The number on your dish does not mean what you think it means.

Two completely different tests. One misleading number.

When you see a lead number attached to a dish, it came from one of two entirely different types of instruments. They are not interchangeable. You cannot compare their results without understanding what each one actually measures.

Portable XRF scanner

A handheld device (Niton XL, Olympus Vanta, and similar) fires X-rays at a solid surface. The atoms in the material fluoresce back at characteristic wavelengths. Software translates that signal into a concentration in ppm (parts per million by weight). It is measuring the bulk lead content of whatever is under the beam: paint, glaze, metal, decorated ceramic. It does not tell you whether that lead is mobile. It does not tell you what chemical form it is in. It does not tell you how much of it ends up in your food. It is a composition measurement, not an exposure measurement.

Lab liquid analysis (ICP-MS, ICP-OES, atomic absorption)

A liquid sample is introduced to a plasma or flame and the instrument counts individual metal ions in solution. The sample might be water, wine, digested food, blood, urine, a dust wipe dissolved in acid. Results come back in µg/L (micrograms per liter), also called ppb. This is an exposure-relevant measurement when the liquid is something you consumed or came from your body.

What 800 ppm on a dish actually produces.

I tested an 800 ppm dish. Poured 250 mL of liquid over it. Sent the liquid to a lab for ICP-MS analysis. Result: 2 ppb (0.002 µg/mL) of dissolved lead in the liquid.

The FDA leach limit for flatware is 3.0 µg/mL (3.0 ppm = 3.0 mg/L). My result was 0.002 µg/mL. That is 1,500 times below the FDA flatware limit. And the FDA limit is itself a standard based on soaking a dish in 4% acetic acid (stronger than vinegar, more acidic than most food) for 24 hours at room temperature. No real meal replicates those conditions. Actual use exposure from that 800 ppm dish is a small fraction of an already-negligible test result.

Government-collected paired data backs this up. The NYC Department of Health tested the same items with both portable XRF and ASTM C738 leach tests on the same day:

High XRF does not automatically produce high leach. The material matrix, the firing temperature, the chemical form of the lead, and the conditions all matter. The aluminum pot at 4,065 ppm passes. The glazed plate at 260,000 ppm leaches 833 times the flatware limit. XRF alone cannot tell you which situation you are in.

Even very high-lead items lose the ability to leach over time.

Lead crystal contains roughly 24% lead oxide by weight, around 240,000 ppm by XRF. A published study found that the first leach test (4% acetic acid, 24 hours) released 3.1 µg/cm² of lead. The second test two months later: 0.4 µg/cm². After 10 months of normal use: 0.05 µg/cm². The surface forms a lead-depleted passivation layer after repeated contact with liquid. The item is the same item with the same XRF reading. The leach is 60 times lower.

A ceramic glaze at 2,000 ppm goes through the same process. It depletes faster, on the first wash.

The reagent is doing the chemistry for you.

FluoroSpec uses methylammonium bromide (MABr) dissolved in isopropanol. When it contacts lead that is chemically accessible, lead that can give up Pb²⁺ ions at the surface, it forms methylammonium lead bromide perovskite quantum dots (MAPbBr₃) that fluoresce bright green.

If the dish does not glow, the reagent found no significant free lead ions on the surface. The lead, whatever total amount exists in the bulk material, is not accessible under normal use conditions. No glow means the test answered your actual question: is there reactive lead on the surface of this object that food contact could mobilize? The answer is no.

Real XRF readings. Incomplete interpretation.

Tamara Rubin has tested thousands of items with a portable XRF scanner. Her equipment is real. She knows how to operate it. Her raw readings are real readings. The problem is the interpretation layer that stops at the ppm number and calls it an exposure verdict.

She posts surface XRF ppm and labels items "UNSAFE" or assigns them verdict language based on ppm thresholds. Not one of those verdicts has ever been paired with a leach test. Not one has been independently verified by an accredited lab. There is no dose calculation, no contact-time adjustment, no consideration of whether the lead is in a fired glaze matrix or a surface decoration. The NYC Department of Health, which does paired XRF and leach testing by protocol on the same items, routinely finds that high XRF numbers and high leach numbers are not the same items.

This does not mean her flagged items are safe. Some of them are genuinely dangerous, particularly handmade glazed pottery from high-risk origins and vintage items with painted decorations. It means the ppm number she posts is not the number you need to understand your actual risk. You need the leach test, or you need a surface reactivity test like FluoroSpec, to bridge the gap.

The FDA leach limits are real regulations. They are also decades out of date.

The FDA's leach limits for ceramic tableware come from Compliance Policy Guide Sec. 545.450. The original 1971 standard started at 7 µg/mL (7 ppm) across all tableware. The current limits, tightened over subsequent decades, are:

µg/mL = ppm = mg/L for dilute solutions. These are all the same unit expressed differently. The original 1971 limit of 7 µg/mL has been tightened to 0.5 µg/mL for the most-used vessel types. That is progress. It is still not calibrated to modern understanding of lead toxicology.

These standards were set against a background blood lead level of roughly 15 µg/dL in the general US population. The current CDC reference value is 3.5 µg/dL. The science has moved substantially. The regulations have not. A pass on the FDA leach test does not mean an item is safe by modern standards. It means it met standards written for a population already carrying 4 times the lead load we consider elevated today.

At the same time, an item that barely exceeds the FDA limit is a fundamentally different situation from an item that leaches 2,500 mg/L. The limit is a floor, not a ceiling, and reading anything close to the floor requires calibration against the full picture of your lead exposure, not just that one dish.

Sources: FDA CPG Sec. 545.450 · FDA Lead in Food and Foodwares · Hellinger et al., Journal of Community Health (PMC) (historical 7 ppm origin)