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Some insight into karat gold metallurgy

#1

Hello everyone, I thought I’d post this here even though it’s not really a question. It was however extremely helpful to me as someone who makes all my own alloys and will be helpful to others who do the same (such as Austin The Jewellery guy)

I found a fascinating research paper on karat metallurgy. It outlined how gold alloys May be classified by two parameters, gold purity and the ratio of silver in the master alloy. For 10-14 karat golds it classified them into three types, type one alloys were soft easily worked and non age harden-able, type two were tougher but still soft and age harden-able and type three were hard and not very ductile. Type one alloys had from 0-10% silver or 90-100% silver. Type two had 10-25% silver or 75-90% silver and type three was in the middle. This explains the use of zinc which has a softening effect by allowing higher copper content and less silver pushing yellow alloys into the type one or two range rather than three which is especially important at 10k. Not to mention that in low karats zinc additions improve colour.

For 18K the higher gold content means alloys that would fall into the type three range work like type two 14K alloys. At 22K all values are easily worked.

#2

If your piece is going to have complicated soldering, or many solderings, or very small parts, or soldering gold and silver parts together, I strongly recommend not using zinc in your alloy.

Janet in Jerusalem

#3

I was under the impression almost all low-mid karat gold that are commercially produced have zinc in them. How would you achieve alloys that aren’t overly hard and are decently yellow without some zinc in the lower karats, especially 10K?

Do you have any recipes you find work for you in the 10-14 karat?

#4

I’m not familiar with commercially-produced alloys. I work in 14K and 18K and have always made up the standard alloys for my yellow gold: equal parts of copper and silver for the non-gold part. I don’t know of any goldsmiths who put zinc in their gold alloys. Zinc may be used in gold alloys used for casting, but it’s not traditionally used for fabrication alloys.

Janet in Jerusalem

#5

Ah, I see. If you remember a while ago you mentioned that the difference between .960 silver was like the difference between 14K and 18K gold. I wondered what you meant by this because most hardness values for 18K and 14K are about the same. Turns these were the softer zinc containing 14K alloys! Anyway thanks for sharing your thoughts.

#6

Hello - I pulled this and hope it helps explains some things. Archives are great!

#7

Actually, I have come across this very article and bookmarked it! :slight_smile: good to review before actually buying the Gold.

#8

Working properties are not necessarily in accord with karat. It all depends on the particular alloy. When people generalize they’re usually referring to an alloy where the non gold part is equal parts silver and copper. At least this was traditionally the case in the past. In the standard 14 and 18 karat alloys (equal parts silver and copper), the 14 karat is considerably springier and the 18 karat is considerably softer-- very similar to the difference between 925 silver and 960 silver. If you’re not making a piece which requires maximum ductility and malleability, then the difference would not be noticeable. It’s important for example when you’re working with very fine wires (0.30mm or less).

Janet in Jerusalem

#9

What solder do you use for 14K gold or 18K gold composed of equal parts silver and copper for the non gold part? Do you make your own solder?

#10

Yo can add brass ( zinc with copper) let me get back to you with some formulas I’m camping at the moment

#11

Yes I would love some formulas, thank you! :slight_smile:

#12

I buy my gold solder. Have purchased from different companies over the years. I always keep a careful record of both melting points and flow points of each solder. Of course it’s important to know the melting points and flow points of your sheet and wire as well.

Janet in Jerusalem

#13

What would be melting and flow Point of a 14K alloy composed of 58.5% Au, 20.5% Ag, 21% Cu? (You will tell me to use a phase diagram :slight_smile:)

I have attempted to read phase diagrams before but someone will have to teach me how to use one. It’s just to hard to teach myself. Alloying my own metals from scratch is already quite complex. If you know the melt and flow point of said 14K I appreciate knowing it.

I like riogrande’s solder, their 14K hard flows at 804 Celsius.

#14

I have elsewhere given you links to literally hundreds of sites which explain to jewelers how to read gold and silver alloy phase diagrams. If for some reason you are unable to learn from any of these articles and videos, created precisely for people such as yourself asking the same questions you are asking, you can always take a course locally. Our goal should be to learn how to answer our own questions. You should probably try to get out of the “someone will have to teach me” mode and into a “I must go and learn” mode :slight_smile: . Words simply cannot describe how happy you will be when you can read phase diagrams, given your interest in alloying and relevant issues. You will be able to see in an instant the melting point of any and all combinations of Au/Ag/Cu! And then you won’t have to keep asking people to look these things up for you…:-)…

#15

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Using those phase diagrams at the top (above the ternary plot) I determined the Solidus point of a 14K yellow with 58.5% Au, 20.5% Ag, 21% Cu to be around 800 degrees and it’s liquidus around 840 degrees.

The 10K I was using a few months ago with 41.7% Au, 35.9% Cu and 22.4% Ag had a Solidus around 760 and liquidus around 850.

This would mean with rio’s solders I would need to use 10K easy (Solidus 682, liquidus 746) and 14K medium (Solidus 707, liquidus 754)

#16

Here is a link to the article if anyone is interested.

#17

Hi Janet,
I can’t speak for individual goldsmiths like yourself who are adept at making their own alloys.
Most US manufacturers of alloys used in jewelry industry put zinc in the formulas. Without zinc addition, copper could oxidize and copper oxide streaks are a headache to get rid of in sheets during manufacturing. Wires made with zinc-free alloys, if they contain brittle copper oxides, tend to break during drawing. Most importantly, combinations of copper, silver and zinc allows the manufacturers to get that particular color (customer asks for) especially in yellow golds.
Even in Europe, they use zinc but tend to restrict it to about 2% in 14K casting grains.

#18

Perhaps I should point out that all goldsmiths used to make their own alloys-- it was a basic gold smithing skill…:-)…

Personally, I wouldn’t buy sheet or wire from a manufacturer who doesn’t know how to prevent copper oxides in his melts… And it’s not clear why you would think zinc prevents copper from oxidizing… In any case, the standard non-zinc alloys (AuAgCU) have worked well for goldsmith’s for centuries…:-)… Guess I’m just old school…;-)…

I do a lot of gold and sterling together in the same piece with many very small, delicate parts. Zinc in the alloys would make this type of soldering much more difficult, and in some instances, impossible. If you’re doing standard stuff with minimal soldering, the zinc in your allies should present no problem.

Janet in Jerusalem

#19

Aren’t alloys with zinc easier to solder as yellow colours made without zinc tend to have lower, not higher, melting points than those with zinc additions?

#20

I agree, goldsmiths should be making their own alloys. It’s like a baker who only uses cake mix to make thier cakes. :slight_smile: