What is Vitrelle?
Corelle plates, bowls and open handle cups are products of a glass lamination process, formed from a three-layered sheet comprising two different types of glass. The sheet's core layer is white opal glass sandwiched between a transparent top and bottom skin. The layers provide superior strength while allowing it to be very thin, like plywood is quite strong without being very thick. The brand name of this glass-laminate is Vitrelle.
In more technical terms, the two kinds of glass possess different coefficients of thermal expansion. This mismatch means that the outer two layers are in compression (pushing) and the core layer is in a state of tension (pulling), with the stresses delicately balanced with each other. This enables a glass-laminate to resist damage from impacts that might break ordinary glass of a similar thickness. But Vitrelle is not completely immune to breakage, and when the threshold is crossed it will shatter more forcefully than ordinary glass.
Small defects on the edges of Corelle 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 Vitrelle's manufacturing process, the two types of glass flow from two different melters, meeting 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 travels through rollers that flatten it to the correct thickness. The continuous ribbon passes 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, similar to 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. The cut edge is smoothed and sealed by fire polishing where hot flames just briefly melt the edge of the glass.
After decorating, 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 developing 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, white-hot gobs of homogeneous glass are dropped into a mould and cast into shape when the top half of the mould presses down upon it. It is possible to make almost any type of dish with this method, including closed handle cups. The only requirement is that the interior of the item must be widest at the top and narrowest at the bottom, a slight conical shape, or "draft angle".
Some thin Pyrex products 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.
Corning and the Craft of Innovation, Margaret B.W. Graham and Alec T. Shuldiner.
"Apparatus for Forming Glass Articles", James W. Giffen, United States Patent 3,231,356
"Opal Glass Compositions", William H. Dumbaugh, James E. Flannery, George B. Hares, United States Patent 3,661,601
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