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Melting 9ct and 18ct gold together


#1

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


#2
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
www.studioarete.com


#3

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


#4

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


#5

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


#6

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


#7

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*(0.3755+0.752)/(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


#8

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 :-

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

Regards Charles A.


#9

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


#10
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


#11

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: http://ganoksin.com/blog/primitive/

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


#12
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.


#13

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

Best
Gill


#14

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?


#15

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


#16

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


#17
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


#18
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


#19

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:
http://www.ganoksin.com/gnkurl/whoswhoingoldandsilver to see the
latest in modern work.

David Cruickshank (Australia)
jewellerydavidcruickshank.com.au


#20
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