Hello Ganoksin!
I have a question for you folks doing alloying at home or in the shop. I am interested in trying some of the different silver master alloys in addition to just traditional 7.25% copper sterling silver. However, one thing that keeps coming to mind is the accuracy of the silver content in these DIY sterling alloys. I know some people go a little under with the copper or alloy portion of the sterling so they don’t go below 92.5% silver but how much should I realistically aim for? Do I need a super precise scale? My main concern is eventually selling a piece of jewelry that comes out under 92.5 percent silver. I’ve seen a few videos of people selling jewelry and it coming out slightly under what it is stamped. In these videos the jeweler buying the piece claims they got scammed while it very well may have been a measuring error. It seems to me the scale I use to weigh 5 ounces of silver would not be the best scale to measure grams of copper for alloying as it would need to be sensitive to smaller weights. Just curious how you all deal with this and accuracy of alloying yourself.
Thank you!
Allowances for undercarating are pretty small. FTC allows 3 parts perthousand for unsoldered gold and 4 parts per thousand for unsoldered silver. For soldered items, the tolerances increase to 7 and 8 respectively to allow for some solder being of lower carat. If I were alloying, I would make sure to have an accurate scale and go a little closer to, say 92.75% for silver. This way you are sure not to run afoul of the FTC. -royjohn
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your scale has to be sensitive, accurate and precise. These terms are scientific. Google them to find out the meaning. Acurracy is not the same as precision. Mettler Toledo scales are very accurate, precise and sensitive. They are also the most expensive. Any metal to weigh must be absolutely clean and dry. Oxide build up on copper will throw the weight off. If you are using fine silver, copper also must be fine. Roy John’s suggestion to go slightly higher on silver is advice well taken. Since you are dealing parts per thousand, a scale’s sensitivity should in milligrams per gram. Precision is how often you get the same measurement with each measurement. Precision is the average of the scatter of multiple measurements.. Accuracy is how close to the true to composition the scale can weigh. Good scales can be calibrated for accuracy.
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One place to get accurate scales at low cost is your local gun shop. Reloaders routinely work to 1/10 of a grain. 1 gr = 0.06479891 g The drawback is most of these max out at 500 - 1000 grains, but might work for alloying.
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Most of the master alloys for silver have zinc in some quantity in them. So a little will burn off during pre-alloying and casting phases, resulting in an alloy with slightly more silver than you originally weighed.
Also if you under-heat the metal, some of the copper will not mix and stay at the bottom of the crucible, again resulting in a high ag alloy.
So basically even if you make minor mistakes you will be safe legally. The problem here is the silver loses some of the alloy benefits (hardness mostly, tarnish resistance for heavily recycled metal).
Best practice IMO is to have an accurate scale and measure slightly above purity. I use .928
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