The Phoenicians invented an iron tube for blowing this substance.  
Fred Zachariasen came up with namesake rules for how this material is formed.  A highly stable form of this material is made through the Vycor process.  In the Renaissance, the most highly valued type of this good was made from cogoli and was improved by the addition of manganese oxide.  When industrially making this material, it's common to use a batch containing as much as 60% of low-value (*) cullet.  It's not silk, but Venetian craftsmen who made this good worked on the island of Murano and forbidden from leaving the city.  A type of this material made for labs and kitchens is known as Pyrex.  This material is traditionally made by combining fluxes with sand. The Phoenicians invented an iron tube for blowing this substance.  For 10 points, name this material whose clear variety was essential in the invention of spectacles.
Fred Zachariasen came up with namesake rules for how this material is formed.  
glass