On Food and Cooking

in a thick cast iron pan or passes through
infrared-transparent glass than is
transmitted by thin stainless steel. So in a
moderate oven, a cast-iron water bath may
reach 195ºF/87ºC, a glass bath 185ºF/ 83ºC,
and a stainless one 180ºF/80ºC. If the pans
are covered with foil, then evaporative
cooling is prevented, and all of them will
come to a full boil.
Custards are tenderest when heated
gently, and so are best cooked in an open
water bath — one, however, that is sure to
reach at least 185ºF; otherwise the mix
may never completely set. Many cooks
take the precaution of folding a kitchen
towel in the bottom of the water bath so
that the custard cups or dish won’t be in
direct contact with the hot pan, but this can
backfire: the towel prevents the water from
circulating under the cups, so the water
trapped there reaches the boil and rocks the
cups around. A wire rack works better.