Answer to Question #12263 Submitted to "Ask the Experts"

I went to the pottery festival in Mashiko, Japan, recently and purchased some beautiful bowls, plates, and cups. Some of these were made in Ibaraki Prefecture using local clay, water, and glazes, as well as local timber to fire them. Does pottery absorb radiation, and are these plates safe to eat from?

Your question about radioactivity in your Mashiko pottery expresses concerns about exposure to radiation raised by many people, especially in Japan after the reactor failures at Fukushima. Apparently, release of radioactive materials in the air from the Fukushima reactors has resulted in fallout of these materials in the Ibaraki and Tochigi Prefectures to the northwest of Fukushima, including the wooded forests of Mashiko.

The radionuclide of concern is cesium-137 (¹³⁷Cs). This radionuclide's chemical behavior is similar to sodium and potassium, which are natural components of soil and all living materials. As a radioactive element, ¹³⁷Cs decays very slowly with a half-life of about 30 years, and thus it will remain in the environment for a long time. Traces of this radionuclide can be found in all trees and food products as a result of atmospheric testing of nuclear devices several decades ago.

Finding traces of ¹³⁷Cs, however, is not an indicator of hazard because our ability to measure small amounts of radioactivity is exceptionally good. Actually, all of our food has detectable amounts of many radioactive materials including uranium, thorium, radium, and radioactive potassium. Thus, our bodies are radioactive from the radioactive elements in all the food we eat.

The question regarding Mashiko pottery is whether the wood ash from contaminated trees in Mashiko forests can transfer during processing in the kiln to the glaze of the ceramic pottery. The Japan Ministry of Agriculture, Forestry, and Fisheries has assumed, being cautious for the sake of safety, that contaminated wood ash could be transferred in a kiln to the glaze on pottery and subsequently transferred to food. This agency has therefore imposed restrictions on the amount of ¹³⁷Cs in wood ash that would be acceptable for pottery kilns. Consequently, most kilns in Mashiko are not using wood from local forests. However, even if local contaminated wood was used, at the temperature of a pottery kiln, the cesium would be vaporized and therefore would not be transferred to a ceramic glaze.

All of this means that it is very unlikely that the pottery which you recently purchased from Mashiko will have any significant amounts of radioactive materials from Fukushima. As you can realize, from the large prevailing fears of radiation in Japan, if the Mashiko pottery was contaminated there would be a great reluctance to buy this pottery and the whole pottery business in Mashiko would be gone. Actually, after reviewing many websites for sales of Mashiko pottery, I find that there is not a single mention of any radioactive contamination and Mashiko pottery seems to be as popular as ever.

In conclusion, I offer the following answers regarding your questions about radioactive contamination of Mashiko pottery. First, pottery does not absorb radiation. Radioactive materials from wood ash in kilns might be incorporated in a ceramic glaze, but this is very unlikely for reasons given above. Second, since it is very unlikely that your pottery has any radioactive contamination in the glaze, your plates are perfectly safe to eat from. I hope you will find these answers helpful for your concerns about radiation.

Ray Johnson, MS, PSE, PE, FHPS, DAAHP, CHP