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Ok so just to see I am getting al of this straight.... 1 Troy Oz of
Gold, + Alloys [to make it 18kt] = 38.8 grams Is this correct?
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No. Not sure where you went wrong.

But 18K gold is 75 percent gold, right? In simpler terms, that’s 3

parts gold to 1 part alloy. So for any given amount of fine gold,

take it’s weight, and divide by 3. The result is the amount of alloy

to add.

In the case of one troy ounce, it looks like this:

In troy weight, 1 Oz Troy - 20 dwt. divide by 3 = 6.66 dwt. So add

6.66 dwt alloy to your 1 oz of gold to get 26.66 dwt of 18K gold

In grams, it’s like this:

One troy ounce = 31.1 grams. Divide by 3 to get 10.36. Add 10.36

grams alloy to your 31.1 grams (one ounce) fine gold to yield 41.46

grams 18K.

Your result is not far off what you’d have ended up with if you’d

made the mistake of forgetting that a troy ounce is 31.1 grams rather

than the lesser 28.35 grams to the avoir. ounce. But even then, it’s

off. If you had 1 avoirdupoids ounce of gold, ie 28.35 grams, and you

alloyed it to 18K, you’d divide 28.35 by 3 to get 9.45 grams alloy

which would give you 37.8 grams 18K.

If the above (the divide by 3 bit) isn’t clear, do it this way.

Any gold alloy is some ratio of gold to alloy. You can do it with

Karats, or the millesimal markings. The latter is 750 gor 18K gold,

or 583 for exactly 14K, though many slightly over karat at 585 to

allow for error or solder, etc. Either way, the difference between

the gold mark and 1000 is the percentage of alloy.

In 18K, which is really simple, it’s a ratio then, of 750 gold to

250 alloy. Or 75 percent gold to 25 percent alloy. Just another way

to say the same thing. Basic algebra tells you that the ratio of 750

to 250 can be most simply stated as 3:1, which is why I used those

numbers in the above example.

For 14K, the process is similar. The ratio of gold to alloy is 585

gold to 415, if stated in millesimal numbers, which is parts per

thousand. Or in percentages it’s 58.5 % gold to 41.5% alloy. Or in

Karats, it 14 parts gold to 10 parts alloy. If you like, you can

reduce that ratio further to 7 parts gold to 5 parts alloy (again,

basic algebra…) So for a given weight of fine gold to be alloyed to

14K, you take the weight of fine gold, and divide by whichever of

the ratio numbers (585, 58.5%, 14, or 7) that you prefer, and then

multiply by the corresponding ratio for alloy (415, 41.5%, 10, or 5).

Just make sure that the two numbers you use are in the same ratio.

You can’t do it with 585:10, for example. ) Anyway, the resulting

number is the weight, in whatever units of measure you used for the

fine gold, of the alloy you need to add to get 14K.

None of this needs to be memorized as any sort of formula. Once you

understand what the calculation is doing and why, then it’s simple,

especially if you have a cheap pocket calculator, you simply plug in

the numbers quickly and easily.

By extension, you can get more complex to change from one alloy to

another, rather than starting with fine gold.

Suppose you have a certain weight of 18K, and wish to alloy it down

to 14K.

Take the weight of the 18K, and divide by 4. You do that because you

know that 18K is 3 parts gold to 1 part alloy, so 4 parts total.

Understand? Now you know the quantity of alloy (one part of the 4),

and the quantity of gold (3 parts of the 4) Going back to the

original process, using just the calculated amount of fine gold in

your 18K, determine the amount of alloy needed to make 14K. Got that?

Now subtract the portion of alloy that’s already in the 18K, and the

result is the amount of alloy you need to add to reduce the 18K to

14K.

Of course, it gets more complex if the alloy itself differs from one

karat to the other, such as if your 18K is alloyed with equal parts

copper and silver, but you want a 14K with 3 parts copper to 2 parts

silver, with 2 percent zinc. This gets more complex, but in the end,

it’s the same sort of process.

You’d do that be breaking down your original quantity of 18K not

just to weights of gold and alloy, but weights of gold, silver, and

copper.

There are several approaches from there, but I’d start by

determining how much silver and zinc to add to get the alloy that’s

already there, to the desired end alloy composition, without yet

worrying about the gold. After you’ve got that, then figure how much

alloy your existing quantity of fine gold (in your existing gold

alloy) needs in order to to get your desired end karat.

If the total alloy you end up with after converting the existing

alloy to what you want, is less than the total required alloy for the

karat you wish, then you take just the needed additional alloy and

calculate what portions of copper, silver, and zinc you need to add

to bring the total alloy up to the needed quantity. Your working with

a number of adjustments: The amount of silver or copper or zinc

needed to convert the existing base metal alloy to the composition

needed for the alloy, then the amount of additional

silver/copper/zinc, in the desired alloy composition, you need to add

to bring the total alloy up to the needed amount for the amount of

fine gold you’ve already got in the existing karat gold. Add all

these numbers together, and you then know what to add to your 18K to

get your desired 14K composition.

it can get a little confusing, especially to describe, but it’s

actually a lot simpler than the above paragraph was to write.

Or perhaps you might find that after adjusting the alloy (for gold

color, etc), you’ve now got too much alloy for the quantity of gold

you started with. In that case, take the amount of alloy that you

need for the gold you’ve already got, and subtract from the total

alloy. That excess alloy now needs an appropriate amount of

additional fine gold added. The process to calculate this is

essentially the same as all these other calculations. For this sort

of thing, you’ll usually need a pencil and paper in addition to the

calculator, to keep the numbers straight, but in the end, it’s simply

a matter of working with the ratios. It helps to be sober and awake,

so you don’t confuse yourself as to which numbers mean which portions

of metal, but in the end, it’s still just 9th grade algebra. Pretty

basic math.

You can, if you wish, condense all the above into one of several

nice neat formulas into which you plug numbers. But then you have to

remember the formula.

Or it’s not hard to put together a simple spreadsheet on the

computer to calculate it for you. But that only works well if you’ve

got your computer in your workshop, or don’t mind going out of the

shop to do the math…

I prefer to simply remember the WHY of what’s being calculated, and

then simple logic tells you WHAT to do with the numbers, without

having to memorize formulas.

There is one other consideration to keep in mind. Any existing gold

alloy is a multiple ratio of the various componants. An 18K pale

yellow gold, for example, might be 750 gold, 225 silver, and only 25

parts copper. If you were trying to convert this to a 14K rose gold,

you might have a problem, if the desired rose gold formula had only

a little silver. Your existing silver in the 18K isn’t going to go

away, so to get the desired ratio of copper to silver for what you

might wish in that 14K, you might have to add a LOT of copper before

the total silver was only the minor componant in the needed small

ratio. Then you’ve got a LOT of alloy, so you might then have to add

quite a bit of gold before you’re back up to 14K. In this example,

even though you’re lowering the karat from 18K to 14K, in order to

change the color as you wish, you end up having to add a bunch more

fine gold in addition to copper, to get there. And if you’re

converting from one alloy to another where the desired end alloy

contains NONE of one of the constituents of the original alloy, then

you simply cannot do it. Then you have to have your existing alloy

refined, to extract the fine gold, which you then alloy down as

desired. The common example of this situation would be raising the

karat of an existing alloy that might have zinc or another deoxidizer

or additive or contaminant, when the desired end alloy has none of

it. Or trying to start with a white gold and end up with a yellow

gold or vice versa. Can’t generally do it because although you can

dilute the undesired portion with lots more gold and alloy, it

doesn’t go away totally.

Hope that helps.

Peter Rowe