Choosing the Right Material:

http://www.rayotek.com/tech-specs/material-comparisons.htm

This site compares the various high temp glass materials on the market. The temperature ranges are listed as well as the max temperatures each are able to tolerable tolerate for instances. 

It also lists the pros and cons of the various materials. 

Metal doors with furnace observation windows:

http://www.gageglass.net/observation/saf-t-eye.html#Dimensions

Blast gates, high temperature NPT threaded sights, etc

http://www.boilersupplies.com/ports/index.html

and

http://www.tate-jones.com/peepsights.html