Fine weight and job work, worked through

Every job-work bill comes down to one figure: how much pure metal, the fine weight, changed hands. Here is how it is worked out, with numbers you can check on a calculator. The rates below are only examples.

The four words

• Gross weight: what the piece weighs on the scale.
• Purity (tunch): how much of it is pure metal, as a percentage. 22K is 91.6%; 18K is 75%; silver 925 is 92.5%.
• Wastage: the extra percentage the karigar or the shop charges for metal lost in making.
• Fine weight: the pure metal the bill is settled in.

The formula

Fine = gross × (purity + wastage) ÷ 100, cut, not rounded, at three decimals: the trade drops what is past the milligram rather than rounding it up.

Example: a silver casting of 127.860 g at 93% purity with 5% wastage. 127.860 × 98 ÷ 100 = 125.3028, so the fine weight is 125.302 g.

A sale in gold

A 22K chain weighing 10.000 g holds 10.000 × 91.6 ÷ 100 = 9.160 g of fine gold. A shop can charge either on the gross weight at the 22K rate, or on the fine weight at the 24K rate. The two give close but different totals, so a bill should always say which it used.

A karigar's account

You give a karigar 100.000 g of fine gold. He brings back jewellery weighing 98.000 g at 92% purity, and you allow him 6% wastage. You credit him 98.000 × (92 + 6) ÷ 100 = 96.040 g of fine. He still owes you 3.960 g of fine gold, and that balance carries to his next job.

That running fine balance, kept per party beside the rupee balance, is what a jeweller's book is really for. A general billing app treats purity and wastage only as a way to arrive at a price, and loses the metal account.

Checking a bill by hand

1. Multiply gross by (purity + wastage), divide by 100.
2. Drop everything past the third decimal.
3. Add or take away from the party's fine balance, on the side the metal went.