Why a water bath is important when cooking custards and flans

From Harold McGee’s

The Surprising Science of Water Baths

Most cooks know that oven heat can be moderated with a water bath.  Though the oven may be at 350°F, the liquid water can’t exceed 212 °F/100°C, the temperature at which it boils and turns from liquid into vapor.  Less well known is the fact that the water temperature can vary over a range of 40°F depending on the pan containing the water and whether it’s covered.  A pan of water is heated by the oven, but it’s simultaneously cooled as water molecules evaporate from the surface.  The actual water temperature is determined by the balance between heating of the water mass through the pan, and evaporative cooling at the water surface.  More heat accumulates 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 195°F/87°C, a glass bath 185°F/83°C, and a stainless one180°F/80°C.  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 185°F; otherwise the mix may never completely set.  Many cooks take the precaution of folding a kitchen towel in the bottom of a 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.