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the safety of iron-rich glazes

updated fri 6 mar 98


Stuart Altmann on thu 5 mar 98

"Apparently extra iron is extremely bad for kids--but...I could not tell you
what amounts are...What of a person who...'always' has a cup of black coffee
in a black cup at his elbow....?"
--Lili Krakowski

If iron is the only potential toxin in your glazes, then none are lethal,
even saturated iron glazes. How so? Here's the worst case scenario for that
coffee drinker. Suppose that the coffee is in a large mug, glazed inside
with 15 grams (dry matter) of an iron rich or even saturated iron glaze (say
12% RIO = red iron oxide, like the Red Reserve glaze that I posted today).
Suppose also that this coffee is so strong (and this glaze so weak) that
every bit of iron is leached from the glaze into the coffee, leaving behind
only the white base glaze. (That's not just worst case, its unrealistic,
but never mind.) Finally, suppose that all of the iron has been converted
into a fairly soluble form that is as readily absorbed by the gut as is the
iron in ferrous sulfate (again, completely unrealistic) and that all of the
coffee is consumed by an adult weighing a mere 40 kg (88 lb.).

How much iron did the coffee drinker take in? Well, 12% of the 15 g of
glaze is 1.8 g RIO. What's the lethal dose of iron for an adult? Between
200 and 250 mg of ferrous sulfate (or its equivalent) per kg of body mass
(National Research Council, 1977 monograph on iron), so for someone weighing
40 kg the lethal dose would be 8-10 g ferrous sulfate or its equivalent. In
short, even in this grossly exaggerated, worst case scenario, you wouldn't
take in even a quarter of the amount of iron that might kill you. Of
course, if our coffee drinker used a new black mug every day, instead of
being freaked out by the bleached cup, things could be different. Well,
suppose a two-year-old drank all the coffee. Would it kill the child? No,
the lethal dose for a two-year-old is about three grams.

For adults, deleterious effects of daily iron intakes of 25-75 mg are
unlikely in healthy persons (Finch and Monsen 1972). However, much larger
doses of iron can be dangerous. There are approximately 2000 cases of iron
poisoning each year in the United States, mainly in young children who
ingest the medicinal iron supplements of their parents (NRC, 1980 RDA

The amount of iron obtained from iron cooking pots and pans can be enough to
reduce the chance of iron-deficient anemia but is ordinarily not dangerous.
Yet, some bantu Africans who each day drink a gallon or more of kaffir beer
that has been brewed in iron pots may take in 100 mg or more of iron a day
and suffer from siderosis, that is, from iron toxicity. The same problem
occurs when wine and hard cider are brewed in iron vessels (Davidson et al
1972). Perhaps fermentation leaches iron from iron cooking pots; alcohol
is known to increase iron absorption in the gut (Brown 1964). So, except
for, say, beer steins or fermentation pots, iron-rich glazes can safely be
used on food containers.

Stuart Altmann

P. S. For about 25 years, the dinner plates in our house have had iron-rich
glazes, as have our coffee carafe and some of our cups and bowls. In
addition, we use a cast iron frying pan and a cast iron stewing pot. Yet,
no iron toxicity..and no iron-deficient anemia, either!