Hello Justin,

First of all, a question like this should be answered by an expert in materials science, and I’m not. Bear that in mind when you read the following.

Generally, you should not take it for granted that coffee mugs are oven safe. Quite simply, some are and some are not.

However, high quality coffee mugs produced for the catering industry; especially the white ones with no decor, are normally oven safe. Nevertheless, I will not give you any guarantee.

Basically, coffee mugs cannot be regarded as oven safe, unless the manufacturer have stated so with a stamp underneath or otherwise have published some kind of guarantee, and the manufacturer is a respected and well-known entity within the industry.

Why coffee mugs aren’t always oven safe

A coffee mug isn’t always what it seems. Coffee mugs come in many varieties, are made from different materials, differ in decoration, and so on.

Moreover, coffee, tea, hot chocolate, and other hot drinks commonly served in mugs, are seldom; read never, served at temperatures above 100 ºC / 212 ºF. However, due to competition, special on sale items, and so on, coffee mugs of low quality might not be able to withstand temperatures much higher than this. The reason is simply that manufacturers, or the manufacturers’ customers, want to save a few pennies to gain a market advantage. There might also be a consumer demand for cheap low quality coffee mugs.

Materials

Most coffee mugs are made from some kind of ceramic. Others are made from wood, plastic, clay, pewter, steel, and a long list of other materials. Obviously, some of these materials will never be reckoned as oven safe. To keep it simple, I’ll stick to ceramic for the reminding part of this answer, although other materials might be oven safe.

Ceramics commonly found in a kitchen environment include Earthenware, Stoneware, Porcelain, and Bone china. Of these Earthenware and Bone china are seldom, read never, reckoned as oven safe, although this isn’t completely true as far as Bone china is concerned. Nevertheless, only a complete idiot would use a Bone china mug to make French Onion soup au gratin, cup cakes or any other kind of food.

That leaves us with Stoneware and Porcelain. Both Stoneware and porcelain are normally oven safe, unless decor elements have been added after the finale glaze is applied and the coffee mug is baked in a furnace.

One final remark needs to be added about coffee mugs made out of thin porcelain. Although these mugs might very well withstand the heat, it’s advisable to ensure that they are not exposed to any other kind of force while hot, e.g. handled without care.

Further information about Coffee Mugs, and links to other resources, can be found at Wikipedia: Coffee Mug.

Bottom line

When you use both racks at once, your cooking pattern will become somewhat uneven.

Normally, you will only see this as a serious effect in thin, rapidly cooking foods like cookies. Some cookies on the tray may be browned and done, while others are still pale and underdone. This is why cookie recipes almost always specify swapping the trays between the racks, and rotating them front to back, half way through. The idea is to balance out the influence of these various affects.

For thicker and longer cooking foods like a casserole or a braise, internal conduction or convection will tend to even out the influence of the local hot and cool spots over time, making use of both racks fairly irrelevent to the outcome.

Background

Ovens transfer heat into food in two modes:

• Infrared radiation (much like a heat lamp puts out) warms the surface of the foods (or their containers) directly
• The hot air in contact with the food (or their container) conducts heat into the surface

Furtheremore, the conduction of heat into the food from the air is strongly influenced by the convection (circulation of the air due to temperature differentials). If the air were pefectly still (which it never is), then the part that is in contact with the food would soon come into thermal equilibrium with the food. As air is a very poor thermal conductor, it would then take some time for it to heat up from the adjectent air, and the food would cook very slowly.

In the real world, air also changes density when the temperature changes, becoming denser and sinking as it cools, and becoming less dense and rising when it is heated. This causes the air in the oven to circulate, even if there are no fans, and is called convection. It increases the ability of the air to transfer heat energy into the food.

There are also ovens which have fans to force air circulation, technincally called forced air convection ovens, although often simply called "convection ovens" for short.

Understanding these modes of heat transfer allow you to understand what happens when you use two racks within the oven:

Shadowing

As radiant heat is a major mode of heating, and infrared is light (just of a wavelength too long for people to see), it can be blocked and cast a shadow.

When you use only a single rack, the food (or its container) is bathed in the radiant heat from all of the walls (and floor and ceiling).

As soon as you use two racks, the food on one rack absorbs and blocks the radiant heat from at least one direction from getting to the food on the other rack.

This will cause the baking to slow down slightly, and change the pattern by which heat reaches the food.

Convection patterns

When you use more racks, the pattern of natural convection within the oven will change. Its like being in a city where the wind patterns are affected by the shape of the buildings. This can create hotter (more air going by) and colder (less air going by) spots within the oven.

If you have forced air convection (a fan), this affect is reduced but not eliminated, because the air still must circulate past the foods to actually transfer heat energy into them.

A note on on oven design

All ovens have radiant heat as a major cooking modality. The hot air from burner or heating element heats the walls of the oven, which then begin to emit infrared radiation back to the foods.

Some ovens also directly expose their heating element within the main oven cavity. In this case, infrared can be an even stronger mode, and become very directional.

This is particularly true of electric ovens with directly exposed coils. The shadowing affect becomes stronger in this type of oven.