QUESTION: WHEN WRAPPING POTATOES WITH FOIL ( for baked potatoes ), SHOULD THE SHINY OR THE DULL SIDE BE FACING OUT?
When aluminum foil is being manufactured, two layers of aluminum go through the rolling process at the same time, face to face. The two surfaces (top and bottom) that are in contact with the highly polished steel rollers, get a shiny finish due to contact with the rollers. The two inner surfaces (face to face) do not come in contact with the rollers, and have a matte (dull) finish.
In other words, the two surfaces, a dull one and a shiny one, are just a coincidence of the efficient manufacturing process of making two rolls at the same time. It is not done for any particular purpose.
(There are specialty foils that are made one roll at a time, where both sides come in contact with the polished rollers, and so both sides are shiny. It is just more efficient (cheaper) to make two rolls at the same time.)
To understand heat and cooking times: * Convection is the transfer of heat in a fluid (liquid or gas) from a warmer area to a cooler area. This is how an normal oven works. The heat is transferred from the hot air to the cooler potato. (All ovens are 'convection' ovens - ovens with the designation 'convection oven' simply have a fan that moves the air faster, which causes faster heat transfer - both regular and 'convection ovens cook food by convection).
* Radiant heat is heat transferred by radiation. Sunlight is a source of radiant heat. So are microwaves. So are the elements in a toaster. So are the glowing coals of a fire. Aluminum foil reflects radiant heat.
OVENS When baking a potato, the heat that cooks the potato in an oven is primarily by convection, not radiant heat. (The amount of radiant heat emitted by the walls of the oven is much less significant to the cooking process). The temperatures involved are generally less than 500 degrees F. Metals are efficient conductors of heat, and the foil is in direct contact with the potato, the fact that the potato is wrapped in foil (or not wrapped) is much more important than whether the dull or shiny side faces out. There might be a difference in cooking time between dull and shiny sides of the foil, but it would be measured in seconds, or at most maybe a minute.
RADIANT HEAT - GLOWING COALS AND BROILERS Cooking with glowing coals or in a broiler, where the heat source emits visible light, and the temperatures involved are generally 1000 degrees F. and above is different. As long as the potatoes are fairly close to the radiant heat source (a few inches) the potato is being cooked by a combination of radiant and convective energy. A foil wrapped potato reflects some of the radiant heat - so the potato will not be overcooked on the outside and undercooked on the inside. But again, the difference in the amount of heat reflected by the dull or shiny side of the foil is relatively insignificant. But it is still present, and the shiny side out potato will take longer to heat, again, measured in seconds or possibly a minute or so.
I hope I have explained this without being too confusing.