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How do I know if my clock has radium?

Many antique clocks and watches have radium-based luminous paint on the hands and dials. This allows the timepieces to glow in the dark so they can be read at night. However, radium is radioactive and can pose health risks if the paint is flaking or peeling, allowing radium dust to be released. Here are some signs that indicate your vintage clock may contain radium paint.

What is radium paint?

Radium-based luminous paint was commonly used in watches, clocks, and other instruments from the early 1900s through the 1960s. The paint consists of a mixture of radium and phosphor. The radium emits radiation which causes the phosphor to glow, creating a long-lasting luminous effect.

There were a few different types of radium paint used over the years:

  • Undark – Developed in 1903, the first radium paint. Contained a high percentage of radium.
  • Luminous Processes – Lower radium content, manufactured starting in 1916.
  • Radium Luminous Material Corporation (RMC) – Produced after 1938, lower radium content.

The highest radium content is found in products manufactured before 1930. However, all radium paint poses some degree of risk.

Why is radium paint dangerous?

Radium is an unstable radioactive element that gives off radon gas as it decays. Radon and radium can increase cancer risk with prolonged exposure. When old radium paint deteriorates into fine dust, it can be unintentionally inhaled or ingested. The radium enters the body and continues emitting radiation internally.

In the 1920s, a group of women working in watch factories painting dials with radium paint developed severe health complications, such as bone necrosis and anemia, that became known as “radium jaw.” This revealed the dangers of radium exposure.

Signs that a clock has radium paint

Here are some signs that can indicate if an antique clock contains radium paint:

  • Glow in the dark – If the clock face, hands, and numbers exhibit a glow after the lights are turned off, radium paint is likely present.
  • Manufacture date – Was the clock made before 1965? If so, it may contain radium paint, although not all clocks from this era do.
  • Brand – Brands such as Rolex, Bulova, and Elgin commonly used radium lume.
  • Crackling or flaking paint – Old deteriorating paint can be a warning sign. Look for cracking, flaking, chalkiness, or paint dust.
  • Cloudy crystal – The crystal can become foggy over time due to radium particles inside the case.
  • Dial repainting – If luminescent paint was added to a dial long after manufacture, it is likely radium-based.

Using a Geiger counter to test for radium

The best way to determine if a clock contains dangerous levels of radium is to use a Geiger counter to measure radiation levels. Geiger counters detect gamma rays and beta particles emitted by radioactive materials. Here are some tips for testing a clock with a Geiger counter:

  • Place the Geiger counter probe window close to or in contact with the clock face and hands. Test multiple areas.
  • Note any elevated radiation readings. Background radiation is normal, but readings significantly above background indicate radioactivity.
  • Pay particular attention to cracked, flaking, or powdery areas where paint deterioration may have occurred.
  • Test the crystal, as radium dust may be present inside the case.
  • For most Geiger counters, readings above 5 microsieverts per hour (μSv/hr) indicate a potential radium hazard requiring caution.

If elevated radium readings are detected, it’s best to remove the clock from where people are present for extended periods. Seek professional testing to determine radium levels and if remediation is required.

Safety precautions for radium clocks

If you have an antique clock that may contain radium paint, here are some basic precautions to take:

  • Place the clock in a room that is not occupied most of the time.
  • Increase ventilation around the clock to dilute any radon gas emitted.
  • Do not touch deteriorated paint areas. Wear gloves if handling.
  • Clean up any fallen paint chips or dust with a damp cloth.
  • Do not brush, sand, or disturb the painted dial/hands.
  • Do not open or repair the clock yourself. Hire a professional.
  • Consider replacing old luminescent dial paint with non-radioactive paint.

Proper disposal of radium clocks

If a clock contains radium paint and needs to be disposed of, there are specific guidelines to follow:

  • Contact your local/state radioactive materials regulatory agency to discuss disposal options.
  • Do not throw a radium clock in household trash. This could contaminate a landfill.
  • The clock must go to a licensed radioactive waste facility or radioactive materials handler.
  • Transportation regulations apply when moving a radioactive source like a radium clock.

Proper disposal protects people’s health and prevents contamination of property or the environment. Never attempt to dismantle or discard a radium clock without following hazardous waste regulations.


Antique clocks with glowing hands and numbers may look neat, but the luminosity was often created with radium paint that can pose health risks over time. This radioactive paint was commonly used for many decades before the dangers of radium were fully understood. When evaluating an older clock, look for signs it may contain radium paint. Use a Geiger counter to be sure. Take precautions when handling radium clocks, and dispose of them properly if needed. This protects people’s safety and prevents environmental contamination from the radioactivity.