance. Other alloys may contain germanium, but these are limited to contents below 0.5% germanium because of our patent protection and they have to rely on supplemental additions of zinc and tin to try to match the performance of Argentium silver alloys.
Traditional sterling silver – This is the simple 92,5% silver, 7.5% copper combination. A good basic alloy with good hardness. For silversmiths the problems of firescale and tarnish are well documented.
Spinning silvers – Historically these alloys had part of the copper content of the traditional sterling silver composition replaced with cadmium (usually about 2%). This gave an alloy which was about 10-15HV lower in hardness than traditional sterling silver which had excellent deep drawing and stamping characteristics. Cadmium containing silvers are now prohibited by worldwide legislation and attempts to create similar alloys by simply replacing the cadmium with tin or zinc failed because the oxides of the tin and zinc, formed when torch annealing, were very hard to remove. Some manufacturers now use Britannia silver (95.8% silver, 4.2% copper) as a spinning silver because of its lower hardness.
Deox silver alloys – These are alloys developed primarily for casting applications and their properties were recently reviewed in an excellent paper presented at the 2010 Santa Fe Symposium by Joerg Fischer Buhner (click here to download pdf). Some of the copper content of these alloys is replaced with zinc and/or silicon with the aim of giving bright, firestain free castings. While the silicon and zinc additions do limit the formation of firestain it is not always a complete success. The higher silicon content alloys can be more difficult to cast consistently and cannot be fabricated easily limiting their use; whereas zinc is well documented to fume at typical investment casting temperatures. These alloys are an improvement on the traditional silver composition for investment casters but have no significant benefits for the practicing silversmith working with sheet and wire.
Platinum group metal additions – these are additions of either gold, palladium or platinum which replace some of the copper content of the traditional sterling silver composition. Aside from the considerable cost implication of replacing copper with a precious metal it has yet to be demonstrated that an alloy which contains an addition of one of these elements has a significantly improved tarnish resistance compared to the deox alloys detailed earlier. Gold additions make the alloy more yellow; platinum creates difficulties on melting and increases the potential for hard spots to form and silver alloys containing palladium have shown sensitivity to ultraviolet light when tested under ‘showroom’ conditions.
Alloys containing germanium - the mechanism by which the germanium content of Argentium silver alloys protects against tarnish and firestain by forming a transparent germanium oxide is something that I will discuss in another blog. Our work has shown that the composition of the Argentium silver alloys with about a 1% germanium addition optimises the mechanical working and casting characteristics of the Argentium silver alloys while giving exceptional tarnish resistance and firestain resistance. Other alloys may contain germanium, but these are limited to contents below 0.5% germanium because of our patent protection and they have to rely on supplemental additions of zinc and tin to try to match the performance of Argentium silver alloys.
Friday, 18 March 2011
New Members
We would like to welcome all of the new Members of the Guild. We have had a wonderful response to the website going live and look forward to the site evolving with information and showcasing the wonderful range of work being made in Argentium.
Please feel free to send us your working tips and details of any Argentium related events that you would like advertised. If you have a short article that you would like to be included in the 'Monthly Feature' on this blog – please email info@argentiumguild.com.
Please do not worry if you are unable to send images immediately for the Gallery - we can upload or change your image at any time.
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