Precious Metals, Not all Golds are the same.
Updated: Feb 7, 2020
"Can't you just melt it down and....." "Is white gold an Isotope?" "What's the difference between Red and Rose gold?" These are questions I've been asked many times, some are easy answers others not so much.
So where do I start on this? The first step is to explain that gold used in jewellery is usually an alloy;
"Alloy: a metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion."
"Pure" gold is theoretically 24ct and 100% pure gold, we will use this as it keeps things simple but in reality "pure" gold is usually 99.999% pure (this gets very technical, so we will leave it there). Pure, fine or 24ct gold is a lovely orangy yellow colour, but very soft.
So we have 24ct pure gold, but we know 9ct gold, 18ct gold, 22ct gold so what are these? I've also included some alloys that were legal standards in the UK.
9ct gold: 9 parts gold to 15 parts other metals or 37.5% gold, 62.5% other metals.
12ct gold: 12 parts gold to 12 parts other metals or 50% gold, 50% other metals. (disc.1932*)
14ct gold: 14 parts gold to 10 parts other metals or 58.5% gold, 41.5% other metals.
15ct gold: 15 parts gold to 9 parts other metals or 62.5% gold, 37.5% other metals.(disc.1932*)
18ct gold: 18 parts gold to 6 parts other metals or 75% gold, 25% other metals.
22ct gold: 22 parts gold to 2 parts other metals or 91.6% gold, 8.4% other metals.
* Many sources state this was discontinued in 1931, but pieces were last hallmarked in 1931 and the standard no longer assayed in 1932. If you have an interest in hallmarks, I have a Pinterest board dedicated to interesting Hallmarks I find which you can view here.
OK, so we know what the gold content is, but what about the other stuff? The other metals come down to what you want the metal to look like and what you want to use it for. The main differences are to do with colour which most of us are familiar with;
Yellow: A mixture of Silver and Copper is usual, often with some Zinc. This is basically the same for most carats.
Rose/Red gold: Copper is added, usually with a little silver. The higher the carat, the more subtle the colour. 9ct Rose/Red will have a much more red colour than 18ct Rose/Red. The colour variation can be made more subtle by adding more silver and less copper (Rose), but most commercial suppliers don't bother with these subtle differences. I have dabbled with alloying my own metal and it's a bit nerve wracking, so I try not to.
White gold: Silver in the lower carats as there is enough to hide the yellow sufficiently, but not completely (think adding 62.5% white paint to 37.5% yellow paint = Pale yellow). In higher carats such as 18ct gold, Palladium is often used in the better alloys. Palladium has a high pigment if you like, so you can often get a whiter colour than with a 9ct white gold alloy. Palladium is much more expensive than silver (and gold gram for gram) so it isn't used on lower alloys. Much of the white gold jewellery produced is plated over the top with Rhodium to give a whiter appearance.
Those are the basics, but there are also these;
Green Gold: Usually an 18ct alloy containing 75% gold with silver and copper in varying degrees and Cadmium. Cadmium is no longer used as it is highly toxic. Care must always be taken when working on older pieces of green gold.
Blue Gold: Gallium or Indium in the mix. Not usually used in jewellery as it is very limited carat wise to around 11ct and not really workable.
Purple Gold: Aluminium content of around 21% to 79% gold. VERY brittle and doesn't solder well. You can cast things from it, but don't expect more from it.
Blue and Purple are very rarely found in jewellery, but green is having a bit of a comeback.
This is an 18ct gold tobacco box Made from 18ct Yellow (main) and 18ct Green gold for the decoration. In the flesh it looks more rose and off yellow.
So that's the difference between Red and Rose covered and also the fact that white gold is not an isotope. So what about the "Can't you just melt it down?" question. I get asked this a lot when it comes to re-making jewellery, with it usually relating to "can't you just melt it down and cast what I want" being the most prominent. There is a straight forward answer to this, no, but the reasons are these;
1. When you cast a piece, you often need to use over 5x the amount of metal you need for the channels into a mould and some other technical bits.
2. The next question is "Can't you just add a bit to it?". I'm not keen on this as if metal for recycling has a small quantity of impurities, this mixes in to my metal also and could render it a lower carat than it was, meaning I've wasted a lot of metal or I have to refine it and start again.
3. If there is enough metal, I would need to remove any solder joins, repairs and all traces of dirt before melting. Any one of these things can cause issues in the final piece. Chains are a straight forward no as there are too many joins.
4. Casting isn't always the answer.
So how about melting it down and hammering and rolling to dimensions? Yes, this is sometimes possible, but there can be issues again. It's all down to the recipes used for the alloys. Think of it as a combination of flour and fat, make breadcrumbs and you have a crumble topping, add water and you have pastry, add and egg and water and you have a rich pastry, add yeast and sugar you have bread. If you want gold to cast well, add silicon, but it doesn't stretch well. If you want to make wire that you can stretch, add a little Indium, this alloy I find creates a rough casting. Unless you know the recipe you are dealing with, it can be tricky.
All of these different recipes can be difficult to deal with as they have changed over time for different reasons, for example gold wire made in the 1800's didn't contain Indium as it wasn't discovered until 1863 and not alloyed with gold until much later (1950's I think, but not positive). Customers often prefer the look of the "Old Rose Gold" rather than new, when this "Old Rose" was first made, it was simply known as gold and a purity. The recipe was a little heavy on copper.
I'm hoping to add pictures of the different colours side by side at some point, but please bear with me on this.
I hope this answers a few questions.