THE TECHNIQUE OF MATERIAL

Angela van der Burght

MATERIAL: the physical substance or tangible elements used to produce a work of art, artefact, form or object; building material, raw material used to make something, which can be worked or fashioned, such as wood, textiles, plastic, stone, paint or, for example:

GLASS: a generic term covering a virtually infinite variety of substances of different compositions existing in a vitreous state. To produce glass one needs silicon dioxide or sand. Only sand however, is not suitable for setting up for instance glass bottle fabrication. What is needed is a glass melting medium. Therefore sodium (Na2O) or potassium (K2O) is used. To make glass for processing one needs a solidifier such as calcium (CaO). Since 1830 six parts of sand, one part soda and one part lime has been considered a good combination. Nowadays glass factories use more and more old glass or cullet, which will be mixed thoroughly with the other components.

Glass is one of the first man-made materials to have been created. It is multifunctional and multi-interpretable in a manner unmatched by any other material, and has numerous characteristic properties. In the realm of the visual arts, glass is frequently interpreted as a primary matter, quasi-protoplasm or proto-material, and is thus deployed for the purposes of dematerialising art. Materialisation is the process whereby energy is converted into matter; dematerialisation – the converse process – repudiates the intrinsic value of the material, as in the case of a statue made of glass instead of bronze.

Glass is an ingredient, a working material, an industrial material, imitation material, a base material, an insulating material, a packaging material and a recycling/upcycling material and can be categorised by composition, special qualities, function, appearance, the original processing, working, later processing, brands and trade marks.
N.B.: The misconception persists that glass, as an amorphous material, can have no structure. However, neutron researchers from the University of Cambridge, working with the reactor located at the Institute Laue Langevin in Grenoble, Switzerland, demonstrated five years ago that it not only exists in a haphazard disorder but also possesses a structure: glass is virtually crystalline. Another misconception is the thought that antique glass would sag and become more thick at the bottom during time: Edgar Dutra Zanotto of the Federal University of Sao Carlos in Brazil calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount. Cathedral glass would require a period "well beyond the age of the universe," he says. His study demonstrates what many scientists had reasoned earlier: "You would have to bring normal glass to 350° Celsius in order to begin to see changes," says William C. LaCourse, assistant director of the NSF Industry-University Center for Glass Research at Alfred (N.Y.) University.

* Boundary material: since glass is transparent and a person perceiving it may either ignore it in optical terms or, on the contrary, zoom in on it and look through it, it is known as a boundary material. It makes the space behind it visible, but at the same time also isolates that space from the space in front of it. Florian Lechner was the first to use this term.
* Imitation material: like no other material, glass imitates qualities of other materials and shapes. Maybe because it has no shape of its own it can be ‘pressed’ into so many sensible and irrational forms and shapes. As imitation gem-stone, imitation rock crystal, imitation of stone, ceramics and even metal, usually the shape was imitated first as with glass cameos, and then followed the stone and shell techniques or the pressed Bakelite until its own independent shape was understood. The light bulb followed the candle, cooking devices ceramic and metal pottery, the first sculptures of glass those from plaster and marble or bronze.
* Man-made material: artificial, put together and made by a human intermediary. Examples of such man-made materials include glass, paper, bronze and plastic. N.B.: All transparent plastics, such as acrylic, polycarbonate and Perspex are also known as types of glass or quasi-glass.
* Natural glass: manifests itself in nature in the form of thunderstone or fulgurite, obsidian, tektite and pumice or volcanic foam glass, and consists of reaction products of sand, sodium and calcium compounds.
* Special glass: glass which, unlike flat glass, glass fibre and hollow glass (like vessels and containers), is defined not by the form in which it appears but by the process applied, involving specially adapted properties. Thus, it may have a high level of chemical and thermal durability and possess suitable optical, electrochemical and other special qualities of the kind applied in the field of chemistry, in laboratories, in pharmaceuticals, electrical engineering, optics and lighting technology and in the home. The types of glass falling within this category include sintered glass granulates and billets used for insulation, high-voltage insulators, lamp glass, optical glass and spectacle glass, glass ceramics and foam glass.
-Optical glass: special, extremely pure glass with well-defined properties originally produced for the manufacture of lenses, spectacle glasses and prisms. After the fusing, the hot glass is homogenised in special melting furnaces fitted with a stirring device, or in smaller furnaces, and is then cast in a steel mould into stones, tiles, rods, triangular shapes, bars and cubes. Special quantity-measuring and pressing installations may also be used to produce unfinished pieces or blanks for spectacle glasses, lenses and prisms. For further details, see: The Optical Qualities of Glass and Spectacle Glass.
*Matrix: literally mother soil, natural mineral with enclosed or embedded fossil, metal and crystal, all surrounding a solid form like a mould or blowing form, material like concrete and glue which embeds and assembles another material such as mosaic and the ground material from a composite material.
Matrix is also the signalisation system on motorways for traffic regulation by optical systems.
N.B.: Matter is the general substance of which something exists, whilst its base material and materiality represents its tangible physicality.
* Work materials: industrial materials – subdivided into the following categories:
Composite materials: materials and special glasses used in processes requiring, strength and light, such as those in the nautical, aeronautical and astronautically fields, they consist of homogeneous structures composed from a continuous matrix or parent material in which glass fibre is embedded or dispersed.
– Glass ceramics: this type of glass is produced by stimulating the formation of crystal to just the right extent during the cooling process, thereby giving it special properties such as the ability to withstand temperatures of almost 800° C without expanding. It is used in stoves, diesel exhaust filters, cooking rings and hotplates (Ceran), mirror carriers in telescopes (Zerodur), dentistry materials, types of photosensitive glass, printing moulds and perforation plates.
– Open-pore glass: by contrast with foam glass, the cells are not closed but open, with a hollow-space volume of nearly 90%, the absorption capacity being significantly increased. Low-capillary membrane glass or micro-porous glass may be used in installations for the collection and purification of seawater and kidney dialysis machines.
– Porous glass or foam glass is obtained by mixing the unpurified batch with extra gas or gasifiers which create the air bubbles in the frit. This cell structure brings about an increase in the power insulating and impelling the heat. Sintered porous glass is used in the production of water filters such as Siporax for cleaning aquariums. Construction modules such as building stones are often executed in black.
Meta materials: (from the Greek word meta, meaning to change into) these are compound materials, the properties of which have been substantially altered. Thus, flat glass and copper structures, combined to form a lens, may serve not only to focus light but also to obtain a reverse Doppler effect and a negative refractive index.
Hybrids: materials which, over the last ten years, have followed on from composite materials and in which textiles, glass, metal and ceramics are combined. They may be produced in a number of different ways: as a specially made-up composite material, by means of a coating and stratified as a laminate and sandwich. Hybrids containing glass have various different properties: they are fire-resistant and fire-retardant and serve to insulate heat and sound, are suitable for steam cleaning, are lightweight and yet durable. E-glass hybrids, for example, are used inter alia in silencers such as glass fibre wall hangings which largely prevent the creation of static electricity in spaces containing a large number of computers. For further information, see > The Technique of Glass Fibre.

Base materials: unworked raw materials referable to whatever is or may be produced from them and forming a principal component of something:
* Glass formers: various chemical substances, including, primarily, oxygenates or oxides, are capable of being used in the formation of glass. Once they have cooled down after being heated, they become virtually petrified, but without crystallisation. Such glass formers also possess this quality when other metal compounds are mixed in with them, even though the latter may alter the physical and chemical composition and the range of application of this type of glass.
* Melt: up until the 18th century, the molten mass was produced from sand, soda, potash and lime, sometimes with metal oxides being added. Nowadays, approximately 60 out of the 90 oxygenates or oxides found in the world are used for the composition of the mixture or batch, together with decolourising agents and colouring agents or ingredients such as iron oxide, cobalt oxide, copper oxide and gold; fluidifying agents such as soda; stabilisers such as lime, refining agents designed to make the glass homogeneous and intermediants such as lead.
Silicon dioxide (silica): a composition of silicon and acid. The earth's crust contains 59% silicon oxide, the chief component of 95% of all stones. Silicon dioxide contains three silicate minerals of crystalline varieties, such as quartz, tridymite and cristobalite and is used in the construction of buildings and roads in the form of cement, concrete and mortar; in sandstone; as a grinding and polishing material for glass, diamond and stone; in casts and moulds; in the production of glass and ceramics; in silicones; as a fireproof material, fire clay or silicate refractories, and as precious stones. Well-known materials include quartz, feldspar, silicon dioxide, clay, mica and tallow.
Silicon (from the Latin words silex and silica’s, meaning flint): a chemical element within the carbon group; just like quartz, it can be transformed into glass by heating.
Silicon gel or dehydrated silicic acid: a porous, non-crystalline form of silicon dioxide used as a cleaning material, to thicken liquids and to manufacture water glass.
Silicates or silicic acid salts or stones: silicic acid compounds occurring in nature, such as lithium, sodium, potassium, magnesium, lime, strontium, barium, manganese, iron and aluminium. N.B.: Silicone is one of an extensive class of different chemical compounds which are produced in the form of liquids, resins and elastomers and belong to the group of polymers. Silicone resins are used in protective coatings and electrical insulating varnishes, for the laminating and sealing of textile glass and as a component of glass adhesives and glues.
* Recycled glass: in a glass aggregate system, conveyor, shredder, glass crusher and pulverizer equipment, glass of one kind of glass type can be crushed to produce cullet and granulates for re-use in kiln forming techniques and industry. Well known in art glass is the re-use of Coca-Cola bottles, old window glass and old special glass. Industry uses the glass for glass products like cullet, fibre glass, marbles, eco-glass, glass beads, jewellery, marblite, pressed glass, sintered mosaic tile, terrazzo, floor and roof tiles; as glass-sand for sand blasting grit, landfill cover, and replacement natural sand on beaches etc; aggregate for glasphalt, landscaping, glass concrete and drainage. Glass sand is used as White blast in abrasives, in frictionators and as flux/binder in ceramics.
* The purifying process: the stage reached in the melting at which large air bubbles are released from the fused material by the addition of refining agents. The bubbles pass through the hot frit, ensuring a good mix and a homogeneous mass, whereby they simultaneously gather together the small air bubbles and bring them to the surface.
* Cullet (scraps of broken or waste glass gathered for remelting) is gathered by every glass factory itself, in the form of splinters, shards or scraps, or bought in and added to the mix. It functions as a fluidifying agent, making the sand and pellets in the fusion more fluid. As long ago as the time of the Phoenicians, cullet was recycled by gathering scraps of glass according to colour groups and returning them to the melting furnace. In 2002, Maltha Glass Recycling supplemented its hollow glass collection and processing activities by opening the first flat glass recycling installation at Kaulille, Belgium. 10,000 tons of cullet are processed there and moved to Lommel since 2009. Fine glass is the pure window glass that can be directly processed into granules for the glass and glass wool industry, whilst safety glass, wire glass and insulation glass must first be stripped of any linings and sealants which it may contain.
- Opacifiers: chemicals in the batch causing milk white sub-translucent glass as for instance arsenic, antimony, phosphates, fluorides and zircon which all produce a white appearance by forming crystals during the annealing process. For very dense-white opaque glass tin oxide, titanium and zirconium are mainly used for a glass type which in English is called enamel type Opal.
- Second quality: in the salesrooms of most glass factories visitors can buy second quality articles with minor defects offered at reduced prices. Standard articles in the collection are the first choice, so the best possible quality. In industrial production the Clear Line is camera-controlled glass on cosmetic and critical defects while the Top Line is guaranteed camera-controlled, with inferior products eliminated.
- Durability: glass may be affected by pollution: rust (for example in stations), concrete, acid rain, exhaust fumes, carbon monoxide (as well as exhaled air and the burning of incense and candles in churches) all affect positions 1 and 2. Float glass and Antique glass are resistant to pollution provided that they are regularly washed and are not constantly allowed to get steamed up or covered with moisture, thereby preventing leaching and weathering. If metal oxides are added to the glass mass, this will increase resistance to climatic factors. See further for self-cleaning glass the Technique of refining Floatglass.
- Defect: in the chemical sense of the term, an unevenness in the crystal lattice of the glass and, in its general sense, a fault in the glass-making process, such as the presence of stones, seeds (i.e. small bubbles) and dirt. Defects arise from the use of impure base materials and dirty tools and from poor mixing, fusing and refining.
* Break: the glass is broken. A crack is the start for breaking. A star after a stone impact in car glass can be repaired by a vacuum pump and two-component glue, to prevent a crack. Glass debris is found by archaeologists in cesspools and disposal pits revealing broken household ware and fragments of broken glass objects that can be assembled with or without supporters and dividers.
* Cracking and crazing: crazing is an unwanted fault affecting the enamel or glazing, whilst crackling is an intentional effect found in crackleware.
N.B.: for further information regarding cases in which the glass surface has to crackle, see the entries entitled "Frosted Glass" and "Auftemperen" in the section on Originating Techniques; this involves the fracturing of an intermediate layer of tempered glass in sandwiched safety glass.
* Devitrification: glass maintained for a lengthy period at the melting temperature which, on account of the viscosity, has the tendency to adopt a more rigid, crystalline structure. The composition and coefficient of expansion of the crystals are altered, so that they render the glass opaque and blank, ultimately building up a tension and splitting. Even new glass which is initially clear but is later used for melting and fusing purposes and is not specially made with that in mind frequently exhibits this quality. The use of special glass, a better cooling-down curve and the application of an overglaze (in German: Glazurspray) may prevent this effect.
* Sick glass (in German: Glaskrankheit) is mostly caused by poor compounding of the mix, with the potassium content being too high. Extremely fine hairline cracks occur in the glass surface, which becomes blank; these subsequently cause the glass to deteriorate. Over the course of many decades, the glass content of precious, rare and old crystal and glass objects becomes stabilised, and if the object in question is suddenly moved to a dry environment dehumidification occurs. Consequently, for conservation purposes, such objects are kept by museums in special containers at a specific constant temperature and level of humidity, with a view to delaying the deterioration process for as long as possible.
* Seedy glass or ‘gray glass’ is the term used in relation to Leerdam glass to describe small bubbles in the glass caused by an excessively low fusing temperature or insufficient refining agent in the mix.
N.B.: Frank Lloyd Wright used genuine antique glass, calling it "seedy", in a process whereby the small "seeds" were deliberately created in the frit.
- Closed bubbles: enclosed air, mostly not visible to human eyes.
- Open bubble: is burst open at the surface of the glass wall.
- Blasenstrecke (air trap or air twist): German for a chain of air bubbles one after the other or overlapping
- Stones: an opaque inclusion and knots are transparent inclusions
* Weeping glass: produces, in humid conditions and as the result of the extraction of alkali, small drops of water with a slimy surface.
* Hairline cracks: a great many studio glass objects exhibit hairline cracks in their thicker attachments, resulting from the incorrect decontraction of the glass or bad annealing. A scratch is superficial damage not penetrating the glass wall.
* Hydrophobic or water-repellent properties: by using special coatings such as Clearshield, a protective layer can be superimposed on the surface of float glass, making it smoother, water-repellent and non-polar, and thus less demanding of maintenance. On account of the adhesion, position 1 cannot be coated in any other way, and suction devices, when pressed on to the glass, are no longer capable of sticking to it.
* Irising: glass which has remained in the ground for a long time – especially Roman glass, in the case of which no lime was added to the mix – exhibits a rainbow lustre as a result of a chemical reaction occurring over many years. Later on, this lustre effect was specially added to glass by means of treating it with saline sulphides.
* Scale or furring: carafes, fish bowls and flower vases which are in use frequently exhibit white rings which have penetrated deep into the glass. Such furring can be removed only by the application of a light acid solution.
* Cleaning: antique glass and crystal objects and stained-glass windows must be kept clean by being washed in a light, tepid soap solution so as to remove dirt and deposits. Dishwashers should not be used, since the chemicals in them are too aggressive. The glass should be allowed to dry on its own in the air, or a linen dishtowel may be used.
* Leaching or washing out: soluble materials present in a substance may be detached by repeated treatment with a watery solution separating them from the insoluble elements. The glass must be stored in a dry place and the separating and packaging materials must have a low pH value, so as to prevent leaching and protect the glass from the effects of the weather.
* Weathering: decomposition and the formation of mould through the effects of dampness. Window glass can be seriously marked by the effects of acid seeping out of brickwork and girders, and the only thing to do is to polish it up again and wash it after each rain shower.

Main glass types or glass composition: depending on the composition, the following types of glass are produced: calcium soda glass, sodium calcium glass, borosilicate glass, barium (strontium) silicate glass, lead glass, borosilicate glass and special glass such as quartz glass, Pyrex and Duran.
N.B.: In the case of controlled crystallisation, the properties of the glass are altered, producing glass ceramics and photosensitive types of glass.
-Sodium calcium glass: applied for bulk goods as the most used glass type as packaging glass, flat glass and lighting glass.
-Calcium soda glass: is the type used for floatglass
-Borosilicate glass: glass types with low expansion coefficient with good corrosion resistance and very useful for laboratory glass and for instance headlights. Borofloat is a flat glass Brand of Schott.
-Lead glass: as crystal glass with a high refractive index (for service and drinking glass not allowed since 2003 in Europe) and very high electricity resistance. Wikipedia: Lead glass is a variety of glass in which lead replaces the calcium content of a typica lpotash glass. Lead glass contains typically 18–40 weight% lead(II) oxide (PbO), while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO. Lead glass is desirable owing to its decorative properties.
Originally discovered by Englishman George Ravenscroft in 1674, the technique of adding lead oxide (in quantities of between10 and 30%) improved the appearance of the glass and made it easier to melt using sea-coal as a furnace fuel. This technique also increased "working period" making the glass easier to manipulate.The term lead crystal is, by technicality, not an accurate term to describe lead glass, as glass, an amorphous solid, lacks a crystalline structure. The use of the term lead crystal remains popular for historical and commercial reasons. It is retained from the Venetian word cristallo to describe the rock crystal imitated by Murano glassmakers. This naming convention has been maintained to the present day to describe decorative hollow glass and table-ware.
Lead crystal glassware was formerly used to store and serve drinks, but due to the potential health risks of lead, this is rare nowadays. One alternative material is crystal glass, in which barium oxide, zinc oxide, or potassium oxide are employed instead of lead oxide. Lead-free crystal has a similar refractive index to lead crystal, but it is lighter and it has less dispersive power.
In the European Union, labeling of "crystal" products is regulated by Council Directive 69/493/EEC, which defines four categories, depending on the chemical composition and properties of the material. Only glass products containing at least 24% of lead oxide may be referred to as "lead crystal". Products with less lead oxide, or glass products with other metal oxides used in place of lead oxide, must be labeled "crystallin" or "crystal glass" and in Dutch Sonoor glas. Full Lead Crystal contains 30 % lead or more.
See also the Techniques of Glass Cutting, Glass Grinding and Glass Polishing.
-Quartz glass: high melting point and corrosion resistance, even component flint glass types with extreme low expansion coefficient and hard to process. Is used for sophisticated products as lightning, optical communication fibres and special laboratory glass.
-Barium silicate glass: used for monitors and television screens because it blocks x-ray waves emitted from this kind of tubes.
NB: California's Proposition 65 requires a warning of potential dangers, including birth defects, from consuming food or beverages served or kept in full lead crystal.

Manifestations: the three main forms in which glass manifestates:
-Hollow glass: with the invention of the glass blowpipe around 200 B.C., hollow glass began to be produced, taking the form of glass containers or vats and open three-dimensional vessels such as drinking glasses and glass sets, ornamental glass, container glass and illuminating glassware. The most important techniques are the development techniques of mouth blowing, machine blowing and compressing and the processing techniques of core-forming, pâte-de-verre and flame working. Measurements are dictated by definitions as function, hand made, machine produced, amount of melt in glass furnace, opening shape of furnace and glory hole, measurements annealing kilns, blowing pipe and composition working team.
-Grootwerk: Dutch for large hollow vessels, for hand blown work as storage bottles, medicine and essence bottles, Balloons, Carboys or Demijohns.
* Hollow glass can have thick or thin walls.
-Wall thickness: the difference between thick and thin in one wall.
-Construction hollow glass is used in relation to building blocks and roof tiles.
* Tube glass can be produced in very fine and very large measurements as capillary glass tubes or tanks and tubes for milk plants. The Borosilicate glass used can be protected of both sides of the glass wall in transparent or opaque thermoplastic cyclooienfirs copolymer.
- Flat glass: this term signifies all types of glass produced in a flat form. See also the references to: table-cast glass, drawn glass, float glass, centrifuged glass, plate glass, figured glass and genuine antique glass, which are all types of flat glass. Until 1900, window glass was virtually exclusively mouth-blown. The cylinders (Dutch: Mof) of what is now known as genuine antique glass, after being blown, were cut open along a length of sometimes as much as two metres and flattened in a flattening kiln or smoothing kiln. For further information, see the entries on drawing glass, float glass, glass rolling and figurating, centrifuging and winding glass.
- Glazing: providing windows with panes fitted into rabbet or rebate in frames, wall, door or window done by a glazier. Glazing may also be structural, sometimes offering special features such as reduced energy consumption, thermal insulation, low emissivity, sound-proofing, enhanced safety, design and decoration features or cladding for outside walls.
* Dimensions and measurements:
-Production measurement: the standard dimensions produced by a glass factory.
-Utilisation measurement: the dimensions after the cutting up of the production measurement.
-Fitting measurement: determination of the dimensions so that the utilisation measurement fits into the groove or rabbet and fastening. What is measured is the clearance measurement, namely the dimensions of the visible glass in a groove, and the groove measurement, being the dimensions of the clearance measurement + the glass needed to fit into the groove. The fitting measurement of an interior, such as a container form or the vessel held inside a piece of silver, is known as the interior measurement.
-Module: is the standard measurement and form.
-Thin glass: varies between 0.40 and 2.90 mm in thickness and is used for displays such as LCD-screens in telecom technology and IT displays, EL or electroluminescent, Fed or Field Emission Displays and Touch screens but also photo protecting glass, cosmetic and automotive mirrors, medical applications as micro slides for data storage as discs and glass masters and slide protection glass, wrist clock protection glass and sun cells.
Ultra Thin Glass is available at 0,03mm to 1,10mm (0,00118" to 0,0433") thickness.
-Thick glass: flat glass having a thickness of 20 mm or more.
-Glass elements: the pieces used for the assembly of the different glass sheets by means of sandwiching or mounting to produce insulation and safety glass.
-Twin skin or second skin: when a closed Binnenblad (Dutch) or doublate –the built wall behind glass lining– of a building is clad with plain or worked, transparent or opaque façade glass this cladding or dressing is called the glass skin. At first a glass façade was known as a flees façade or curtain façade, but now after modern developments in building the preferred term is glass façade and more specifically, an aluminium glass façade for an aluminium flees façade. The book Façades in Glass and Aluminium by Mick Eekhout describes all technical possibilities and glossaries.
-Light: a window above or next to a door; an opening in a wall, ceiling or roof.
* Position: the place or position in which the two sides of a glass window or sheet of glass are located from the standpoint of the person perceiving them, indicated by numbers. Stand in a room and look outwards through a glass window. Position 1 is the outside, i.e. the rear side of the glass which is furthest from you and which comes into contact with the air outside. Position 2 is the side of that pane of glass which faces inwards. Position 3 is the rear side of a second sheet of glass and position 4 is its near side. Glass structures exist which are built up out of three or more layers. Position 5 is the rear side of the third sheet of glass, and so on.
-Placement position: the position achieved by working flat glass in the course of placing it:
-Painted side: position 1, in the case of flat glass the rear side, which is painted.
-Clear side: position 2 (the side of the pane that faces inwards) in the case of flat glass the rear side of which is painted.
* Protection glass: for protecting for instance stained glass windows and leaded glass windows one can choose between several options:
-Doublate: a shard of glass from a window gets its own protective piece of glass in the same shape and form, put together into the lead cames.
-Protecting glass with outside ventilation: all windows stay in their original rabbet or groove and the clear protecting float glass, metal wire glass and synthetic slabs as Polycarbonate is placed outside installed some centimetres from the wall.
-Protecting glass with inside ventilation: in the rabbet clear and/or anti-reflecting glass is installed. In the inside a second rabbet is made for the stained and leaded glass panels. When there is space enough in the new restored mullion (stone) here a double rabbet can be made. Slits at the top and bottom allow for the ventilation.

Forms of flat glass:
- Glass sheet: mouth-blown cylinder glass which is cut open and stretched, having on its two short sides and on one long side a serrated edge and on one long side a round-fused edge and machine-produced flat glass which is cut free from the glass ribbon and possibly cut up into special or standard measurements before being further worked and processed or placed in position.
- Pane: a rectangular or square glass sheet of window glass set in the framework of a window. A fixed pane sits in a groove or rabbet. See also the Techniques of stained and Leaded Glass.
- Ruitje or small pane: a small pane-shaped piece of glass (in the form of a rhombus or parallelogram standing on one sharp point), rectangular or square, forming part of a leaded panel or directly set in a wooden subdivision.
- Window: in its general sense, this term signifies the opening through which daylight and air is able to penetrate a closed room and the structure, composed of glass, wood and casement, serving to close up that opening; the opening through which things can be seen; a window for looking through, made of transparent material. In its special sense, it signifies a stained glass window, panel or a lattice/leaded light.
The following are different types of windows: rose windows and rosette windows; rib tracery; trefoil and quatrefoil plate tracery; mouchette, curvilinear or flowing, intersecting, geometrical flamboyant, perpendicular-style, rectilinear and lancet windows.
-Slab or Plate: these words are used as general terms in relation to the form: flat, hard, mostly rectangular or square of a kind of material with solid thickness compared to its measurement as marble, lead or glass slab or plate. It is also a protecting material, a print of an engraved and etched metal or stone slab or thick plank. Glass plate is used for glass negatives with photosensitive material, a glass record or a heavy flat glass for working and tabletops. The slab material from Bisazza is called Luxor and Petrea, fused glass and Aventurine and granite flint, polished and sparkling like marble and used as laminating or furnishing material for furniture and interior design.
-All glass is the use in architectural glass where no visible structures are used but transparent glue, PVB-foil and the like.

N.B.: Flat is used as general word related to the form especially the wide and flat side as for shallow dinner and kitchen ware; a plane two-dimensional surface and non-worked as on material or a text.
* Flat glass subdivisions: the glass industry subdivides the various types of products according to the following categories: construction, automobile, transport, household appliances, cooling apparatus and optical instruments.
- massive glass or solid glass: glass formed by hand, compressed, sand-cast and shape-cast, optical and crystal glass. Artists frequently use optical full glass in order to cut and polish sculptures in the round and to carve them.

Families of products: these subdivide products in larger groups. It is impossible to name them all as for instance Prizes, Souvenirs, Gifts, Cosmetic glass and Memorabilia but the following are the most important groups:
Architectural glass or Glass in Architecture: glass used for walls, floors, roofs, doors, ceilings, staircases, bridges, elevators, balustrades, staircase, windows, dividing walls, lanterns, domes
Cosmetic glass: artificial eyes
Decorative glass: Christmas tree decorations, angel's hair, decorative garden lamps, ornaments, Gartenkugel, fruits and vegetables
Dinnerware: storage articles, filling articles, pouring articles, drinking articles
Drinking glasses: baluster glasses, Berkemeier glasses, beer glasses, cocktail and liqueur glasses, champagne glasses, cognac glasses, dice shakers, Doorloper, Doornstokje, drinking sets, drinking horns, Drinkuit, flutes, Hansje in de kelder, Klokglas, whisky glasses, tumblers, Knobbelbeker, Knotsbeker, Kometenbeker, lemonade glasses, water glasses, Angster, Dradenbeker, beer jugs, goblets, Humpen, cabbage stalks, Kutrolf, Netbeker, Noppenbeker, rummers, rhytons, Scheuer, Spechter, Stangenglas, stem glasses, Toverbeker, wine glasses, Vleugelglas, Traanglas, Slingerglas, Slangenglas
Friggers: (Schersglas in Dutch; Bitzen, Tantern, Murksen, Privateln or Schinden in German) a compressed glass article used for some other purpose, e.g. a clown's hat used as a candle snuffer or a pistol used as a whisky bottle. In America, such articles are called whimsies, referring to the whim of the glass-maker: Fopglas, Dubbelkelkglas, walking sticks, swords, musical instruments such as trumpets and horns, Drinklaars, pistols, Molenbeker
Funeral glass art (in German: Sepulkraalglas, in Dutch: Funeraire kunst: grave monuments, urns, reliquaries, grave gifts
Glass accents: fairy lights, chimneys
Gift articles: ashtrays, candlesticks, vases, flower bowls
Grootwerk: Bowlstel, cheese covers, Fruittest, fish bowls, battery containers, patelle bowls
Illumination: light bulbs, strip lighting, candlesticks, candelabras
Industrial, Laboratory, Surgical and Chemical glass: Laatkop (Dutch) or letting bowl, glass for letting blood, (German: Schröpfköpfe), sputum glass, distillation flask, tube, tank, ampoules, bottle, injectabilia, pipette, drop counter, feeding bottle, retort, pill box, eye rinsing device, thermometer, barometer, looking glass, water level meter, paint rubber, cartridges, tambour copy machines
Interior decoration: wall cladding, tables, tiles, cupboards, working and table tops, shower door and panels, mirrors, folding screens, fire places, fire-screens, handrails, staircase steps, fireplace screens
Kitchen glassware: heat-resistant glass, towel-rails, butter dishes, singing tea-kettles, herb-pots, cruet stands
Office accessories: paperweights, Kroonbol, inkstands, postage stamp rolls
Optical glass: spectacle glass, lenses
Packaging glass: bottles, cellar bottles, preserving jars, rose water bottles, wine bottles, demijohns, carboys, storage jars, pouring jugs, miniature bottles, Pakfles, medicine bottles, apothecary's jars, Bockbeutel bottles
Sacral glass: monstrances, crucifixes, holy water fonts, rosaries, reliquaries, pilgrims' flasks, mass jug, altar pieces, candle sticks, crucifixes, ex-voto
Small figures and sculptures: religious figures, animal sculptures, car mascots, bookends
Tableware: breakfast services, tea services, and jubilee services, drinking glasses
Ornamental glass: flower pots, decanter, annual commemorative tumblers, decanters, carafes, fish bowls, tazzas (ornamental dishes), perfume bottles, jewelry, furniture
Toys: piggy banks, dolls' tea services, woven miniatures, dolls' eyes, marbles
Utilitarian glass: bulb glasses, water pipes, eggcups, bulbs/floats, Stopbol, Strijkpaddenstoel, piano feet, hyacinth glasses, Autovaasje, glass inlays or liners in silverware
Window glass: glass which is coloured in the mass, clear or blank, figured glass, float glass, layered and silvered glass. See> Flatglass
N.B.: If the translators could not find the English word, we left the names in German and Dutch.

Semi-finished products: semi-finished items and pre-form components produced from the processing of base materials, which can be further used in all glass techniques. These include:
Ballotini or micro-pearls measuring less than 0.2 mm are embedded in paint, glue or plastic in the manufacture of reflecting road signs, road marking strips and projection screens.
Bevelled pieces and bevelled clusters (in German: Facetten and Facetbilder): blank glass elements, which are facet-ground and polished, and may or may not be mirrored. They are mostly processed in the leaded glass industry.
Billets, pellets and granules: a ball-shaped, cushion-shaped or granule-shaped semi-finished product in the form of small ingots, balls or granules, used as a base material for melting in melting furnaces and kilns so that the glass-maker no longer has to compose his mixture himself.
Blank: unworked, unfinished metals and semi-manufactured glass articles such as the pre-form or embryo used in the Grail technique or spectacle glass in its compressed but as yet unpolished form.
Bull's-eye glass: in medieval times, these wound mallets of glass, between 10 and 12 cm in diameter, were known in German as Schewe or Scheube. Nowadays, articles of bull's-eye glass (known in Flemish as cieven) are much coarser and thicker, and are currently formed or compressed by hand, being either clear or in all sorts of different light colours.
Composition glass: crystal parts as beads, rosettes and small plates for assembling into lusters and chandeliers.
Confetti: are small flakes of wafer-thin blown glass. Since they are so thin, the transparent and opalescent colours are very light in tone.
Crown glass (in German: Mondglas): mouth-blown, wound discs which, divided into pieces, produced straight panes and the attachment part with a navel or bull's-eye or bullion. Examples of nineteenth-century crown glass include discs as large as 94 cm in diameter!
Cullet and patties: consist of irregular scraps and shards of broken glass or thick glass, possibly produced from recycled glass and suitable for remelting as part of the technique of open casting and kiln-forming and used as a base material for the mix for the glass fusing kiln.
Cylinders (Dutch: moffen) are blown by hand. As with genuine antique glass, sheets of glass are produced from the frit for use in the application of the leaded glass and stained glass techniques, from colorescent glass (in German: Farbglas) which is blown into cylinders in a flattening kiln to become a flat sheet.
Dalle or glass tile is an inch-thick, hand-cast glass slab within an iron frame which is used in the production of glass in concrete. In its standard form, it measures 30 x 20 cm, and may be produced as a special edition in a size of over one square metre. Such glass tiles are found in numerous different colours.
Fibre/filament: see the special chapter on this topic, in particular the entries in it entitled: Composition threads, Technological fibres, Reinforcement, Carbon and graphite fibres, Fireproof oxide fibres, Silicon carbide, Whiskers, Laminate and sandwich, Geotextiles, Three-dimensional textiles, Moulded articles, 3-D weaving and plaiting: weaving, knitting, bobbin and plaiting techniques, Aluminium silicate fibres, Micropore, Faserpapier, Ceramic fibrous mats, Moist felt, Ceraboard, Silicate fabrics, Form and insulation pastes, Faserplast and Fire cement: see The Technique of Glass Fibre.
Flakes: are thinner and smaller than Confetti, still, and are mixed by means of lacquer for use as reinforcement for the paintwork on large objects such as bridges, waterworks and flood barriers.
Frit: the semi-finished product used in kiln-forming techniques; it tends to be molten glass which has been finely crushed, and the colours are generally compatible with each other. They consist of different sizes of granules, varying from powder to 0.2 mm, fine frit measuring from 0.2 to 1.2 mm, medium frit measuring from 1.2 to 2.7 mm and coarse frit measuring from 2.7 to 5.2 mm.
N.B.: Emile Gallé called the technique whereby frit is fused marqueterie de verre, frequently combining this with patinage, the patination technique whereby he used, in addition to granules, other materials, chemicals, ash or minerals which were removed with a clear layer of glass.
Glass fibre: the name given to a considerable length of thinly drawn out glass, whereby the flexible thread becomes stiff as a result of cooling down. By analogy with the techniques used in the textile industry, the drawing, entwining and laying on of a thread is known as spinning, whilst the twisting of the thread itself (using an S-twist or a Z-twist) is called twisting and the plaiting of two or more threads is known as twining. The interweaving of threads is called weaving.
N.B.: Glass spinning is also a technique whereby small objects such as Christmas decorations and toys are produced – mainly from blank, zig-zag drawn glass fibres.
Glass rod: full glass taken from a considerable length of thicker glass; a bar is shorter, even thicker and stiffer than a rod, and is used for the overlay of glass and sometimes also in the assembly of items to form glass objects and sculptures. The single-coloured rods
Effetre, Moretti for beadmaking and Kugler Zapfen or rods (invented by Klaus Kügler) for making flashed glass or melting these ‘soft’ glass rods with a torch to make beads, sculptures, buttons or marbles and are used as a semi-finished product for the production of mosaic rods, mosaic composition rods and filigree rods. As regards Murrini, florets and spiral stems, see: The Techniques of Glass Mosaic, Filigree and Millefiori.
Glass powder: the finest form of pure crushed glass.
Glass tube: before any further processing, rods were drawn from the hot mass for the production of balls, neon tubes, glass filaments, fibres and millefiori and filigree rods. As semi-finished products for the firing process, rods, bars and tubes were manufactured by hand and, later on, by machine: the horizontally produced rods and tubes (drawn upwards) are those manufactured using the Maetz-Schuller and Corning techniques, whilst those produced vertically (drawn downwards) are found in the Vello and Hänlein, Danner and Philips techniques. It was only in 1912 that the continuous production of tube glass became possible thanks to Danner's invention, the Danner technique. A continuously flowing "thread" on an iron rod twists hot glass out of the frit, and air is blown in at the end of this slightly oblique pipe, causing the thread to open and a tube to be formed. In the production of rods, no air is blown in, so that the "threads" remain solid. Modern furnace-workers can choose between soft glass or the hardened type of borosilicate glass known under the brand names of Duran (Germany), Pyrex (USA), Murano (Italy) and Vulkan (Austria). Hard and soft types of glass are also subdivided according to their coefficient of expansion. For further details, see Kiln forming techniques.
Ingot: (Dutch: Gieteling) cast primary glass roll, half globe or bar for further processing: the larger variant of billets, mainly traded in the form of a rod, bread, ox hide or roll or as bulk form when cast by mould based methods.
Norman slab glass: produced in 1889 by blowing a glass post, like a bottle, into a rectangular box form. The corners were considerably thicker than the facets. After these glass forms had cooled down, the facets were cut free, in such a way as to create a rectangular plate of flat glass from the four walls and the square base. Such glass is also known in English as Prior Early English, after its inventor, E.S. Prior, an Arts & Crafts architect. The colours were clear, slightly grained with mostly red swirling marks or of a very deep hue.
Nugget: a piece of glass resembling a small lump of gold, often used as glass jewellery.
Optical glass elements such as lenses, prisms, reflectors, spheres (in German: Muggelsteinen) and polished stones; compressed, cast or slumped three-dimensional forms such as flowers, beetles, fir cones, jewels or imitation precious stones were composed to form jewel glass windows.
Overlay cups: (in German: Überfangtrichter): coloured, ready-made glass cups used for the production of items in application of the overlay technique.
Pellets: for the production of glass filaments and glass fibres, a standardised code is used to designate the different special pellets of E glass, an alkali-free glass with a high boric oxide and aluminium content which is very resistant to water. Where the fibres need to be resistant to acid, S glass is used. This is an alkali-calcium glass with an increased borosilicate content. D glass is used where the dielectric properties (relating to the properties resulting from the presence of an electric field) are an important factor, whilst K glass pellets are specially produced from crystal glass by the Lauscha glass fibre factory. Pellets are the glass balls for melting furnaces in transparent and coloured forms.
Pizza: is poured and flattened glass in the form of a doughy base, like that of a pizza, and is the semi-finished product used for the slicing of tesserae or mosaic stones. For further information, see > The Technique of Glass Mosaic.
Tellerscheiben: were produced in the 17th century from wound, plate-shaped discs showing typical spirals.

PHYSICAL AND CHEMICAL PROPERTIES:
Electrical properties or insulating properties of glass, used in electrotechnology and electronics.
Resistance to corrosion: Pyrex and Duran are types of corrosion-proof glass, resistant to acids, saline solutions, organic fluids and aggressive gases.
Water resistance: glass dissolves in water, and its resistance is reflected in hydrolytic classifications ranging from glass which is chemically highly resistant to glass which is very susceptible to corrosion.
Insulating properties or heat and electricity insulation capacity: glass is a material possessing, at room temperature, a high insulation capacity in respect of electrical voltage, whilst the thermal conduction coefficient of glass is low compared to that of metals.

Thermal properties:
Viscosity: the resistance of a material to becoming fluid. A material with high viscosity is very stiff, whereas one with low viscosity, such as liquid gas, flows easily.
Melting behaviour: the viscosity or syrupiness and thin liquidity of the frit is determined by the temperature, causing the molecular units to adhere more or less strongly to each other:
- Melting temperature: at this temperature, the melt is so thinly liquid that it can be homogeneously fused, free from bubbles;
- Working temperature: at this temperature, the design processes, such as bending, drawing, pressing and blowing, take place.
Short glass: the range within which glass can be processed depends on the temperature of the melt at which the glass can be given its shape and form and is limited since the glass solidifies quickly.
Long glass may be processed over a longer period of time, as for example in the case of the Venetian glass techniques: the temperature interval is greater than 300° C.
- Processing temperature: at this temperature, the glass changes shape upon softening by virtue of its own weight, as in the case of the burning in of paint, fusing, slumping and sagging.
Annealing, or carefully controlled cooling: the tension is reduced by a process of controlled cooling down to the annealing point – in other words, the point at which the tension has gone out of the glass – which is different with each type of glass. Thereafter, it is kept at that temperature in accordance with its thickness, coefficient of expansion, the warmth emanating from the annealing furnace, the dimensions of the surface, the possible existence of the fired form or fire mould and the tension desired. A specific cooling-off curve is adhered to for each type of glass and object, whereby the object in question, following that curve, is allowed to cool down to room temperature in a slow, controlled manner. In the course of that cooling-off process, some objects and their moulds remain for months, or even as long as one year, in the annealing furnace. Factory glass is cooled in a cooling tunnel. The tension in a piece of glass can be ascertained by means of a strain viewer.
* Annealing soak: this involves the temperature being kept at a certain level over a specific period of time, so that everything in the annealing furnace, such as the frit, the form and the props, or hollow cylindrical ceramic supports, can reach one and the same temperature just before the annealing starts.
* Crash cooling or soak: the cooling-off process whereby the furnace ventilators or dampers, and sometimes even the cover or door of the furnace, are opened in order to prevent devitrification by causing the temperature to come crashing down to the annealing point.
Firing: is the total process of heating up and cooling down objects in application of the furnace forming and stained-glass making techniques, fusing, slumping and sagging.
Fusing, or melting together, is the term used to describe the intense heating-up of glass, whereby the mixing of different components, such as glass fragments or chips melt or fuse together to form a homogeneous mass. Tack fusing is a process in which the temperature is kept at a somewhat lower level and the components are "stuck" closely together. See: The Technique of Fusing.
Slumping/sagging: flat glass that is fired into a given shape is first allowed to "sag"; if it falls further into a shape, the glass plate slumps by reason of taking on the form of the underlying template or firing form. See: The Technique of Slumping and Sagging.
Coefficient of expansion: the value given to a material, for example 96 in the case of glass, which indicates the possible expansion and contraction of the material upon being heated up and cooled down. Pieces of glass having excessively disparate coefficients of expansion will crack and explode after fusing: such types of glass are incompatible. All types of glass having the same chemical composition will be compatible, and may be melted and fused together.

Mechanical properties:
Brittleness: glass is strong when under pressure but weak when exposed to a tractive force. The compressive stress is determined by the extent of any damage to the surface of the glass. A surface which is undamaged or unworked will have the greatest breakage resistance.
Hard glass and soft glass are terms often used to indicate the relative level of the processing temperature, when what is actually meant is the mechanical hardness or softness of the glass. The hardness is indicated in various units such as Moh, Knoop and Vickers.
- The glyptic qualities of glass are used in sculpting and carving. For further details, see the sections on the techniques of Glass Grinding, Glass Cutting, Glass Polishing, Glass Engraving, Glass Etching and Sandblasting.
- Grindability: the mechanical workableness or grindability of glass depends on its composition.
-Tension: the effect caused by drawing out the surface of a material and increasing its mass.
Surface tension is caused where a liquid solidifies and thereby contracts. Glass with a high level of viscosity has a greater surface tension than more liquid glass. As a result of the temperature being raised, glass moves from a fixed (elastic) phase into a plastic phase. Where glass reaches room temperature in the course of a natural process of cooling down, the difference between the temperature of the surface, which will have cooled down much faster, and that of the still warm inner mass may cause so much tension to build up that – even several years later – the glass may break or even explode.

Optical and diaphanous properties:
Transparent, semi-transparent, translucid, diffuse, semi-translucide and opaque: see The Optical Qualities of Glass.

Plastic properties:
The term plastic signifies the giving of a form, as in the expressions "plastic arts" or "plastic surgery". It refers to anything which can be modelled, formed, folded, pressed or kneaded into shape; thus, the plastic quality of glass and plastic materials relates to those materials which may be shaped by force, pressure and heat.
The term plasticity signifies the fact of being plastic, pliability or the ability to be shaped. A Plastic sculpture is one which is produced from soft materials capable of being shaped, as in the art of modelling or certain sculptures in glass which are formed directly from the hot frit.

Translation: James Benn

©Angela van der Burght, 2007

This article will be followed in future with:
- Technique of natural glass
- Technique of transparency
- Hidden glass
- Technique of optic qualities of glass and diaphany

See also > What is glass? http://www.bullseyeglass.com/education/what-is-glass.html on the Bullseye Glass Company website Methodes and Idea