Glass is formed by the rapid cooling of molten substances below their glass transition point without time allowed for regular crystal lattices to form (Matbase 2011; Lenntech 2011). The specific glass that is likely used for these lids is a borosilicate glass, composed of a mixture of silica, boric oxide, and certain oxides; silica is the main component in almost all commercial types of glass, and the boron helps to increase the heat resistance of the glass (Lenntech 2011). Like other types of glass, this glass would also be resistant to corrosion and is entirely non-porous, meaning it will not become stained or degraded by foods nor will it impart any tastes (Matbase 2011; Lenntech 2011). Forming Processes

There are several different possible methods that Le Creuset might use in the actual forging of the aluminum and the stainless steel that make up the composition of these cooking pans. Given the malleability of aluminum and even the stainless steel (especially into the uniform and non-complex disk that is needed for this application), the cold closed-die forging process could be successful here (Totten & MacKenzie 2003). Heated closed-die forging could also be used, though it is not clear that this would have any advantages -- it would simply require more energy expended in creating heat, and slightly less energy needed in the press used to shape the metal pieces (Totten & MacKenzie 2003).

Anodizing is a process that increases the hardness of aluminum, and also creates a less penetrable finish (Kaufman 1999). Basically, aluminum oxides are created in a chemical reaction that is instigated by soaking the aluminum product in an acid bath and applying an electrical current; "home brew" techniques similar (though opposite) to some tips for polishing silver can actually lead to fairly strong anodization, but hard-anodizing processes such as those employed by Le Creuset require much colder temperatures...

The basic process is the same, however, and the aluminum oxide that forms from the aluminum surface itself turns into a layer of aluminum hydrate that actually is embedded half in the surface of the aluminum and half extending out of it, creating a layer that is much harder than the aluminum itself (Kaufman 1999).
Conclusion

At least since the Atone Age, mankind has used certain tools to help them cook their food. Flat rocks used as primitive griddles have clearly come a long way, but humanity is still finding it necessary to develop materials to expand their capabilities in a very basic and essential task for survival -- the preparation of food. The specific properties of aluminum, steel, and glass, as well as the processes that were used in the manufacturing of these products, are the reasons behind the performance of these pans and the legacy of Le Creuset's quality.