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Glass Industry Glossary of Terms

Glass Industry Definitions

The following terms are used in the Glass industry




Acid etching

This process for the decoration of glass involves the application of hydrofluoric acid to the glass surface. Hydrofluoric acid vapours or baths of hydrofluoric acid salts may be used to give glass a matt, frosted appearance (similar to that obtained by surface sandblasting), as found in lighting glass.

Glass designs can be produced by coating the glass with wax and then inscribing the desired pattern through the wax layer. When applied, the acid will corrode the glass but not attack the wax-covered areas.

Acid polishing

A process used in the production of cut crystal to remove the opacity of etched surfaces where decoration has been applied. Items to be polished are immersed in a mixture of demineralized water, sulphuric acid and hydrofluoric acid, and then rinsed. There may be a single short immersion in a stronger solution or, alternatively, a series of immersions in a weaker solution.

Acid stamping

The process of acid etching a trademark or signature into glass after it has been annealed, using a device that resembles a rubber stamp.

Air trap, air lock

An air-filled void, which may be of almost any shape. Air traps in stems are frequently tear-shaped or spirally twisted.

Alkali-borosilicate glass

A special glass used for glass-to-metal seals, particularly suitable when electrical qualities are not important.

Alumina-silicate glass (1)

Alumina (aluminium oxide Al2O3) is added to the glass batch in the form of commonly found feldspars containing alkalis in order to help improve chemical resistance and mechanical strength, and to increase viscosity at lower temperatures.

Alumina-silicate glass (2)

A special glass used for glass-to-metal seals, particularly suitable when operating temperatures of electrical components are high (up to 750°C).


Under natural conditions, the surface of molten glass will cool more rapidly than the centre. This results in internal stresses which may cause the glass sheet or object to crack, shatter or even explode some time later.

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”. Inside the lehr, the glass is allowed to cool to a temperature known as the “annealing point”. When the glass reaches this point, the lehr temperature is stabilized for a specific length of time (depending on the glass type, its thickness, its coefficient of expansion and the amount of residual stress required) to allow stresses present in the glass to relax. This phase is followed by a period of cooling with a pre-defined temperature gradient.

Armour plate glass

Laminated glass, resistant to mechanical shock, composed of at least four panes of glass and usually at least 25 mm thick.


A strong vessel used for the lamination of glass under hugh pressure and controlled temperature conditions.

AZS refractories

Refractory blocks or tiles in varying proportions of alumina-zirconiasilica; initially used for areas where corrosion resistance was important but now used in most parts of the furnace.


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The application of decorative bands of enamel or precious-metal compounds, normally by machine, to containers such as tumblers, cups, cosmetics bottles, etc.

Barium crown glass

Barium crown glasses contain larger proportions of boron oxide and barium oxide with a relatively low SiO2 content. The glass can be stabilised against devitrification and weathering by adding small amounts of substances such as aluminium oxide


A term used to refer to the raw materials required to produce the desired type of glass once they have been weighed and mixed, and are ready for melting.


A process used widely in the production of bowls, plates, ashtrays, etc., whereby the shaped glass article (which may be pre-printed) still in sheet form is placed on a stainless steel, sheet steel or cast iron mould coated with talc or powdered chalk. The temperature is increased until the glass sheet sinks into shape in the mould.


The production, by abrasion, of a sloping edge on the glass sheet.

Commonly used on mirror glass.

Blow-and-blow process

A production process used for glass container manufacturing with forming machines. The elongated gob of molten glass formed by the gob feeder falls into the inverted parison (blank) mould. It is blown down into the mould (settle blow) before being blown from below

(counter blow) back up into the now closed mould. The inverted parison is transferred to an upright position in the blow mould where it is reheated before compressed air is introduced into the parison bubble. During blowing, a vacuum is applied through the mould to suck any trapped air or other gases from the bottom of the mould. A takeout mechanism then lifts the container from the mould.


An iron or steel tube, usually about five feet long, for blowing glass.

Blowpipes have a mouthpiece at one end and are usually fitted at the other end with a metal ring that helps to retain a gather.

Borosilicate glass

Glass made from silica and boric oxide. Such glass is highly resistant to chemical corrosion and temperature change (thermal shock) and is particularly suitable for laboratory ware (test tubes, etc.), domestic cooking ware (oven dishes, etc.), high-power lamps and other technical glass ware. It is also used when glass has to be bonded to metal and low expansion is a key characteristic.

Bottle-making machine

See I.S. machines

Boudin process

A glass rolling process in which glass flow is controlled by the speed of the machine and fed directly onto the rollers over a refractory sill. As the ribbon of glass passes from the forming rollers, it is supported by an air cushion. The process can be adapted in order to introduce wire mesh into the glass ribbon. (See also “Pilkington double-pass wired glass process” and “wired glass).


Gaseous inclusions in the glass melt which are removed by refining (see “fining”). Fining agents are introduced to encourage the formation of larger bubbles which rise more rapidly to the surface of the melt, attracting smaller bubbles on their way.

Larger bubbles which are not removed by fining are known as “blisters”, smaller ones as “seeds” and longitudinally stretched bubbles as “air-lines”. Bubbles in glass are generally considered as defects but may also be intentionally created and used as a form of decoration (see “air twist”)

Bulletproof glass

Armour plate glass which is more than 60 mm thick and which resists penetration by bullets.


Used to heat glass in furnaces of all sizes, burners mix air (or oxygen) and gas (natural gas or liquid petroleum gases) for efficient combustion.


Platinum alloy electrically-heated boxes with numerous nozzles in their bases used as furnaces for the forming of continuous glass fibre.

Glass can be fed into the heated bushing either in its molten state from a forehearth (direct melt) or, alternatively, as marbles to be melted (re-melt process).


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Cast glass

Glass produced by ‘casting’, in other words by pouring molten glass into a mould or by heating glass already contained in the mould until the glass melts and assumes the shape of the mould.

Centrifuging process

A relatively new method for the production of hollow ware such as borosilicate glass columns in chemical plants, funnels, television tubes and other non-rotationally symmetrical items by spinning. Molten glass is fed into a steel mould which rotates at the required speed. At high speeds, the glass can assume almost cylindrical shapes. When the glass has cooled sufficiently, rotation stops and the glass is removed.

Chalcogenide glass

Glass with electrical conductivity characteristics made with the addition of the chalcogen elements (sulphur, selenium and tellurium).


A thin layer which covers the surface of an object. Coatings may be applied to glass in order to alter the appearance or performance of the product in question e.g. anti-reflective coatings applied to auto mirrors to aid vision, coatings with photocatalytic and hydrophilic properties to make self-cleaning windows.

Cold end

The name given to the stage in glass production involving processing when the glass is cold. Cold end processes include grinding, engraving, cutting, etc.

Conductive coating

A glass coating which is electrically conductive. Conductive coatings have been used to produce frost-free windscreens, and in a range of electro-optical applications. One way of producing a conductive coating is by depositing tin salts onto the glass.

Containers, defects

Defects in automatically produced containers are categorized as very critical (Group 1), main faults (Group 2) and secondary (Group 3).

Group 1 defects make the container dangerous and unusable; Group 2 defects make the container unusable; containers with group 1 or group 2 defects must be discarded. Group 3 defects represent a lowering of the quality of the container but do not affect the functionality of the container.

Containers, enameling

The application of enamel as a means of applying decoration and/or labeling to containers. Enamel patterning or labeling is typically applied by automated silk screening; all-over color can be applied by spraying. See also Enamel

Containers, forming

The process of turning a gob of molten glass into a hollow container was first mechanized towards the end of the 19th Century. Fully automatic machines were developed during the first quarter of the 20th Century, principally in the USA, using the blow-and-blow process for narrow-neck ware and the press-and-blow process for wide-neck ware. The landmarks in the development of automatic forming of containers were the gob feeder in 1923, which automated delivery of consistently sized gobs of glass, and the individual section bottle making machine in 1925. The equipment in use today is descended from these innovations.

See also gob feeder, press-and-blow, blow-and-blow, I.S. machine, mould.

Containers, inspection

Inspection of glass containers includes the following: gauging or measuring; inspection for specific faults; proof testing.

Gauging or measuring checks: height, diameter and verticality; choke (inner and outer dimension of the neck); dips and saddles in the finish area (mouth/seal of the container); wall thickness.

Inspection for specific faults: cracks (also known as checks); stones; foreign material (tramps); spikes; birdswings; thin spots.

Proof testing: simulated impact; vertical load.

Crown glass

1. Window glass blown into a crown or hollow globe that is flattened and cut before use. This is produced by reheating and spinning out a bowl-shaped piece of glass (bullion) that causes the glass to extend into a flat disk by centrifugal force. The glass is then cut into the size required

2. One of the two principal types of optical glass used in the production of compound lenses. The Crown glass, which is an alkali-lime silicate optical glass, has a low index of refraction and low dispersion (its Abbe v-value is larger than 50 or 55, depending on its index).


The technique whereby glass is removed from the surface of an object by grinding it with a rotating wheel made of stone, wood, or metal, and an abrasive suspended in liquid.

Cylinder glass

A technique for producing sheet glass dating from the 11th century.

By blowing a hollow glass sphere and swinging it vertically, gravity pulls the glass into a cylindrical “pod” measuring up to 3 metres long, with a width of up to 45 cm. While still hot, the ends of the pod are cut off and the resulting cylinder cut lengthways and laid flat.


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Dalle glass

Coloured glass produced in pot furnaces and cast in moulds to form plates in thicknesses of approximately 25 cms. Dalle glass (“dalle” is French for “tile”) is used in church and decorative glazing, as well as for furnishings such as door handles.

Danner process

A widely used method for the production of glass tubing. The process was developed by an American engineer, Edward Danner, in 1912.

In the Danner process, the glass flow falls onto a rotating, slightly downward pointing mandrel. Air is blown down a shaft through the middle of the mandrel, thus creating a hollow space in the glass as it is drawn off the end of the mandrel by a tractor mechanism. The diameter and thickness of the glass tubing can be controlled by regulating the strength of the air flow through the mandrel and the speed of the drawing machine.

Day tank

A glass-containing vessel made from refractory blocks mainly used for the melting of batch for coloured glass, crystal glass and soft special glasses. Day tanks are refilled with batch daily, with melting usually done at night and glass production the following day. Used for producing larger quantities of glass than is possible with pot furnaces (see “pot”). The type of glass to be melted can be changed at short notice.


The name used to describe the batch feeding compartment within the furnace. The molten glass is covered with the batch material as it flows through the compartment.


A raw material compound (CaCO3 + MgCO3) of calcium carbonate and magnesium oxide, which helps lower the melting temperature in the production of flat glass.

Domestic ware

The collective term for glass containers used in the home (oven dishes, bowls, jars, etc.).

Drawn glass

A process for making sheet glass by drawing the molten glass as a sheet directly from the furnace. The thickness of the glass is determined by the drawing rate.


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The shaping or finishing of the edges of a glass surface, usually by grinding with an abrasive wheel.


A metal conductor through which electricity enters or leaves an electrolyte, gas, vacuum, etc.).


Carving or moulding in relief. The forming or application of figures or patterns to an object so that they stand out from the surface.


A vitreous substance made of finely powdered glass colored with metallic oxide and suspended in an oily medium for ease of application with a brush. The medium burns away during firing in a low-temperature muffle kiln (about 965-1300°F or 500-700°C).

Sometimes, several rings are required to fuse the different colors of an elaborately enameled object.


The production of a design in glass by cutting into the glass surface.

Engraving methods include copper wheel engraving, diamond or tungsten point engraving, acid etching and sand blasting.


A process for the production of continuous strips or rods of material such as glass and also the butyl used in the sealing of insulating glass units. The material, molten in the case of glass, is forced through a die and cut to the required length.


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A mechanism mounted on the casing of the forehearth which delivers the glass in gobs. The rate of flow of the molten glass is regulated by the use of different sized orifices in the feeder spout and by a plunger which pushes the glass through the orifice. Shears for the cutting of the glass flow into gobs are operated through the same cam system as that of the plunger to ensure constant gob size.


Also known as “felspar”. Any of a group of aluminium silicates of potassium, sodium, or calcium. Used in the batch as a means of adding alumina to the molten glass.

Fibre glass

Very fine strands of glass (normally with a high boric oxide and content) used in the form of glass wool for insulation, glass fibre for matting, etc., and also for the reinforcement of plastics.

The principal production process involves blowing jets of steam or air onto molten glass as it emerges from a tank furnace through very small diameter nozzles.


The process by which gaseous inclusions are removed from the glass melt after all batch materials have been added. Fining agents induce the formation of large bubbles which collect smaller bubbles as they rise to the surface.


The process of bringing a glass furnace up to its operational temperature and then maintaining the temperature.

Flat glass

All types of glass (rolled, float, plate, etc.) produced in a flat form, regardless of the method of production.

Float process

A method for the production of high-quality sheet glass whereby a ribbon of molten glass is fed across a bath of heated liquid, usually molten tin, in a carefully controlled atmosphere. The process was developed by the UK firm Pilkington Brothers.


A duct or channel for conveying heat or exhaust gases.


A substance that lowers the melting temperature of another substance. For example, a flux is added to the batch in order to facilitate the fusing of the silica. Fluxes are also added to enamels in order to lower their fusion point to below that of the glass body to which they are to be applied. Potash and soda are fluxes.

Foam glass

Glass with a high bubble content, produced by adding additional gases or gas forming substances to the glass melt. The resulting glass has a very low density but a high compressive strength and dimensional stability, making it particularly suitable for thermally and acoustically insulating construction materials.


A refractory tank whose function is to receive glass from the furnace, reduce its temperature to the desired level and discharge it to the feeder mechanism at a uniform temperature. The forehearth usually consists of two sections: a cooling section with burners and cooling ducts which allow the cooling process to be regulated, and a conditioning (equalising) section generally equipped only with burners which ensure uniform temperature distribution through the glass flow as it enters the feeder.


The initial phase of melting batch. For many modern glasses, the materials must be heated to a temperature of about 2450°F 1400°C). This is followed by a maturing period, during which the molten glass cools to a working temperature of about 2000°F (1100°C).


The process of giving a glass surface a matt finish, thus reducing transparency. Frosting may be by means of acid treatment (pouring hydrofluoric acid onto the glass), sandblasting, special glue application and subsequent removal, or mechanical etching with a grinding wheel.


An enclosed structure for the production and application of heat. In glassmaking, furnaces are used for melting the batch, maintaining pots of glass in a molted state, and reheating partly formed objects at the glory hole.

Furnace, pot

A pot furnace consists of a melting chamber lined with refractory brick, a vaulted roof or “crown” of silica brick, and external walls made of insulating brick. Below the upper chamber in which there may be as many as twelve melting pots, there is a lower section for the pre-heating of the fuel gas.

Pot furnaces are used today in the manufacture of mouth-blown glass objects and special glasses.

Furnace, tank

See Tank.


(1) The process of founding or melting the batch; (2) heating pieces of glass in a kiln or furnace until they bond (see casting and kiln forming); (3) heating enameled glasses until the enamel bonds with the surface of the object.

Fusing glass-to-glass

Glasses of different compositions can be fused together for decorative purposes and also in the sealing of electrical, medical and industrial components. The fusion temperature for soda-lime glasses is generally between 760°C and 820°C. Particular attention must be paid to the thermal expansion coefficients of different glass types.


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A homogeneous material with a random, liquidlike (non-crystalline) molecular structure. The manufacturing process requires that the raw materials be heated to a temperature sufficient to produce a completely used melt, which, when cooled rapidly, becomes rigid without crystallizing.


Materials produced from glass which have a polycrystalline structure.

Most offer advantages of low thermal expansion, making them suitable for uses such as cookware. Others have high physical strength and can be machined like metals.

Glory hole

A hole in the side of a glass furnace, used to reheat glass that is being fashioned or decorated. The glory hole is also used to fire-polish cast glass to remove imperfections remaining from the mould.


A drop of still molten glass formed by the cutting of the stream of glass as it flows from the forehearth through a feeder into a spout/orifice of variable diameter; the greater the diameter, the larger the gob. The gobs are fed into the forming machine to be moulded into bottles and other glass objects.

Gob feeder

A machine mounted at the end of the forehearth that dispenses gobs of molten glass of consistent size and weight for forming into glass containers. From the spout of the forehearth the molten glass flow out through an orifice, the size of which influences the flow rate of the glass. A cylindrical plunger moves up and down to accelerate or slow the flow of molten glass through the orifice. Linked to the motion of the plunger is a shear that cuts the molten glass into gobs at the correct point in relation to the plunger action. The gobs are then fed down chutes to the forming machine.


The removal of glass with abrasives or abrasive (grinding) wheels in order to shape, polish or otherwise finish both flat and hollow glass.

Grinding processes include milling, sawing, edging and drilling.


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Heat resistant glass

Glass which has a low coefficient of expansion and which is therefore less liable to thermal shock. Borosilicate glass is the most common type of heat resistant glass.

Heating up

Raising the temperature within the furnace to the required operating temperature under strictly controlled conditions, ensuring the homogenous expansion of refractory materials.

Hollow ware

Made generally of soda-lime glass, but also of crystal, lead crystal and special glasses, hollow ware includes a wide variety of containers and receptacles: container glass (bottles, jars, medical and packaging glass), tableware (drinking glasses, bowls, etc.), construction hollow ware (glass building blocks, etc.), medico-technical glassware (laboratory equipment, tubing, etc.) and lighting glass (lamps, bulbs, etc.).

Hot spot

Inside the furnace, the hot spot is that area on the surface of the melt which has reached the maximum temperature (at which batch reactions have been completed and dissolved gases have been reduced to acceptable levels). Also known as the “spring” or “source”.


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I.S. machine

I.S. (independent/individual section) container 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.

Infrared lamp

An incandescent lamp working at a low filament temperature and consequently emitting relatively high amounts of infrared radiation.

Infrared bulbs are usually made of borosilicate glass with molybdenum or tungsten wires.


The unwanted entry of air into a furnace through expansion-created gaps in the furnace superstructure or through other areas such as burner ports, regenerators and exhaust flues. Inleakage can result in decreased efficiency and increased fuel costs.


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An oven used to process a substance by burning, drying, or heating.

In contemporary glassworking kilns are used to fuse enamel and for kilnforming processes such as slumping.

Kiln forming

The process of fusing or shaping glass (usually in or over a mould) by heating it in a kiln.


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Laminated glass

Laminated (or compound) glass consists of two or more sheets of glass with one or more viscous plastic layers “sandwiched” between the glass panes. The solid joining of the glasses takes place in a pressurised vessel called an autoclave. In the autoclave, under simultaneous heating of the already processed layers of glass and special plastic, lamination occurs.

When laminated safety glass breaks, the pieces remain attached to the internal plastic layer and the glass remains transparent.


Two distinct types of lathe exist, although both basically consist of a horizontal shaft rotated by a motor. The first type uses the shaft to spin an abrasive wheel (often at high speed) in order to cut, score or polish glass; the second type uses the shaft to rotate a piece of glass so that it can be heated and manipulated.

Lead crystal

The type of glass produced when lime in the batch is replaced by lead oxide. The omposition 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.


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.

In smaller workshops, the lehr may be a simple kiln with a shelf for the glassware rather than a moving belt, and with electronic controls to programme the temperature cycle required.

Libbey-Owens process

A method for the production of sheet glass by means of a continuous drawing process. Devised by the American, Colburn, and further developed with the support of the US glassmaker Libbey-Owens, the process was patented in 1905, and was first used for commercial production in 1917.

The glass ribbon is drawn vertically from the tank for about 70 cm by a metal “bait” before being bent over a roller into the horizontal plane ready for cutting and annealing. The drawing speed with the Libbey-Owens process is twice that of the Fourcault process.


A sedimentary rock composed mainly of calcium carbonate which is added to the batch to provide calcium oxide.

Low emissivity on Low-E glass

Commonly known as “low-E” glass and often used in double and triple glazing units, this window glass has a special thin-film metallic or oxide coating which allows the passage of short-wave solar energy into a building but prevents long-wave energy produced by heating systems and lighting from escaping outside. Low-E glass thus allows light to enter while also providing thermal insulation.


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The fluid glass produced by melting a batch of raw materials.


Polished glass with a reflective coating of silver deposited on the back.

Mixed feed

The transfer of the various ingredients of the batch into the mixer by means of hoists, buckets and conveyor systems.


A form, normally made of wood or metal, used for shaping and/or decorating molten glass. Some moulds (e.g., dip moulds impart a pattern to the parison, which is then withdrawn, and blown and tooled to the desired shape and size; other moulds are used to give the object its final form, with or without decoration.

Mould, block

A particular type of mould produced in a single piece of cast iron, hollowed into a specific shape using a cold-deformation process.

Used in the production of pressed glass hollow ware.

Mould, dip

A cylindrical, one-piece mould that is open at the top so that the gather can be dipped into it and then inflated.

Mould, optic

An open mould with a patterned interior in which a parison of glass is inserted, then inflated to decorate the surface.

Muffle kiln

A low-temperature kiln for refiring glass to fuse enamel, fix gilding, and produce luster. See Kiln.


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Narrow-neck ware

Glass containers, such as bottles, whose opening is tapered and of smaller diameter than the body of the vessel.


In the production of glass containers, the tool coupled with the blank mould (parison) which gives the shape to the neck of the container.

During the shaping process in the IS machine, the neck ring transports the glass container into the blow mould (or finishing mould).


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Owens-Illinois coating techniques

Techniques developed by the Owens Illinois company for the surface treatment of glass containers. The two main types are stearate- and polyethylene-based.

Stearate treatment using polyoxyethylene monostearate gives good lubricity and reduced friction. It is water-soluble and not waterrepellent, facilitating the application of labels. Stearic acid is of vegetable origin, making this type of coating also suitable for kosher foods. However, since the stearate is destroyed by firing or any subsequent processing, the treatment needs to be repeated after the firing of applied colour labels.

Polyethylene coatings are deposited from a water emulsion but are not water-soluble and can thus withstand washing and pasteurising. Although they are slightly water-repellent, most glues used ensure satisfactory adhesion. Polyethylene coatings are transparent and give high lustre and lubricity to the glass surface.


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A small block of compressed matter. Pre-weighed and mixed batch materials are available in the form of pellets.


The preparation of materials, e.g. batch ingredients, in pellet form (see also “pellet”).


See “vial”.

Philips process

A variation of the Schuller up-draw process (patented in Germany in 1931) for the mechanical manufacture of glass tubing and rod.

Plate glass

Flat glass made by the casting or rolling of molten glass which is then mechanically ground and polished to produce a smooth and transparent sheet.


A tool used in the production of glass containers during the first stage of shape forming in the IS machine. The task of the plunger is to help give the glass container its final shape inside the parison (or blank mould).


A fire clay container placed in the furnace in which the batch of glass ingredients is fused, and kept molten. The glass worker gathers directly from the pot.

Pressed glass

Glassware formed by placing a blob of molten glass in a metal mould, then pressing it with a metal plunger or “follower” to form the inside shape. The resultant piece, termed “mould-pressed,” has an interior form independent of the exterior, in contrast to mouldblown glass, whose interior corresponds to the outer form. The process of pressing glass was first mechanized in the United States between 1820 and 1830.

Preston test

The means of verifying the bursting strength of a glass container during automatic inspection.


The base of a glass bottle (particularly of a wine bottle) which is pushed upwards inside the bottle during the forming process.


An instrument used to measure the temperature inside the furnace or kiln.


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Raw materials, for basic refractories

Basic refractories are made up of various mixes of periclase (magnesium oxide), chromite (chrome ore) and forsterite (olivine).

Bonding agents can also be added so that refractories can be shaped.


The abbreviated form of “reduction-oxidation”. The term “redox equilibria” is used to refer to the balance between reduction and oxidation in the glass furnace.

Redox equilibra

Used to refer to the balance between reduction and oxidation in the glass furnace.


Refining ensures that a homogenous glass is produced during founding by eliminating bubbles (see also “bubbles”). Refining is achieved through the action of certain chemicals (refining agents) added to the batch recipe and also by keeping the glass above the liquidus temperature so that the bubbles rise to the surface.

Refractive index

A standard of measurement used particularly to establish the qualities of optical glass. The index is the ratio of the sine of the angle of incidence of a ray of light to the sine of the angle of refraction (the change in direction when a ray of light passes from one medium to another) by the glass. The second medium normally used to establish the index is a vacuum.


Material capable of withstanding extremely high temperatures and thus used in furnaces for industries such as glass and steel where raw materials have to be heated to a molten form.

Regenerative heating

As in recuperative heating (see “recuperative heating”), waste heat from the furnace is used to pre-heat combustion air.

Regenerative heating is a cyclic process whereby exhaust gases pass over and thus heat up refractory blocks in one of two pre-heating chambers. Once the first chamber has been heated up, exhaust gases are diverted to heat the second chamber, while cold combustion gas is introduced into the first chamber to be pre-heated by the hot refractory blocks. Continuous reversal of this process provides a permanent flow of pre-heated gas for combustion.

Rolled glass

Rolled (or cast) glass is a translucent glass with 50-80% light transmission, depending on its thickness and type of surface. It is used where transparency of the glass sheet is not important or not desired.

To produce rolled glass, molten glass pours from the melting tank over a refractory barrier (the “weir”) and onto the machine slab where it flows under a refractory gate (the “tweel”), which regulates the volume of glass, and then between two water-cooled rollers. The distance between the rollers determines the thickness of the glass.


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Safety glass

Glass which does not disintegrate into sharp and potentially dangerous splinters when it is broken. Safety glass may be produced by laminating (see “laminated glass”) or by tempering (see “tempering”).


The most common form of silica used in making glass. It is collected from the seashore or, preferably, from deposits that have fewer impurities. For most present-day glassmaking, sand must have a low iron content. Before being used in a batch, it is thoroughly washed, heated to remove carbonaceous matter, and screened to obtain uniformly small grains.

Screen printing

A process for the decoration of glass whereby coloured ink 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 colour.

Considerable automation of the process has been developed, thus allowing extremely high printing speeds for even complex designs.

Sheet glass processes

See the definitions for the following processes, listed in order from oldest to most recent: “crown glass” (definition 1), “cylinder glass”, “drawn glass”, “Fourcault process”, “Libbey-Owens process”, “Pittsburgh process”, “float process”. The float process is now the standard method of producing sheet glass world-wide.


Silicon dioxide, a mixture that is the main ingredient of glass. The most common form of silica used in glassmaking has always been sand.


Sodium carbonate. Soda (or alternatively potash) is commonly used as the alkali ingredient of glass. It serves as a flux to reduce the fusion point of the silica when the batch is melted.

Soda ash

Sodium carbonate (Na2CO3), or ‘soda ash’, is the main source of sodium oxide (Na2O), or ‘soda’. This anhydrous, white powder is added to the glass batch, with sodium oxide becoming part of the glass and carbon dioxide being released.

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 colour.

Spinning, hollow ware

A relatively new method for the centrifugal production of hollow ware such as borosilicate glass columns in chemical plants, funnels, television tubes and other non-rotationally symmetrical items. Molten glass is fed into a steel mould which rotates at the required speed. At high speeds, the glass can assume almost cylindrical shapes. When the glass has cooled sufficiently, rotation stops and the glass is removed.

Staple fibre

Short lengths of glass fibre, usually U-shaped, which intertwine and are used, in particular, to create insulation materials.


The collective term given to drinking glasses whose body is connected to the base by a thinner column of glass.

Suck-blow process

A process used for glass container manufacturing with forming machines. Glass is sucked from the tank of molten glass into the parison mould and then cut by shears. A plunger inside the mould produces a hollow space in the glass which is then enlarged by blowing. Following this initial blow, and then reheating, the parison is transferred to the finishing mould for the finishing blow.

Suction cup

A semi-spherical cup of flexible material such as rubber. By pressing the cup onto a glass surface and removing air from inside the cup, the vacuum thus created holds the cup and glass together. Suction cups are used in both the manual and automatic handling and conveyance of glass.


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A large receptacle constructed in a furnace for melting the batch. Tanks replaced pots in larger glass factories in the 19th centry.


See toughening

Thermal conductivity

The passage of heat through a material. Insulation materials are defined as having ‘low’ thermal conductivity whereas metallic materials generally have ‘high’ thermal conductivity.

Thermal shock testing

Assessing the effects on a material of rapid temperature change. In glass, the shock may derive from the external surface of glass expanding or contracting more rapidly than the interior surface as a result of heating or cooling. Any such difference may lead to cracking or shattering.


A pair of different metals in contact at a point, generating a thermo-electric voltage which can serve as a measure of temperature.

The wires are encased in a protective sheath that can be introduced as a probe into the glass furnace or kiln.


Special process of solidification of a glass sheet in order to make it particularly resistant to breakages. The process may be physical (thermal) or chemical. In the former, the glass sheet is heated to a temperature just below its softening point and then immediately cooled by special jets of cold-air. These harden the surface of the glass, giving the inside more time to cool. This allows the external layer to crystallize into a wider lattice while the inside solidifies with greater compression than in the crystal lattice. The result is a sheet of glass which is two or three times stronger than untempered glass and which, upon breakage, shatters into tiny pieces with blunt edges (the most common applications are for automotive glass).

The chemical process, on the other hand, is based on the so-called ion-stuffing technique. Different chemical elements possess different ionic radii and therefore different densities. Hence, if glass containing sodium is cooled slowly in a salt bath of molten potassium, the sodium ions will migrate from the glass to the salt, while the potassium ions will move to the surface of the glass where, due to their wider radium, they create a denser and therefore stronger surface layer (of no less than 0.1 mm).

Glass sheets which have been chemically tempered are five to eight times stronger than those which have not undergone any tempering process.

Tube-drawing process

See danner process


Hollow rods of glass used especially in the production of laboratory/medical equipment (ampoules, vials, etc.) and fluorescent lighting.


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Vapour deposition of thin fims

The term covers a wide range of techniques for applying a thin film on the surface of the glass to change its technical or aesthetic properties e.g. scratch resistance, solar control. The methods employed to deposit the film include spraying onto hot glass, condensation in a vacuum and evaporation of the film material by heating.

Vello process

A drawing process used for the production of glass tubing. Glass from the furnace forehearth flows down through an orifice (ring) within which is a rotating conical-ended shaft (or mandrel) over and around which the glass flows. The tube-shaped glass is pulled from the end of the shaft by a tractor machine and turned through 90° into a horizontal position ready for cutting.

Venturi tubes

Short pieces of narrow tube between wider sections of tube, used for exerting suction or measuring flow rates and invented by the Italian physicist G. B. Venturi, who died in 1822.


A small cylindrical glass vessel especially for holding liquid medicines.


The quality or state of being viscous; the physical property of a liquid or semi-liquid that enables it to develop and maintain a certain amount of shearing stress dependent upon the velocity of flow and then to offer continued resistance to flow.


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Waste gas analysis

Gases emitted by the melt in the furnace can be analysed either in the furnace itself (in order to assess melting efficiency, for example) or as they are discharged from the furnace stack (above all, for pollution control purposes).

Furnace gas testing may be performed with Orsat equipment (gases are absorbed selectively as they pass through a series of specific solvents) or by means of instrumental analysis. Paramagnetic detection may be used for oxygen analysis, and infrared absorption for carbon dioxide analysis. Mass spectrometry or gas chromatography are also used to analyse gas mixtures.

Waste-heat recovery

An economy measure whereby the heat of exhaust gases is used in a cyclic process to pre-heat combustion air and/or fuel-gas. (see “regenerative heating”).


Changes on the surface of glass caused by chemical reaction with the environment. Weathering usually involves the leaching of alkali from the glass by water, leaving behind siliceous weathering products that are often laminar.

Wired glass

Flat rolled glass reinforced with wire mesh and used especially for glass doors and roofing to prevent objects from smashing through the glass and also to hold pieces of broken glass together. By holding the glass together, it can also protect against break-in and the spreading of fire.

Wired glass is produced by continuously feeding wire mesh from a roller into the molten glass ribbon just before it undergoes cooling.

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