Are coffee mugs oven safe?
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.
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.
Heh? Earthenware *is* oven safe. There are lots of earthenware dishes intended specifically for the oven, I've used them often, and I own one (pic in http://cooking.stackexchange.com/questions/17779/how-to-take-care-of-glazed-earthenware). Maybe there are some glazes used on mugs and not baking dishes which have a chance of leaking or similar, but in itself, earthenware doesn't mind high temperatures.
@rumtscho, you are perfectly right. There is a lot of cookware made of Earthenware; glazed and unglazed. However, Earthenware needs to be handled with care. See FuzzyChef’s advice for more info. Although Earthenware is used for cooking, it’s not oven safe, quite simply because its oven safeness is defined by the way it’s used, or rather the experience of the user, and not by the material.