|Q: [I've got a piece made of 22karat gold, and I want to cast some14karat gold. Can I add something to it to make the lower-karat alloy, and if so, what, and how much?]
A: You can just add enough copper and silver to lower the karat to what you
want. The ratio of copper to silver will determine the color. I happen to like
about half and half, which is paler than the standard alloys, which have more
copper. For casting, use enough brass as part of the copper to introduce some
zinc, as a deoxidizer. Pure gold is 24karat, which means 24 parts out of twenty-four are gold. 22k means that 22 parts out of 24 are gold, and in a 14k gold, only 14 parts out of the 24 are gold, the rest usually being silver, copper and zinc, although other metals may be used to affect the color.
The calculation required goes like this:
The weight of the 22K you have now, times 22/24 equals the fine gold content,
with the extra 2/24 being the existing alloy content.
Take the gold content now, and multiply it by 10/14 to get the total non gold alloy required for that gold to be14K. Subtract from that result the 2/24 (from your first calculations) that is already present, and you know what weight of alloy to add to make the whole thing 14K.
Then you determine what amounts of that resulting alloy weight you wish to be copper, silver, or brass... Generally, you want about 6/7 percent zinc for a casting alloy, if you want a zinc deoxidizer. Standard yellow gold alloys usually have about six parts copper for each part silver. Most yellow brass isabout 30 percent zinc. So 1 part silver, 3 parts copper, and 3 parts brass will give you, in the end, a 14K gold alloy fairly close to standard yellow casting alloys.
Be sure to use a non-leaded brass. This is the brass used for things like drawn tube, spun items, drawn wire, and the like, rather than machined brass objects, which sometimes add some lead to increase machinability.
Note that the above calculations are "off the top of my head." Check them with
a calculator before melting anything... I've made mistakes before...