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Formula to raise silver fineness

Does anyone have the formula to figure the amount of pure silver
needed to raise the fineness of coin silver or lower to sterling
silver. I have some .625 fine Olympic coins that I want to raise to
.925. I know that 1 oz of .625 silver should have approximately 12.5
dwt ( 19.4375 grams ) of fine silver and 7.5 dwt ( 11.6625 grams ) of
alloy so to raise 1 oz of.625 to .925 I should need 4 ozs of pure
which would give me 92.5 dwt of silver and 7.5 dwt of alloy but there
must be an easy mathematical formula to use.

Thanks
Greg DeMark
http://www.natureinspiredjewelry.com

Here’s the general formula:
RP = Required Purity (0.925 for Sterling Silver)
CP = Coin Purity (0.625 in your case)
CW = Weight of coin silver
PW = Weight of pure silver

RP = (CPCW + PW)/(CW + PW)
Cross multiplying gives: RP
CW + RPPW = CPCW + PW
Re-arranging gives: PW*(1 - RP) = CW*(RP - CP)
Therefore: PW = CW*(RP - CP)/(1 - RP)

Since you have 1oz of 0.625 silver and you want 0.925 silver, the
weight of pure silver required is: 1*(0.925-0.625)/(1-0.925) =
0.3/0.075 = 4

Regards, Gary Wooding

Okay

Mass of fine silver = (M(R - H))/(1000 - R)

This equation will give you the mass of fine silver required to
raise the Olympic coin silver to Sterling silver.

M = The weight of metal on hand (in grams)

H = The precious metal content of the metal on hand in parts per
thousand

R = The precious metal content of the required metal

Slot in your figures, and you should be good to go

and example of using the equation :-

Let’s say you have 10 grams of coin silver, and you want to know how
much fine silver you need to improve the precious metal content.

(10(925 - 625) / (1000 - 925) = 3000 / 75 = 40 grams of fine silver.

Regards Charles A.

Thank you for sharing this Charles and Gary. Two from
the many great minds and artists on Orchid.

Greg DeMark