Hi,

I’ve been asked what happens to 9 & 18 carat gold when melting

together, does the gold become 9 or 18 carat gold? This would be for

hallmarking in the UK.

Best regards

Gill

Hi,

I’ve been asked what happens to 9 & 18 carat gold when melting

together, does the gold become 9 or 18 carat gold? This would be for

hallmarking in the UK.

Best regards

Gill

`I've been asked what happens to 9 & 18 carat gold when melting together, does the gold become 9 or 18 carat gold? This would be for hallmarking in the UK.`

There are formulas to calculate resulting purity of an alloy, but

they are of little use to beginner. Let me offer simpler explanation.

10 grams of 18k contain 7.5 grams gold and 2.5 grams silver or other

metals. 10 grams of 9k contain 3.75 grams gold and 6.25 grams of

other metals. Alloying it together would give you 20 grams of alloy,

containing 11.25 grams of gold, and 8.75 grams of other metal. To

convert result to karat system we can use proportion - 11.25 relates

to 20 as “X” relates to 24 (pure gold is 24k) Rearranging we get X =

(11.25 * 24) / 20 = 13.5k

Leonid Surpin

your final concentration would depend on the proportion of 9 and 18

in the mix. When you would melt them together, they would combine to

form a concentration of gold between 9 and 18. If you used exactly

equal amounts and they were actually correctly marked then you would

have 13.5 gold.

John

If you melt together equal weights of 9 & 18 carat gold you will get

13.5 carat - not a lot of use to anybody except the refiner. To make

9 carat from your mixture you can add silver and copper with a

combined weight equal to the weight of the 18 carat gold. 40/60 fine

silver to copper is a suitable starting point assuming it is all

yellow gold. When I am re-alloying I usually aim to be slightly over

carat since the thought of a piece I have a dozen hours in coming

back from the assay office having been flattened with a big hammer

does not appeal.

Andy Parker

the notations 9k and 18k are directly related to the content of cold

in an alloy.

9k is 37.5 % gold or 9 parts per 24

18k is 75% gold or 18 parts per 24

If you are using yellow gold traditionally the other parts are fine

silver and copper in different amounts depending on the colour you

want to achieve.

so to answer you question.

if you had 10 grams of 9k 3.75gm of it is gold and 6.25gm is alloys

10 grams of 18k 7.5gm of gold and 2.5 gm of alloy

doing the math you will have 11.25 gm of gold and 9.75 gm of alloy

divide 11.25 by 20 (the total material) and you get the percentage

of 56.25

24k is the standard so 56.25% of 24 is 13.5

in this case you end up with 13.5k. you need to figure the

percentage of gold to alloy you have in total that will dictate what

karat you end up with

Les

That all depends on how much of each metal you have. With UK

hallmarking you may have a problem, as you might end up with 11K or

15K or something else. There are other considerations as well.

Combining metals of differing alloys can produce undesired and

unexpected results, such as porosity, weird colors, and difficult to

handle working properties, such as brittleness. Unless there is a

sentimental reason for combining the metals and you explain the

potential problems to your customer and they’re OK with it, I would

avoid doing it.

Dave Phelps

9ct gold is 9/24 (0.375) pure gold, by weight, and 18ct is 18/24

(0.75) pure gold, by weight.

If you mix 1gm of 9ct with 1gm of 18ct, the resulting alloy will

weigh 2gm and will contain 0.375+0.75 = 1.125gm of pure gold, so the

new alloy is 1.125/2 = 0.5625 pure gold, which is 13.5ct.

To work out the new carat (NC) of mixing Ngms of 9ct with Egms of

18ct, use the formula NC = 24*(0.375N+0.75E)/(N+E)

For example, if you mix 5gms of 9ct with 2gms of 18ct, the new carat

will be 24*(0.375*5+0.75*2)/(2+5) = 24*(1.875+1.5)/7 = 24*3.375/7 =

11.57ct

Because the only allowable carats in UK are 9,14,18 and 22, and the

Assay Office will only accept the next lower carat, this would have

to be marked as 9ct.

IHTH

Regards, Gary Wooding

Hi Gil,

Let’s do it the easy way

18ct is 750 ppt, and 9ct is 375 ppt

Would work out to be about 562 ppt… which is less than 14ct, but

more than 10ct…so you could stamp it 416 or 10ct in Australia.

Things that would be a better option :-

- Send the lot to a refiner
- Make something out of those carat golds
- Increase the carat of the gold by adding more gold
- Reduce the carat of the gold by adding more silver and copper.

Regards Charles A.

The gold will become something in between 9 carat and 18 carat, so

you have to have it hallmarked as 9 carat. If you send it to the

Assay Office as 18 carat it will fail the assaying and be crushed!

Annette

`The gold will become something in between 9 carat and 18 carat, so you have to have it hallmarked as 9 carat. If you send it to the Assay Office as 18 carat it will fail the assaying and be crushed!`

This is a popular myth and is untrue. What actually happens is that

the assay office phones to ask what they should do: mark it as a

lower carat or return it.

Regards, Gary Wooding

If you alloy an equal amount of 9ct and 18ct, you’ll end up with

something about 13ct, and therefore only markable as 9ct in the UK.

I don’t have it to hand, but there is a clever equation for alloying

up and down using mixtures of premade alloys to reach the right

percentages. So that, eg, you could mix 9 and 18 to get 14ct, or 9

and 22 to get 18. The long-winded way is a bit of mathematical trial

and error.Take your weight of 9ct, say it’s 10g. We divide that by

24 carats, and multiply it by 9 carats - we now know that it contains

3,75g of pure gold. The remainder is 6.25g of silver, copper and

other elements. Now do the same with the 18ct, say 10g again. This

time, divide by 24, multiply by 18 - that gives us 7.5g of pure

gold, and 2.5g of alloy metals. Add our 9 and 18 scrap together, then

we have 11.25g of gold, and 8.75g of other metals, total weight of

scrap is 20g. To express the carat of the alloy as a percentage,

divide the gold weight (11.25) by the total weight (20), then

multiply this by 100.(11.25/20)*100 = 56.25%That leaves us a little

way short of 14ct, which is hallmarkable, although it’s not a popular

alloy here. The only way to up it is to increase the amount of 18ct

scrap that we have, or reduce the amount of 9ct scrap.Because I’m in

a good mood, I’ve even made you a little spreadsheet, already filled

in with the example about. You can then play around with weights in

the Alloy 1 and Alloy 2 boxes until you hit 58.5%, which is 14ct. No

amount of mixing will give you 9ct or 18 ct. But for the Americans,

it is possible to mix 9ct up to 10ct; that’s no use in the UK,

however. Here’s the spreadsheet: Primitive Method — Exploring jewellery techniques from the early middle ages…

Another thing I recommend is to over-carat slightly - ie. put in a

little bit more 18ct than you need to, to ensure that you definately

will pass the assay. Oh, and remove solder before melting up items,

it can change the working properties of the alloy.

Jamie Hall -

Contemporary and Medieval Metalworker

http://primitive.ganoksin.com

`What actually happens is that the assay office phones to ask what they should do: mark it as a lower carat or return it.`

How do you guys find the Assay Office? We don’t have anything like

that here in Oz, we have a hallmarking standard that protects us as

well as the customer.

Just wondering if the Assay Office is “the Devil you know”.

Regards Charles A.

Marvellous Annette thank you for a nice clear & succinct answer!

Best

Gill

If you can post the exact weight of the 9kt gold and 18kt gold, then

I will computer for you what will be the outcome when combined.

Make sure they are actually 9kt or 18kt gold, relying on a hallmark

is not good. Use the simplest “touchstone assay” method, it works

with an accuracy of 10 to 1000.

Are they both in the same color? Or are you mixing yellow gold with

white gold or rose/red gold?

9ct gold is 375 parts per 1000 gold and 18ct is 750 parts per 1000

gold. So, if you melt 10 grams of 9ct and 10 grams of 18ct together

you will get 20 grams of gold of 562.5 parts per 1000 or close to

14ct. (565/1000) Now, under UK law you can have this mixed gold

assayed as 9ct and it wont matter that there is too much gold as the

assay is the minimum fineness but offer it to assay as say 14ct and

you item will be returned melted down as failed assay. My advice is

if you cannot separate the gold then send it for melt and assay and

buy new casting grain or wire/sheet etc with the cash. There are

several companies that will do this such as Cooksons or Blundells and

you will get a good spot price.

Nick Royall

the 9 kt is somewhere about 38 % the 18kt is 75% 10 dwt of each

would contain 3.8 + 7.5 = 11.5 do some reverse % math from your total

weight of gold alloy and you will get your karat. 20 x 0.58 = 11.6

and 20 x 0.57 =11.4, just one more reason art and jewelry students

should be required to pursue comprehensive studies other than art and

jewelry

`There are formulas to calculate resulting purity of an alloy, but they are of little use to beginner. Let me offer simpler explanation.`

The formula is not difficult.

Suppose you have two gold alloys: one of carat C1 and the other of

carat C2.

You want to know what proportions of each alloy is needed to produce

a new alloy of carat C.

Assume you have W1 units (gms or oz) of C1, and W2 of C2, then, as

explained in an earlier post:

C = 24x(C1xW1/24 + C2xW2/24)/(W1+W2)

or

C = (C1xW1 + C2xW2)/(W1 + W2)

If the required proportion of W2 to W1 is R, then R=W2/W1 or W2=RxW1

and the equation above becomes:

C = (C1xW1 + C2xRxW1)/(W1 + RxW1) = (C1 + RxC2)/(1 + R)

which rearranges as C + CxR = C1 + C2xR or Rx(C - C2) = C1 - C

so R = (C - C1)/(C2 - C)

For example. You have 5gms of 9ct and want to know how much 18ct is

required to make a 14ct alloy.

C1 = 9

C2 = 18

C = 14,

therefore R = (14-9)/(18-14) = 5/4

so you need 5x5/4 gms = 6.25gms of 18ct

If you don’t like the maths, the final formula is easy enough: the

ratio of hi carat to lo carat is equal to the wanted carat minus the

lo carat divided by the hi carat minus the wanted carat.

Regards, Gary Wooding

`How do you guys find the Assay Office? We don't have anything like that here in Oz, we have a hallmarking standard that protects us as well as the customer. Just wondering if the Assay Office is "the Devil you know".`

I’m registered with the Birmingham Assay Office here in UK. They are

very friendly, but also rather expensive for one-off items. To get

one item marked with the statutory marks costs 10 plus the carriage

there and back, plus VAT, which works out at more than 20 all

together. Additional items of the same metal average out at about

0.50 each, but I seldom have more than one.

On one occasion I accidentally attached ear-wires of the wrong

purity to a pair of earrings. They 'phoned me to ask if they should

mark with the next lower carat, or return them. I chose the latter,

fitted the correct ear-wires and they were marked properly.

Regards, Gary Wooding

To contact the assay office: http://www.ganoksin.com/gnkurl/1fp hall

try adding: /assay office Google the Goldsmiths Hall London and, The

Assay Office London To see all the rules and regulations. In fact in

Australia we do ‘not’ have a “Hallmarking” system, technically you

should ‘not’ say the marks are hallmarks as the marked item has not

been individually assayed and marked by a recognised organisation,

we only have a marking system “of trust”. Also see:

The Goldsmiths' Directory - The Goldsmiths' Company to see the

latest in modern work.

David Cruickshank (Australia)

jewellerydavidcruickshank.com.au

`How do you guys find the Assay Office? We don't have anything like that here in Oz, we have a hallmarking standard that protects us as well as the customer. Just wondering if the Assay Office is"the Devil you know".`

I’m registered with Sheffield Assay Office, and they are usually very

good but do sometimes make mistakes, for example putting the hallmark

upside down on a serviette clip I made for the Master Cutler a couple

of years ago, despite me sending a clear diagram of how it should go!

I usually send in as many pieces of work as I can, as there is a

minimum charge of just under 20. If I want work back quickly, it

costs 50% extra for ‘early hall’. Laser marks and extra marks all

cost extra. I get everything hallmarked, even if it does not need it

as it is under the minimum weight, because it gives customers

confidence and is a kind of quality control. Customers also like to

see the date letter mark, as it can commemorate the year of a wedding

or birthday for example.

Annette