Build platform - why glass?

[BenTheRighteous]

So given the "chipped glass" thread I've been posting in lately, I've gotten to wondering why we use glass for the bed.

I know nothing about material science so there may be an obvious reason, but to me it seems like we'd be looking for: strong, flat, low thermal CoE, good thermal conductivity, cheap.

Is boro glass really the best/only choice? There have got to be other materials to satisfy all the criteria. Why don't we use something like machined stainless steel disc instead? Or a giant frying pan? Or a cooking stone.

http://reprap.org/wiki/Heated_Bed#Metal

Seems like no matter what material we pick, we'd still be able to add our "adhesion modifiers" such as glue, tape, etc.

[Jimustanguitar]

I've got the aluminum for the job, but I've still always used glass.
http://www.sandsmachine.com/alumweb.htm

I don't have a problem with glass that makes me want to switch. It's inert, easy to put on and take off, easy to clean up, easy to apply adhesion promoter to, just straight enough yet just flexible enough, etc. I started with plain old window glass and was happy with it except that I had to cut it myself, switched to boro glass afterwards, and haven't had a lick of trouble. There might have been a bad batch in recent history or some other manufacturer's flaw, but glass works great as far as I'm concerned.

People use aluminum beds as heat spreaders instead of the surface to print on.

[0110-m-p]

There are a few nice things about glass vs. something like aluminum.

- It has very low thermal conductivity. This means that it takes longer to heat and cool than something with a high thermal conductivity like aluminum which results in less temperature fluctuation. (I consider this a benefit, but others will argue that high conductivity is better).
- It has a low thermal expansion coefficient. This means that it doesn't change dimensionally as much as another material like aluminum.
- It is a very cheap way to get a very flat build surface. To get aluminum nearly the same flatness as glass you need Mic6 or better plate which costs at least twice as expensive as a borosilicate glass plate.
- Glass is also easy to clean and is a very hard surface that isn't easily damaged. Aluminum and others can be scratched and gouged easily.

Obviously glass has its downfalls though....mostly the fact that they are fragile and can't be easily cut/machined.