How to Read a Cookware Spec Sheet: Grade, Ply, and What a Test Certificate Proves

What do the numbers in a stainless steel grade mean?

Grade numbers describe what the steel is made of. Type 304 is the most common austenitic stainless steel used for food contact. It is often written as 18/8, which means roughly 18% chromium and 8% nickel by weight.

Chromium is what makes stainless steel stainless. It reacts with oxygen to form a very thin, stable oxide film on the surface. That film resists corrosion and re-forms if the surface is scratched.

Nickel keeps the steel in its austenitic structure. That structure gives 304 its toughness, formability, and corrosion resistance. It also makes 304 essentially non-magnetic in its annealed state.

18/10 means the same family of steel with about 10% nickel. Both are sold under the 304 designation, because the standard allows a nickel range. The ratio on a label should match the ratio on the mill paperwork. If a listing says 18/10 and the certificate reads 8% nickel, the listing is wrong.

• 304 or 18/8: about 18% chromium, 8% nickel, non-magnetic when annealed
• 18/10: the same family with about 10% nickel
• 430 or 18/0: about 16 to 18% chromium, little or no nickel, magnetic
• "Surgical grade": a marketing phrase, not a standard grade designation

What does a mill certificate prove, and what doesn't it?

A mill certificate is issued by the steel mill for a batch, or heat, of metal. It lists the measured chemical composition and usually mechanical properties such as tensile strength and hardness. It is good evidence of what the raw sheet or blank is made of.

A mill certificate does not describe the finished pan. It says nothing about how the layers were bonded, how the handle was fixed, or how the pan behaves in an oven. Any heat figure on it refers to the sheet under the mill's test conditions, not to a pan with a handle, rivets, and a lid.

It also only covers the heat it names. A certificate for one blank size does not automatically cover a different blank or a different production run. When a brand shows one, check that the product, size, and batch line up.

What is the difference between a material test, a product test, and a component test?

These are three different kinds of evidence, and they answer three different questions. Mixing them up is the most common way a true test result turns into a misleading claim.

A material test examines the substance itself, such as a grade of steel or a type of plastic. It can show the material is suitable for food contact. It cannot show that a finished item made from it performs in a dishwasher or oven.

A finished-product test examines the item as sold, or a named part of it, under stated conditions. This is the evidence that supports a use claim like microwave-safe or dishwasher-safe.

A component test examines one part, such as a lid, gasket, or handle. A lid can be tested thoroughly and still be only a component. Its result does not transfer to the container, and the container's result does not transfer to the lid.

• Material scope: what the metal or polymer is
• Finished-product scope: how the tested item behaves under the tested conditions
• Component scope: how one named part behaves, and nothing beyond it

What should an oven, dishwasher, or microwave rating tell you?

A use rating is only meaningful with its conditions attached. The conditions belong on the same line as the claim, because that is where people read it.

Look for the specimen first. A container tested without its lid says nothing about the lid. Parts made of polypropylene or silicone have much lower heat limits than steel, so an oven figure reasoned from the steel alone can be dangerous for a lid with plastic parts.

Then look for the method. A dishwasher claim should name a test standard and a cycle count. A microwave claim should state the power level, the time, and whether the lid was on or off. An oven claim should state the specimen and whether the lid was included.

• Specimen: which part, and was the lid included?
• Duration or cycles: one hour, 50 cycles, five heating cycles
• Conditions: power in watts, lid on or off, empty or filled
• Standard or lab report: a named method you can look up

What does ply or layer count mean on cookware?

Clad cookware bonds layers of metal together. On any pan, a common tri-ply build puts an aluminum core between two stainless steel layers. Aluminum spreads heat well, and stainless steel gives a durable cooking surface.

A layer count matters less than where the layers are. Fully clad construction runs the layers up the walls. An encapsulated or disc base puts extra metal only on the bottom. A spec sheet should say which one it is.

Thickness is usually given in millimeters, or as a gauge number for sheet metal, where a higher gauge means thinner metal. Check whether the figure describes the base or the wall. On disc-base pans the two can differ a lot.

Induction is a separate question for any pan. Induction cooktops need a magnetic base, which is why many clad pans use a ferritic steel such as 430 on the outside. A magnet test at home is a reasonable check of a base, though it is not a certificate.

Which cookware claims can you spot as untested?

Some phrases appear on spec sheets without any test behind them. They are not always false, but they are claims to question rather than accept.

• Handles described as staying cool, with no measured handle temperature or time
• Warranty wording with no stated term or conditions
• "Airtight" or spill claims with no stated test method or orientation
• "Surgical grade" with no grade number
• A nickel ratio on the label that differs from the mill certificate
• Oven temperatures stated for a whole item when only the metal body was tested

How does SEIDO write its own spec sheet?

SEIDO writes each claim with its tested specimen alongside it. The skillets' cooking surface is 304 (18/8) stainless steel. The 27 oz (800 ml) lunch box container was lab-tested: microwave-safe with the lid off at a maximum of 1000 W, dishwasher-safe to EN 12875-1 over 50 cycles, and freezer-safe. On the site that reads "Dishwasher safe, handwash recommended." The container is also oven-safe, container only, tested without its lid, and the lid never goes in an oven. The glass lid with its PP frame and silicone gasket is described separately. SEIDO skillets in 8 in (20 cm), 9.5 in (24 cm), and 11 in (28 cm) are oven-safe with the lid off, and each comes with a 5-Year Limited Warranty. Where a claim is not yet backed by a document, it stays off the page.