Not sure if this is the correct place to post this question.
I purchased a beautiful ceramic coffee mug in Rhodes, Greece just a few
weeks ago. I was assured by the shopkeeper that the mug was dishwasher
proof and made without lead. After using the mug just a few times,
the glaze on the inside of the mug has begun to crack into a spiderweb
pattern! At first I wasn't too concerned because I couldn't feel the
cracks. They were below the surface. But now some of them are on the
surface. Is it safe to drink from this mug?

--
TickTock

Tick,

You will probably get some different input on this. The issue with the
"spiderweb" is called crazing. It is caused by a either a slight or severe
"misfit" of the glaze to the clay body. The result of which is the glaze not
contracting/expanding at the appropriate rates. Commercial manufacturers
have chemists that formulate clays and glazes that "fit" each other
perfectly so that they can withstand drastic amounts of thermal shock. Often
times however with handmade pots the fit is not as precise. It _is_ a
defect, but a fairly common one.
As far as the saftey of the mug. If of course you find or feel that
pieces are going to come off then the cup should not be used. In the U.S.
and other countries what the main issue with crazign has become is that, in
theory, if the glaze is crazed, the contents of the vessel can leech through
the cracks in the glaze and get into the clay body itself. The thought then
is that bacteria can grow within the clay and then be leeched out back into
subsequent contents that are put in the cup. So in this scenario the coffee,
milk, and sugar (if you drink regular) can migrate into the clay and begin
to grow. Then bacteria can grow and leech out into your subsequent cups of
coffee down the road. Thats the stance from the most letigious society on
the planet. The level of migration depends on many factors, how vitrified
the clay body is, etc.. I always chucke when this topic comes up in the clay
community as we see people smoking, talking on cell phones excessively,
eating horribly, dumping hundreds of pounds of chemicals on their lawns,
among many other things far more detrimental to ones health.
That said, there are cultures on the planet that have cooked, stored,
prepared, and eaten food and drink from vessels with this defect for
thousands and thousands of years. Raku wear (considered not food safe in the
U.S.) has been used for thousands of years in other cultures. Many, if not
all, potters will work feverishly to try to tune their glazes to thier
claybody to try to overcome this but often times the match is not quite
perfect and the pots are still sold. Often times crazing can happen months
and months and in some cases even years after the pot is made. In your case
it seems a bit more severe. The reason the crazing is so prevalent in the
mug is due to its use. The room temperature mug, filled with scalding hot
coffee exerts a major thermal shock on the mug. The same thing happens with
the dishwasher. The intent of the mug is for coffee so it should be able to
handle the shock. Your mug is not handling it as well as it could. We test
our mugs in a crude manner of freezing the mugs for a day, boiling water,
get the mug from the freezer and fill with the boiling water. We do this
over and over. If we hit 5 or more cycles we feel ok about the fit. Its not
easy to do. Hence we dont make a lot of mugs.
This doesnt give you a concrete answer of wether or not to drink from
the mug but gives you some info to help you make the decision for yourself.

Mark

What you have there is either a bad fitting glaze or a claybody that needs
some tweeking or both.
If the claybody is vitrified and the glaze is within safe limits, i.e. having
enough alumina and silica for its fired temperature and isn't overly loaded
with problem causing oxides, then the high heat of a dishwasher would probably
kill any germs that might get into the cracks.
Crazed glazes are not that uncommon. It doesn't make them very desirable, nor
necessarily unsafe, or safe.
Coffe and tea will stain the cracks and make them more prominent eventually.
Personally, I would take it back to the potter. He may appreciate knowing what
happened. Material batches often change and although this glaze may have worked
for him in the past, slight alterations in his claybody or the raw materials
used in the claybody or glaze may have changed enough to lower the coefficient
of expansion of the glaze which would promote the crazing you're seeing; or it
may be a new glaze that came out looking fine in initial tests and that he
didn't keep around long enough to run it through futher testing that would have
shown up the problem.

Regards,
June