What a US silver mark has to mean
A silver mark in the United States is governed by one short section of a 1906 law, the National Gold and Silver Marking Act, now 15 U.S.C. § 296. It does two things. It fixes what two words mean: “sterling” or “sterling silver” may only be used on an article whose silver parts contain 925 parts per thousand pure silver, and “coin” or “coin silver” only where they contain 900. And it limits how far any mark of fineness, word or number, may overstate the metal.
Nothing in the Act requires silver to be marked at all, and no government office tests it before sale. The law bites when a mark is used: if a piece says sterling, the word has to be true within the tolerance the Act allows.
Two tests, both at once
The first test is taken on a part of the article with no solder attached. For the words sterling and coin, the Act allows “a divergence in the fineness of four one-thousandth parts” from 925 and 900. For a numbered mark, such as 800 or 950, the silver may not be less by more than four parts per thousand than the number. So a sterling spoon’s bowl, cut away from any joint, must assay at 921 or better.
The second test treats the whole article as one piece: all the silver and its alloys, and all the solder used to join the parts, melted and assayed together. That total may not be less by more than ten parts per thousand than the mark. For sterling that is 915; for coin, 890. The Act says the two standards are “concurrent and not alternative”, which means passing one does not excuse failing the other.
Why solder gets its own allowance
Silver articles are assembled. A teapot has a spout, a handle and a foot soldered to its body; a chain has hundreds of joints. Solder has to melt at a lower temperature than the parts it joins, and the Act’s own words describe it as “solder or alloy of inferior fineness used for brazing or uniting the parts”. Testing the whole article with its joints would penalise honest construction, so the law tests the parts without solder to the tighter limit and the whole article to the wider one.
Unlike the gold section, § 295, the silver section draws no distinction between flatware, watchcases and other articles. The same four and ten parts per thousand apply to a fork, a bracelet and a tray.
Sterling, 925 and coin
A mark of “925” and the word “sterling” come to the same numbers under the Act: 921 without solder, 915 with it. The difference is that the word is defined by the statute while the number is simply a statement of fineness, held to the same tolerance.
Coin silver takes its name from coinage, and the Act fixes it at 900. The calculator treats the word exactly as the Act defines it. Abbreviations such as STER are not read. The Act also covers any “colorable imitation” of the words, and whether a given abbreviation is one is a legal question rather than arithmetic.
What the figures cannot tell you
These are the lowest figures a mark may lawfully mean. They do not show that any particular piece meets them; only an assay measures that. Acid tests give a band rather than a fineness to the part per thousand, and a surface reading on a plated or worn piece may not represent the metal underneath. Silver-plated ware may carry marks that look like fineness marks and are not.
The Act applies to articles marked for sale in the United States. A British sterling hallmark works differently: it is struck under the Hallmarking Act 1973 by an assay office that has tested the metal before marking it. The silver purity by country page sets out the UK, US and Indian standards side by side.
Using the weight box
Enter the weight of the silver parts and the calculator multiplies it by the lowest lawful whole-article fineness, giving the least fine silver the mark allows. It is a floor, not an estimate of what is there. Leave out anything that is not silver: many sterling table knives have a steel blade in a hollow silver handle filled with a cement or resin, and weighing the whole knife would count both. Melt value at today’s price is a separate step on the silver value calculator, and what anyone would pay for the piece is a different question again.