Vitrelle

Corelle plates, bowls and open handle cups are products of a glass lamination process, formed from a three-layered sheet comprised of two different types of glass.  This sheet has a white opal core layer and a transparent top and bottom skin, like a sandwich.  The layers provide superior strength while allowing it to be very thin, like plywood is quite strong without being very thick.  The official name of this laminated glass material is Vitrelle.

In more technical terms, the two kinds of glass possess different coefficients of thermal expansion.  Due to this expansion mismatch the outer layers are in compression (pushing) and the core layer is in a state of tension (pulling), and these stresses are in a delicate balance with each other.  What this means for everyday dinnerware is that this glass can take a certain amount of abuse that ordinary glass of a similar thickness would not withstand.  But when the threshold is crossed it will shatter, and do so more forcefully than ordinary glass.

On more complex pieces of Vitrelle, like hook-handle cups, small defects on the edge can provide a glimpse of the two types of glass meeting.  The defect looks lumpy, but feels smooth because of the transparent skin on the surface.

During the manufacturing process, the two types of glass flow from two different melters and they meet at an orifice specially built to combine them carefully into three layers without mixing them together.  Upon exiting the orifice, the ribbon of glass has the consistency of warm toffee, and it passes through rollers which flatten it to the correct thickness.  The continuous ribbon travels to the rotating outer face of the Hub Machine, which resembles a ferris wheel, but with several moulds fitted around its circumference.

Turning the ribbon of hot Vitrelle into Corelle dishes is a process called Thermal Vacuum Forming.  The hot glass sheet is drawn down and stretched by suction into a mould and the edge is cut.  It is very much like using a cookie cutter.  By the time the pieces are taken from the Hub Machine they have cooled enough to solidify and hold their shape.  Edges are smoothed and sealed by fire polishing. 

After decorating, the pieces are sent off to be air tempered, or annealed, a controlled heating and cooling off cycle which further improves the strength of the glass.  Tempering deepens the effect of the compression layer beyond the transparent skin.

Corelle's structure and composition are unique amongst Corning's range of household products.  Vitrelle is not just a thin version of opal Pyrex or Corning Ware, and it is not a glass-ceramic at all.  The opal glass used in the core of Vitrelle is not related to opal Pyrex either.  The glass that gives Vitrelle its consistent pure white colour is a spontaneous opal, an entirely new type of glass invented during Corelle's development.  Spontaneous opal glass turns white almost instantly when it begins to cool and harden.  Conversely, opal Pyrex is thermally opacified, its colour develops when it is re-heated in the annealing process, and its whiteness can vary noticeably from batch to batch.

Forming glass articles on the Hub Machine contrasts with the manufacture of more conventional glass and glass-ceramic products. In making thick pressed Pyrex for example, white-hot 'gobs' of homogeneous glass are dropped into a mould and cast into shape.  It is possible to make almost any type of dish with this method, including closed handle cups.  The only proviso is that the interior of the piece needs to be widest at the top and narrowest at the bottom, a slight conical shape.  In the casting business, this is known as a draft angle. 

Other thin Pyrex pieces are narrower at the top than the bottom, like salt shakers and laboratory flasks.  These are made with a two step blow-moulding process.  The gob of glass is pre-pressed approximately into shape then moved to a second mould where air is piped in, forcing the hot glass into its final shape.

Sources:

Corning and the Craft of Innovation, Margaret B.W. Graham and Alec T. Shuldiner.

United States Patent 3,231,356, "Apparatus for Forming Glass Articles", James W. Giffen.

United States Patent 3,661,601, "Opal Glass Compositions", William H. Dumbaugh, James E. Flannery, George B. Hares.