Why do metal ions appear in Pyrex glass?

[mbrmbrg]

Metal ions in Pyrex--why?

According to http://en.wikipedia.org/wiki/Pyrex#Composition" is composed of oxygen, silicon, calcium, and sodium.

I've Google Scholared to no avail...

Thanks!

 

[Borek]

What purpose does the carbon serve in hemoglobin?

I would say - none. Glass composition was experimentally determined so that it has needed properties. Metal ions were not added for a "purpose".

 

[mbrmbrg]

Glass composition was experimentally determined so that it has needed properties.

From that statement, I would conclude that without the metal ions, glass does not have certain needed properties, and with metal ions, it does. My question is: what are those needed properties that are effected by the various ions?

 

[Borek]

Needed properties are temperature at which glass becomes soft, coefficient of thermal expansion, chemical resistance and so on. But I strongly doubt it is possible to assign single property to a single ion.

 

[mbrmbrg]

I hear that. But I really want to know! (Ah, if only that made a difference...)