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Pyrex: How It's Made

In its earliest form, Pyrex brand ovenware was made of clear borosilicate glass, with each piece being hand blown by individual gaffers. The process was labor-intensive and expensive.

Slumping sales in a depressed market forced Corning to turn to professional consultants who ultimately recommended cost cutting through automation. Thus, after 1929 the production of Pyrex was changed to a machine-pressed glass process. Corning was able to reduce Pyrex prices by as much as half.

Later, in 1936, Corning merged with MacBeth-Evans Glass Co., already a producer of machine-pressed opalware. Wartime government contract production of a durable military mess ware and the subsequent entry into commercial dinnerware and tableware would lead to the Pyrex opal colorware of the late 1940s and beyond.

About Pressed Glass

The components which go into the particular glass formula are first mixed and then heated to a molten state in an over-2500°F furnace. For opalware Pyrex, these included silica sand, soda ash (sodium carbonate), lime (calcium hydroxide), plus quantities of some other raw materials.

Pressed glass is an automation line process. The molding apparatus consists of multiple parts, including a bottom half into which a measured amount of molten glass is deposited, a top half which presses the blob to conform to the shape of both it and the bottom mold half, and often a die or slug which makes an embossed impression such as a trademark or model information. Many bottom molds are mounted on conveyor or rotational mechanisms. Typically, a single top half on a press mechanism is used, and multiple bottom halves, after having received a portion of molten glass appropriately called a "gob", are sequentially positioned under it.

As the molten glass flows from an outlet in the furnace, a mechanism much like a scissors cuts off a predetermined quantity of glass. A side effect of this action is that the metal cutter causes the glass at the point of contact to prematurely begin to cool. Artifacts left in the finished product may include wavy lines, and occasionally in the case of clear glass, areas of slight hazyness. Post-pressing flame polishing may or may not remove these.

Some excess glass squeezes out at the point of separation of the mold halves. This excess, or flashing, is fired off or melted flush by flame jets just prior to the piece's removal from the mold.

The glass is now returned to a solid, but must be put through a final annealing process. By subsequently heating the glass piece to a specific but sub-melting point temperature and then allowing it to cool at a predetermined rate, annealing serves to reduce internal tensions introduced in the manufacturing process, improving durability.

Finally, pieces are hand-inspected for quality and defects. Pieces which do not pass are sent to "cullet" for remelting and recyling into new pieces.

Next: the Pyrex opalware decoration process.