Increasing 800 silver to 925
Calculations and methods to increase the silver grade by adding fine metal.
At a glanceIn Swiss jewelry and silversmithing workshops, recycling antique silverware (cutlery, trays stamped with the 800‰ hallmark) is a common practice to produce new jewelry alloyed to the 925‰ Sterling silver standard. Because 800 silver contains too much copper (20%), artisans must raise its fineness by adding pure fine silver (999‰). This guide provides the exact mass-conservation formula (Pearson's square) and ready-to-use workshop tables.
Metallurgical Principles of Karat Elevation
800 silver consists of 80.0% fine silver and 20.0% copper. Sterling silver legally requires at least 92.5% fine silver and no more than 7.5% copper. Melting alone cannot remove the excess copper. The copper must be diluted by adding pure silver casting grain (999‰ or 999.9‰).
Exact Mathematical Formula (Pearson's Square)
To calculate the exact mass of fine silver to blend with a batch of 800 silver:
Fine Silver (999) to Add = Mass of 800 Silver × [ (Target Fineness − Starting Fineness) ÷ (Fine Silver Fineness − Target Fineness) ]
Substituting the values:
- Starting Fineness = 800‰
- Target Fineness = 925‰
- Fine Silver Casting Grain = 999‰
Fine Silver to Add = Mass of 800 × [ (925 − 800) ÷ (999 − 925) ] = Mass of 800 × (125 ÷ 74) ≈ Mass of 800 × 1.6892
Workshop Rule of Thumb: To upgrade 800 silver to 925 sterling, add approximately 1.69 times its starting weight in pure fine silver!
Ready-to-Use Workshop Calculation Table: 800 ➔ 925 Sterling
The following table displays required fine silver additions for common scrap cutlery batches:
| Starting 800 Silver Weight (g) | Required 999 Fine Silver Addition (g) | Total 925 Sterling Obtained (g) | Total Fine Silver Content (g) | Material Cost of Addition (CHF at 1.00/g spot) |
|---|---|---|---|---|
| 50 g (Teaspoon) | 84.46 g | 134.46 g | 124.38 g | CHF 84.50 |
| 100 g (Dinner fork) | 168.92 g | 268.92 g | 248.75 g | CHF 168.90 |
| 250 g (Cutlery set) | 422.30 g | 672.30 g | 621.88 g | CHF 422.30 |
| 500 g (Silver serving dish) | 844.60 g | 1,344.60 g | 1,243.75 g | CHF 844.60 |
| 1,000 g (1 kg) (Dinner service) | 1,689.20 g | 2,689.20 g | 2,487.50 g | CHF 1,689.20 |
Practical Melting Steps in the Workshop
1. Check knife handles: Antique silver table knives almost always have hollow handles weighted with pitch, resin, or lead. This filler must be completely removed before melting, as lead ruins silver malleability.
2. Crucible loading: Place cut 800 silver pieces at the bottom, topped with fine silver casting grain.
3. Thorough stirring: Because silver ($10.49 g/cm^3$) and copper ($8.96 g/cm^3$) differ in density, stir the liquid pool with a preheated graphite rod for at least 20 seconds before pouring.
4. Safety margin: Swiss precious metal control offices recognize zero negative tolerance. Prudent silversmiths formulate to 928‰ to prevent assay disputes.
Frequently Asked Questions (FAQ): Upgrading Silver
Can historic pre-1968 Swiss coins be used to upgrade scrap?
Pre-1968 Swiss 1/2, 1, 2, and 5 Franc coins are minted in 835 silver. To upgrade 835 silver to 925, the factor is: $(925 - 835) / (999 - 925) = 90 / 74 ≈ 1.216$. You only need to add 1.22 times their weight in fine silver.
Can upgraded silver legally be stamped 'Sterling 925' in Switzerland?
Yes, provided the assayed silver content equals or exceeds 925 parts per thousand and your registered maker's mark is applied.
Buy Investment Gold in Switzerland (VAT-Exempt)
Buy and store physical gold in secure Zurich or Geneva vaults at direct live LBMA spot rates.
Alloy & purity calculator
Alloy and purity raise calculator
Calculate exactly how much fine metal or master alloy to add to reach your target purity.
Add pure Gold/Silver (Fine)
+ 166.67 g
New total weight
266.67 g