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maybe dumb raku question - some answers - long

updated wed 27 sep 00


Bruce Girrell on tue 26 sep 00

Andie Carpenter asked:

>I had always been taught and told that Raku was not only not food safe,
>but not able to hold water, due to the temperature used in the firings.
>I also remember a post from some time ago from someone talking about the
>tons of definitely-not-food-safe things in Raku glazes. ...

> I'm
>confused - Raku isn't food safe, can't hold water, but use it as a
>teacup? What gives? Can anyone give me a quick and simple education in
>Raku and explain what the differences might be?

I kind of thought others would help you on this, so I didn't reply, but I
see that as of yet no one has supplied any answers for you - at least

Let's take the easy one first - can't hold water. Most raku glazes are
formulated to craze so that upon post-fire reduction, they will acquire a
pleasing crackle pattern. These cracks, small as they are, allow fluids to
seep into the clay body. Depending on the extent of the cracking and the
condition of the clay body, you could see seepage on the outside of the
container in a few minutes, overnight, or maybe not at all. Even if you
don't see seepage on the outside of the pot, enough fluid gets into the clay
that a raku container filled with water should never be placed on a piece of
fine furniture.

Since you typically don't take hours to consume a cup of tea and a small
amount of seepage into the cup doesn't matter, then it is not an issue for
this type of usage. It would be a problem for a vase, but not a tea bowl. If
this concerns you, there are sealers that have been discussed on the list
before. Check the archives.

Now the second issue - not food safe. This is actually two issues. One issue
relates to the microcracking that causes the seepage as described above. I'm
probably going to catch hell from the health care professionals on the list
for this, but I say that for personal items this really isn't an issue. The
argument is that bacteria can get into the cracks along with the fluids and
flourish there. My feeling is that the bacteria that do get into the cracks
are bacteria from our own surroundings. These are bacteria that our bodies
live with all the time. Because we're accustomed to them, they don't make us
sick. It's the same reason that people living in a particular area can drink
the local water all they want and don't get sick whereas if you go visit
them and drink the local water you may very well get sick. If you take a
look at the cups in your house, you'll probably find that at least a few of
them have some evidence of crazing and hence are harboring those same little

Let's just say for the sake of argument that I'm wrong, that bacteria in the
cracks of a glaze are dangerous. There is a second thing to consider. Your
body fights off small numbers of dangerous bacteria all the time. You never
even know it. You get sick only when your body receives 1) a large number of
bacteria all at once and can't destroy them quickly enough or 2) the
bacteria are particularly virulent or 3) you are in a particularly weakened
state due to stress of some form (stress can include overwork, temperature
extremes, chemotherapy, ...). The cracks in the bottom of a cup are
relatively hostile places to bacteria - no food or water except for whatever
gets poured into the cup from time to time. Bacteria have a unique defense
to this situation. They form spores or live in a kind of suspended
animation. When a fresh supply of food and water is provided by your pouring
something into the cup, they can come out of their protected state and
multiply. But this takes time. By the time that they can even get into gear
about multiplying, you have finished drinking your tea, washed the cup out
and placed it back in the cupboard. If you receive any bacteria, it is a
small number that your body can easily defeat.

The other issue regarding being food safe has to do with the glaze. Here's
the real problem. Many raku glazes make use of metals that are not good for
you. Besides containing nasties, many raku glazes are not formulated to
produce a good glass that would seal in the metals and prevent them from
leaching into food. A classic copper matte would be an extreme example of
this - almost all copper compound and just enough glass former to hold it
onto the pot.

Raku adds one more complication that is very difficult to overcome. In most
other forms of glazed pottery, you can usually find a formulation that
either eliminates dangerous materials or else immobilizes them somehow,
usually by trapping them in the glass matrix. Once you think that you have a
good glaze, you can send it off for testing to determine if you have any
leaching above acceptable limits. As long as you maintain your recipe and
firing you should be OK with that glaze. The very nature of raku firing
makes it impossible to assume that a glaze produced in one firing will act
like the same glaze in a different firing. The atmosphere in the kiln is
often very uneven. The length of time that pieces are allowed soak at the
glaze melting point often varies. Lord only knows what kinds of variables
are introduced by the post-fire reduction.

So the only thing that you can do to ensure a food safe raku glaze is to
formulate the glaze with materials that are not dangerous to begin with or
are otherwise stable in themselves so that they do not rely on the glass to
prevent leaching.

I'm sorry that was so long. I hope that you realize now that your questions
were anything but dumb. This can be a very difficult arena to play in. I
hope my explanations made sense. And if Monona jumps on my butt about the
bacteria thing (or any part of this), well, listen to her - she's got a lot
more experience in that sort of stuff than I.

Bruce "hope you like your white cups" Girrell