For this glossary we have selected those words which will be most helpful as you learn more about the general subject of glass. Our goal is to define the most frequently used terms you will encounter or used to describe the glass forming processes or types of glass.

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Annealing: Under natural conditions, the surface of molten glass will cool more rapidly than the center. This results in internal stresses which may cause the glass object to crack or shatter. The annealing process is designed to eliminate or limit such stresses by submitting the glass to strictly controlled cooling in a special oven known as a "lehr".

Bakeware: This product category includes products used for food preparation such as casseroles, baking dishes, pie plates, cake pans, mixing and batter bowls and measuring cups. These products are suitable for microwave, convection and conventional oven use as well dishwasher safe. These products are not suitable for stovetop, broiler or toaster oven use. Batch: The name given to the mixture of various raw materials and cullet which is fed into the furnace and is melted into glass.

Bead: An enlarged, rounded top edge of a tumbler or other glass article, usually created by the burn-off or glazing process. (see burn-off) 

Blank: Any preliminary shape of glass that requires further forming, finishing or decorating.

Block Mold: The simplest type of mold for pressing glass, having no joints, they are usually the easiest molds to make and easiest to maintain, no vertical seams.

Blow Mold: The mold used for forming a glass article by using air pressure.

Blown Glass: Typically thinner glass than articles made by pressing, article formed by air pressure or mouth blown.

Borosilicate Glass: Glass is made from silica and boric oxide. Such glass is highly resistant to temperature change (thermal shock) and is particularly suitable for laboratory ware (test tubes, etc.), domestic cooking ware (baking dishes, etc.), high-power lamps and other technical glassware. (see heat resistant glass)

Burn-off: The method whereby a glass tumbler is flame cut to deliver a finish resulting in a smooth bead on the top edge. (see bead)

Crystal Glass: This type of glass is produced when lime in the batch is replaced by lead oxide. The composition of lead crystal is 54-65% silicon dioxide (SiO2), 18-38% lead oxide (PbO), 13-15% soda (Na2O) or potash (K2O), and other oxides. Such glass has a high refractive index and is particularly suited for decoration by cutting.

Crack-off: The method whereby the glass tumbler is scribed to create a fracture point to deliver a finished edge, which usually requires grinding and polishing.

Cullet: Broken or scrap glass which can be re-melted and used again.

Cut Glass: Glassware decorated by grinding figures or patterns by abrasive cutting wheel.

Decorated: This product category includes glass, which has been silk-screened, decaled, sprayed, sandblasted or etched.

Drinkware: This product category includes glass tumblers, goblets, stemware, bar glasses, decanters and mugs/cups.

Finishing: Processes of cracking off, glazing, grinding and polishing glass after it has been blown or pressed.

Fire Polishing: To make the glass smooth, rounded or glossy by heating with an intense flame. (see glazing)

Feeder: A brick channel whose function is to receive glass from the furnace, reduce its temperature to the desired level and allows the molten glass to freely flow to the glass forming machinery.

Frit: A batch material which is melted with other materials or elements in order to achieve colored glass.  

Glass Tank: A melting furnace in which the raw batch materials are melted.

Glazing: Creating a smooth, shiny surface, or glossy appearance with an intense flame. (see fire polishing)

Glazier: The automatic machine used for glazing glass articles.

Gob: A hot portion of glass, cut and delivered to the mold for forming.

Heat Resistant Glass: Glass able to withstand high thermal shock, due to low expansion coefficient or tempering. Glass suitable for microwave or conventional oven use, can not withstand direct flame or electric heat sources. (see tempered or borosilicate glass)

I.S. Machine: (independent/individual section) Container (bottles) forming machines are made up of individual but identical sections placed side by side in line. Each section comprises an arrangement of mechanisms with gears enabling the sections to be started or stopped independently of the others, making the I.S. machine more flexible than continuous- or intermittent-motion rotary machines.

Lehr: A special type of oven or kiln used specifically for annealing glass (see “annealing”). In industrial production, it usually has a moving belt to carry the glass through at controlled speeds, and is divided into different areas each with its own heat source, making it possible to carefully regulate the temperature gradient to which the glass is submitted.

Mold Seam: A mark on the glassware resulting from a mold joint. (see two part mold)

Paste Mold: An iron mold lined with carbon or a paste usually made from a material which when coming in contact with high heat leaves a carbon like surface for the glass to be formed against. The resulting glass article usually has a smooth outside surface.

Pressed Glass: Glassware formed by placing a gob of molten glass in a metal mold, then pressing it with a metal plunger to form the inside shape. The resultant piece has an interior form independent of the exterior. The products made with process typically are thicker and heavier than blown glass products.

Serveware: This product category includes such items as small and large bowls, plates, serving trays, creamers, shakers, butter dishes, sugar bowls, cake plates, pitchers, footed bowls and plates, punch bowls and fountainware.

Soda-lime Glass: The most common type of industrially produced glass. A typical soda-lime glass is composed of silica (71-75%), soda (12-16%) and lime (10-15%), plus small amounts of other materials to provide particular properties such as color.

Silk Screening: A process for the decoration of glass whereby colored paint is forced by a flexible “squeegee” through a fine-mesh screen, or “mask”, (traditionally made of silk, now also made of nylon, polyester and stainless steel) onto the glass surface. A separate mask is used for the application of each color.

Stemware: The collective term given to drinking glasses whose body is connected to the base by a thinner column of glass.Storage: This product category includes small and large jars with glass, wooden, plastic, ceramic or metal covers.

Tempered Glass: A special process of solidification of a glass article in order to make it resistant to thermal shock. The glass is heated to a temperature just below its softening point and then immediately cooled by special jets of cold-air. This hardens the inside and outside surfaces of the glass. This type glass is best suited for bakeware products.

Two Part Mold: This mold type is required to produce products that have engraved patterns on the outside surface. These kinds of molds open and shut on hinge. The result is vertical mold seams. This type mold can be used for pressed or blown glass products.