Help with alloying gold

Can someone please help me… I’m baking my noodle! :exploding_head:

I have 3g of customers own 9ct gold and I need to bring it up to 18ct.

If my 3g of 9ct is 37.5% gold then it contains 1.125g fine gold & 1.875g of presumably copper and fine silver.

I normally alloy 18ct at 75% Gold, 15% copper and 10% silver

How much fine gold do I need to add to bring it up to 18ct? Do I need to add extra copper & silver?

I have alloyed gold before but never this way round and I’m going round in circles trying to do the maths!

There is probably an online calculator that will do this for you…Rob

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According to this: Gold Karat Calculator - United Precious Metal Refining, Inc.

You need 4.51 g of fine gold to bring it up to 18k.


Thanks so much… I can’t believe it never occurred to me that there’d be a calculator online!

If anyone is interested, the math works out like this:

(1.125 + x) / (3 + x) = 0.75

where x is the required pure gold to add.

This works out to x = 1.125 / 0.25 = 4.50 g