By Michelle Walton Photography by Marc Pelletreau

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ContentsForeword

Acknowledgements

Author’s Note

Timeline: A History of Glass from Antiquity to the Late Medieval Period

Introduction

Late AntiquityThe Romans, Byzantines and Sassanians: Rome to Central Asia: 5th-7th Centuries AD

The Establishment of the Islamic EmpireThe Umayyads: Córdoba to Samarkand: 661-750 AD

The Golden AgeThe Abbasids: Egypt and the Levant: 750-850 ADThe Abbasids and Buyids: Iraq and Iran: 750-1258 AD

Rivals of the Old OrderThe Tulunids: Egypt, Syria and the Levant: 868-904 ADThe Fatimids: The Maghreb, Egypt, Syria, Hijaz, Sicily and the Levant: 909-1171 AD

Along the Silk RoadThe Samanids: Eastern Iran and Central Asia: 819-999 AD

The Rise of the Turkic TribesThe Ghaznavids and Ghurids: Eastern Iran and Central Asia: 977-1215 AD

The Great Seljuk EmpireThe Great Seljuks: Iraq, Iran, Central Asia, Turkey and the Eastern Mediterranean: 1040-1194 AD

Glossary

Bibliography

Index

Author’s NoteThis book started life as a prologue for another book. In doing preliminary research for the full catalogue of the glass collection at the Museum of Islamic Art, Qatar, the amount of time spent just looking at glass pieces inspired this lavishly photographed document. I wanted to show everyone what I was seeing, to share an appreciation of the varied nature and beauty of the glass in the museum, a collection which has gone unseen by most, both in the literal and figurative sense. To truly see something, some sense of understanding the material is necessary. Therefore, the intention of this book is also to provide foundational material for those unfamiliar with the subject, a clear comprehension of the nature of glass and what makes early Islamic glass simultaneously a link in the overall historical record and a singular artistic entity.

Yet even in the process of giving information, this is not a comprehensive survey of either the vast subject of medieval Islamic glass or the entirety of the collection at the MIA. It is but a small exploration into the field, and exploration, even while bringing answers, engenders further questions.

The very inception of my interest in ancient and medieval glass was in discovering the delectation of perplexity that is its study; it was not enough to know when and where, I also longed to know how and why. The duality of glass always shows itself: liquid and solid, strength and fragility, rarity and worthlessness, sacred and profane, a mystery inherent within the material. Rather than being a daunting enterprise, the quest for answers drives research ever deeper. In keeping with the dualistic nature of glass, this Book of Answers, for me, is the Book of Questions – a view to what inspires the trajectory of the full catalogue, the continued research and development of the MIA’s glass collection and my own unabashed enthusiasm and affection.

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CompositionIn order to appreciate the history of glass and determine its origin, the study of glass vessels must begin with the deconstruction of the material itself. Like some type of subversion of natural law, glass is unlike any other substance in composition or material ability. Glass is neither solid nor liquid in the classical sense, but is termed an amorphous solid. In describing the non-crystalline arrangement of glass, a close – and delightful – parallel is the chaotic molecular structure of cotton candy floss. Despite its fragile reputation, glass has great inherent strength and durability.

From antiquity to the present day, the majority of common glass has been composed primarily of silica, soda and lime. Silica is most often derived from sand, the main raw ingredient. Although it is possible to create glass from pure silica, the melting point is much too high to be commercially viable; therefore, soda is added to lower the melting temperature of

the silica. This is known as the ‘flux’, the best quality of which is natron (used in ancient Egypt for mummification). An alternate substitution of potash, the ash from certain burnt marine plants or wood, was also used. Unfortunately, glass made from only sand and soda disintegrates when exposed to water. In order to counteract the breakdown, lime is added to the batch as a stabilizer.

The chemistry of glass is a complex subject. Variations in quality and colour can be the result of the strata sources of sand, minute traces of impurities or the crucible environment within the furnace, whether oxidising or reducing. For the artisan, exacting recipes and control of the kiln environment and temperature are essential. Myriad recipes for glass exist from collective history, many of them closely guarded secrets of the glassmaker or guild and arrived at through extensive experimentation. For the modern-day historian, an awareness of the composition assists

Cup or BowlFree-blown glass of natural colour with pinched protrusionsIran3rd – 4th centuryH: 8.5 cm, Dia: 11 cmGL.93

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BottleBlown glass of natural colour with heavy weathering Iraq or Iran9th -10th centuryH: 24 cm, Dia: 5.8 cmGL.313

Science and SocietyBeyond the science of its composition, glass was instrumental in advancing scientific practice itself. Although the foundational theories of science originated in ancient Greece, India, Egypt and Rome, it is to the Islamic world that science owes a debt for much of its practical development. The Greeks based much of their works on the outcome of philosophical premise, logical discourse and mathematics. It was with the advent of the Islamic Golden Age (from approximately 750 to 1257), and its employment of an experimental and quantitative approach, that science went beyond mere observation and classification. Glass played an essential role in this development. Its chemical, physical, and, in particular, optical properties make it suitable – or even essential – for experimental applications. It is non-reactive and corrosion-resistant, affected by few chemicals, including most trade and food acids. It can withstand intense heat or cold or retain heat. Furthermore, it is able to reflect, bend, transmit and absorb light with great accuracy and its transparency enables the clear observation of specimens.

By utilising glass and lenses in experimentation, Islamic scientists and mathematicians such as Jabir ibn Hayyan (c. 721-815), Al-Kindi (c. 801-873), Ibn Sahl (c. 940-1000), Ibn al-Haytham (965-1040) and Avicenna (980-1037), reformed ancient Greek and Indian theorems on optics with expanded theoretical and systematic analyses. They influenced later Islamic and European scholars in the continuing advancement of not only optics, but also physics, ophthalmology, astronomy and the physical sciences. The chemists and doctors who exploited glass for experiments, in medical practice, in the testing and creation of chemical compounds and pharmaceuticals, and in crystallisation, distillation and other functions, are too numerous to list.

As examples of chemists making use of glass instruments, Jabir ibn Hayyan, Al-Kindi and Avicenna used distillation to establish the perfume industry. Al-Kindi’s treatise, Kitab Kimya al-’Otoor wa al-Tas’eedat (The Book of the Chemistry of Perfumes and Distillations), which focused mainly on the preparation of essential oils, salves, aromatic waters and alcohol, gave 107 recipes for perfumes. For several centuries, the Islamic

perfumers dominated the market, exporting regionally as well as to Europe, India and China. Many of the perfumers had a close relationship with glassworks, which designed bottles exclusively for their products.

Later, as the corpus of works on chemistry, medicine, pharmacology, astronomy and optics were translated into Latin and made their way into the West, so too did glass laboratory equipment such as the alembic, which still bears its Arabic name. The advances made, thanks to the functionality and plasticity of glass, unreservedly changed the trajectory of science and perceptions of the natural world, medicine and industry.

The atmosphere supporting developments in science, industry, trade and academic study in the Islamic world is similar to the one that fostered creativity and innovation in glassmaking. Glass objects from this period are a direct reflection of

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Late Antiquity

Although the early date assigned to this glass figurine of a fish may precede the book’s main focus, it shows an element of playful design typical of Roman glass during its heyday. Animal, bird and marine-life subjects were popularly used as small storage vessels or as decorative elements, adhered to the outside surface of a container with a small gob of hot glass.

Fish Ornament Hot-worked natural colour and cobalt blue glassItaly3rd – 4th centuryL: 7 cm, W: 3.5 cm GL.280

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Imperfect Perfection

Jar with HandlesFree-blown glass with applied handles and threading, body heavily weatheredLevantine region5th centuryH: 10.2 cm, Dia: 7 cm GL.107

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The lightly ribbed texture of this small jar was made by mould-blowing. The technique of blowing glass began in Roman Syria in the 1st century BC, quickly becoming the favoured method of manufacture. Molten glass inflated within a mould retains the impression, creating a fast and inexpensive way to shape and decorate glass. With its plasticity, glass can be moulded and shaped into an infinite number of shapes and objects or quickly decorated with intricately patterned motifs.

Cosmetic Jar Mould-blown glass with applied threading, heavy weatheringSyria6th centuryH: 5.4 cm, Dia: 4.9 cmGL.207

The esTablishmenT of The islamic empire

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Late Antiquity

Hexagonal Pilgrim Jug Mould-blown glass with applied handle and tooled spout, heavily weathered Israel/Palestine, most likely from JerusalemLate 6th – early 7th centuryH: 14.6 cmGL.195

Christian and Jewish pilgrim vessels have been discovered in their hundreds, and were once used by devoted travellers for transporting water or earth from the Holy Land. Despite two of the panels’ designs being completely obscured by degenerative weathering, the jug displays a typical combination of Christian Byzantine motifs in sunken relief: a cross atop several steps (also seen on contemporary coinage) or elliptical rings and a series of diamond-shaped lozenges surrounded by a border of circular depressions.

Glass trends followed the demand of the Levantine area, whether from Christian, Muslim or Jewish communities. The Levant, the combined area of modern-day western Syria, Lebanon, Jordan, Israel and Palestine, was in almost constant political flux during the early medieval period. Once the Muslim armies initially conquered the Byzantine territory, the various Arab, Persian and Turkish dynasties vied for dominance of this strategic region along with the regular threat of encroachment of the Crusader States.

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The Establishment of the Islamic Empire

This bottle is representative of a group of distinctive-looking very early Islamic glass objects. Similarly-shaped bottles are commonly seen in Egypt, but the wheel-cut decoration is from Persian contexts, although not typical in execution. The colour is unusual: its milky green surface has an opacity unlike the opaque colouring of glass from other periods. It may be the goal was to imitate jade. Eastward to China, jade was (and still remains) a highly-esteemed and desired hardstone, prized for its variation of colour, hardness and alleged protective

Ovoid BottleFree-blown glass with wheel-cut facets Iran6th – 8th centuryH: 8.2 cm, Dia: 9.4 cmGL.32

powers. Since the hardstone is rare and expensive, glass was seen as a suitable substitute. The small amount of glass produced in China was made into bi disks, burial garments, weaponry inlays and vessels, traditionally items reserved for jade. Despite the lack of intrinsic evidence of the intention of the glassmakers and merchants, this short-lived class of manufacture may have been an experiment to supply jade-like vessels to the eastern market. Alternatively, this type of glass perhaps imitated the look of jade for local demand.

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Imperfect Perfection

Flask Free-blown and wheel-ground glass with wheel-cut facetsIran8th – 9th centuryH: 12 cm, Dia: 9 cmGL.12

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A very small amount of glasswork carried any form of writing or signature of the maker. When they did occur, the inscriptions would normally refer to a blessing for the owner or user. In the early medieval period, artisans almost never signed their work, but the Museum of Islamic Art is fortunate to have an extremely rare example: a small ninth-century juglet, mould-blown with the name of its maker, Muhammad ibn al-Ahzan.

Juglet Mould-blown glass with applied handle and tooled spout, heavily weatheredIraq9th centuryH: 10.5 cm, Dia: 6.6 cmGL.191

JugletMould-blown glass with missing applied handle and tooled spoutIraq9th centuryH: 8.5 cm, Dia: 5.5 cmGL.327

JarMould-blown glass with heavy weatheringIraq9th centuryH: 7.2 cm, Dia: 6.5 cmGL.290

BottleMould-blown colourless glass with applied black trailIraq9th centuryH: 7.5 cm, Dia: 5.6 cmGL.188

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The Golden Age

Incised EwerFree-blown cobalt blue glass with incised linear and foliation patterns and applied handle, heavily repaired and weatheredSyria or Iraq9th – 10th centuryH: 26.5 cm, Dia: 14.9 cmGL.163

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Imperfect Perfection

BowlMould-blown blue-green glassLevantine region9th – 10th centuryH: 8.7 cm, Dia: 18.4 cmGL.36

Cosmetic JarMould-blown glassLevantine region9th – 10th centuryH: 8.2 cm, Dia: 9.3 cmGL.214

Squat BottleMould-blown glassLevantine region9th – 10th centuryH: 6.7 cm, Dia: 7.9 cmGL.385

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Rivals of the Old Order

Molar Flask Free-blown, ground and cut glass with facets and elements, slightly weatheredEgypt9th – 10th centuryH: 7.5 cm, W: 2.2 cmGL.257

One of the first techniques used to shape glass was casting, normally associated with metalworking. It was used to its greatest effect in Achaemenid (Persia, c. 550-330 BC) and Hellenistic (Greek and early Roman) glassmaking. Molten glass was poured into moulds, producing thick-walled vessels when cooled, and was then deeply carved like hardstone. This method of decoration continued even when casting was abandoned and the glass was blown with thick walls and then cut when cooled. During the ninth and tenth centuries, glass was cut in imitation of expensive rock crystal vessels. One of the most commonly produced was the small perfume flasks with characteristic feet resembling the roots of a molar tooth. The ‘molar flask’ was produced from about the ninth to fourteenth centuries and was found across the Islamic territories, from Northern Africa to Central Asia, although most commonly from Egypt. The origin or reason for their unsual shape is not known, although it may have something to do with either their display or a practicality in shipping.

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Along the Silk Road

Facet-cut Bottle Free-blown and wheel-ground turquoise glass with wheel-cut facetsIran9th – 10th centuryH: 18.9 cm, Dia: 10.8 cm GL.288

Often identified with the ornamentation of Sassanian vessels, wheel-cut facets are seen as early as the fifth century BC on Achaemenid bowls, with the style continuing in popularity through the centuries as seen on Roman and Parthian examples. The fashion of lozenge and circular, honeycombed facets survived long into the early Islamic period. The facets were cut deeply into the surface of the glass with a revolving wheel aided by gritty slurry. Although looking similar to Sassa-nian antecedents, the globular body type, propor-tions and balanced composition of these vessels are classically Islamic.

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Many cutting styles were used on glass to mimic precious hardstones, such as cameo cutting. Traditionally, glass of one colour was encased in glass of a different colour, which was then carved to reveal the underlayer. Another method used in the Islamic world involved strategically applying patches of colour onto the vessel to be carved, as seen on this example. Not only was this method faster, but it could also be combined with other decorative

BottleFree-blownglass with incised linear decoration and applied and carved green elementsIran9th – 10th centuryH: 16.8 cm, Dia: 10 cmGL.515

styles, such as etching, or an application of a variety of colours. The two horses formed by carving two patches of green glass look more stylistically sophisticated than many other squared and stylised examples of animal forms manufactured in this way. Alternatively, the etched decoration and shape are typical of Samanid glass. The two elements together create questions about the bottle’s artistic roots.

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Along the Silk Road

Squat Bottle Mould-blown dark green glass, slightly weathered Central Asia 9th – 10th centuryH: 9.8 cm, Dia: 7.6 cmGL.98

Squat Bottle Mould-blown, dark green glass with folded rimCentral Asia9th – 10th centuryH: 10.1 cm, Dia: 7.9 cmGL.253

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The decoration of this flask corresponds to a decolourised and gilded bottle in the British Museum, inscribed to Atabeg ‘Imad al-Din Zang, the founder of the Zangid dynasty and Turkish governor of Mosul (1127-1146). This inscription makes the bottle the only confirmed datable example of gilded glassware in Syria in the twelfth century. Identical gilded pomegranate

FlaskFree-blown cobalt blue glass with applied goldEastern MediterraneanLate 11th – 12th centuryH: 24.2 cm,Dia: 6.3 cmGL.378

trees and birds on the flask connect it to twelfth-century Syria, though there are questions about the dominant artistic inspiration. The form and colour of the cobalt blue flask as well as the style for gilding such vessels with intricate patterns and figures were in high fashion at the time in Greece under Byzantine rule.

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The Great Seljuk Empire

BottleMould-blown decolourised glass made in two parts and assembled, with pressed decoration, applied trails in cobalt blue around mouth and neckIran12th – early 13th centuryH: 19.2 cm, Dia: 9.6 cmGL.530

BottleMould-blown cobalt blue glass from a dip mould, tooled neck and mouthIran12th – early 13th centuryH: 19 cm, Dia: 10.2 cmGL.521

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Abrasion is a mechanical technique of grinding with a wheel or some other device to create shallow decoration on the surface of glass which is often left unpolished.

Alembic (Arabic, al-anbiq, ‘the still’) is a shaped connector used as part of the equipment for distilling.

Alkali is a soluble salt composed of potassium carbonate or sodium carbonate. It is one of the essential ingredients for glass, making up about 15 to 20 per cent of the batch. The alkali is a flux, which reduces the melting point of silica, the main ingredient of glass.

Ancient glass is a generalised term used to refer to glass made since its earliest discovery to late antiquity, c. sixteenth century BC to c. 450 AD. This time period includes (but is not limited to) ancient Mesopotamia, Egypt, Phoenicia, Greece and Hellenistic societies, Persia and the Roman Empire.

Annealing is the process of slowly cooling a finished vessel in a separate section of the furnace. It is essential for glassmaking; if allowed to cool too quickly, glass is highly strained and may break when cool.

Applied decoration uses heated glass elements (such as trails or blobs) that are applied during the shaping of a glass object, and are either left in relief or marvered flush into the surface.

Appliqué is a small disk attached to the hot surface of a glass object for decorative purposes. Appliqués can have previously mould-pressed decoration, or be stamped after they are attached.

Arabesque is an intricate ornamental pattern of scrolling and interlaced vegetal elements.

Batch is the mixture of raw materials (silica, soda or potash, and lime as well as other ingredients such as colourants), melted in a crucible or tank to make glass. Cullet can also be added to help the melting process.

Blank is a cooled glass object requiring further forming or decoration to be finished.

Blowing is the manufacturing technique invented in about the first century BC to first century AD, of forming an object by inflating a gather of molten glass on the end of a blowpipe.

Blowpipes are iron or steel tubes, usually four to five feet long, for blowing glass.

Cameo glass describes the method of layering contrasting colours of glass which is then carved, cut, or engraved through the upper layers to produce a design that stands out from the background. The ancient Romans were first to use this technique in imitation of layered hardstones, such as agate.

Canes are single or multiple-coloured rods which are bundled and fused together to form designs that are visible when seen in cross section.

Carving is the removal of surface material from an object with hand-held tools.

Casting is the generic name for a variety of techniques used to form glass in a mould, often having precursory methods in metalwork.

Cold working is a collective term for various techniques employed to shape or decorate glass when it has completely cooled; examples are wheel engraving and cutting.

Coloured glass is made with impurities in the batch ingredients or by actively colouring glass using one of three main processes: utilising dissolved metallic oxide, forming a dispersion of a substance in a colloidal state, or forming opaque colours by suspending pigment particles.

Combed decoration appears as wavy, festooned, feathery, or zigzag patterns in two or more colours, and is produced by applying threads of opaque glass of contrasting colour to the heated glass body. The threads are rolled flush into the vessel by marvering, after which they are combed or dragged.

Crucibles are containers in which batch is fused and kept molten. The glassworker gathers glass directly from the crucible.

Cullet is raw glass or pieces of broken glass from a cooled melt, or scrap glass intended for recycling.

Cupping glasses are small cups in which a partial vacuum is created for cupping, to draw blood to the skin’s surface, usually for bloodletting.

Cut or Cutting is a technique of removing material from the surface of an object by grinding it with rotating wheels to shape or decorate the glass.

Decolouriser is a substance used to remove or offset the green, blue or brown tones found naturally in glass from iron impurities in the batch or impurities in the crucible or elsewhere in the production process.

Dip moulds are cylindrical or truncated conical one-piece moulds with patterned interiors, also known as optic moulds. A gather of glass is dipped into it and then inflated.

Faceting uses grinding and polishing to give an object’s surface a pattern of planes or facets.

Firing is the action of melting the batch into glass, reheating unfinished glassware to be worked, or reheating glassware to fuse enamel or gilding. The melting of the

batch requires temperatures of about 2400°-2750°F (1300°-1500°C).

Flux is a substance that lowers the melting point of another material. Potash and soda are fluxes.

Foot is a word to describe the bottom or base of a vessel; the part on which it rests.

Forest glass is a general term to identify glass made in the rural glasshouses of central and northern Europe in the late Middle Ages. Most forest glass was made with potash from the wood burned in glass furnaces. Iron impurities in the sand give it a characteristic green tint.

Free-blown glass is shaped by inflation with a blowpipe and manipulation with tools.

Furnaces are enclosed structures that utilise heat for the production of glass, and are used for melting the batch, keeping the glass in crucibles in a molten state, and reheating partly formed objects.

Fusing is the process of melting the batch or heating pieces of glass in a kiln or furnace until they bond.

Gather is a mass of molten glass (also called a gob) collected on the end of a blowpipe, pontil, or gathering iron; the word also refers to the action of collecting the molten glass with a tool.

Gilding is the process of decorating glass with gold leaf, gold paint, or gold dust. The gilding is applied, then attached to the glass by heat. Gold leaf can be picked up on a gather of hot glass.

Glass is matter with an amorphous (non-crystalline) molecular structure: it is a solid material that sometimes acts like a liquid. The manufacturing process requires the raw materials to be heated at a high temperature to completely fuse the ingredients, which become rigid when cooled.

Gob is a mass of molten glass, usually small and newly gathered from the furnace.

Gold sandwich glass is a descriptive term to describe several types of glass objects decorated with designs cut and/or engraved in gold leaf, which are sandwiched between two fused layers of glass.

Grinding describes a technique of removing the surface layer(s) of an object mechanically, such as with a rotating wheel.

HIMT glass is a special subgroup of glass identified by its sand with High Iron, Manganese and Titanium levels in analysis. Examples of this type have been found across Europe, the Levant and Northern Africa, first created sometime around the fourth century.

Glossary

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Hot-worked is the generic term for glass that is manipulated while it is hot.

Incalmo (Italian) is the process of assembling a vessel by fusing two or more adjoining sections, and is particularly distinctive when the pieces are in contrasting colours. The technique, which requires great skill, was first practised as early as the ninth century in the Islamic world.

Inlay is any material embedded in the surface of a larger object.

Iridescence, on ancient and medieval glass, is the rainbow-like effect caused by interference effects of light reflected from several layers of weathered glass.

Kick is the term for the concavity in the base of a vessel, usually made by depressing the base with a tool. It strengthens the bottom of the vessel and reduces its capacity.

Lime is calcined limestone, which gives stability to the batch when added in small quantities.

Lustre is a shiny metallic effect made by painting the surface with metallic oxides that have been dissolved in acid and mixed with an oily medium. Firing in oxygen-free conditions at a temperature of about 1150°F (600°C) causes the metal to deposit in a thin film that, after cleaning, has a distinctive shiny surface.

Marver (from French marbre, ‘marble’) is a smooth, flat surface where softened glass is rolled in order to smooth it or to consolidate applied decoration. It also refers to the action of rolling softened glass on a marver.

Medieval glass is a generalised term used to refer to glass made from the end of Classical Antiquity until the beginning of the Renaissance, from the fifth to the fifteenth centuries.

Melt describes molten, fluid glass produced by melting the batch.

Metallic oxide is the oxide of a metal. Oxides can be used to colour glass and enamel, or to produce lustred or iridised surfaces. The result depends on the oxide used, the composition of the glass and the presence or absence of oxygen in the furnace.

Molar flasks are small cosmetic flasks with feet resembling the roots of a tooth. The molar flask is a characteristic type of ninth to fourteenth century Islamic perfume bottle, often having wheel-cut decoration.

Mosque lamp is the common name for a lamp shaped like an inverted bell, with three or more handles from which it was suspended. Although many mosque lamps claim this shape, the type was also heavily produced for secular settings.

Moulds are forms used for shaping and/or decorating molten glass. Some moulds only imprint a pattern while others give the object its final form, with or without decoration.

Mould blowing is the process of inflating a gather of hot glass into a mould. The glass is forced against the inner surfaces of the mould and assumes its shape, together with the mould’s decoration.

Mosaic is a surface decoration created through the use of tesserae, many small, adjoining pieces of multicoloured materials, such as stone or glass.

Mosaic glass describes objects made from preformed slices of canes placed together (often in a mould) and heated until they fuse. For ancient and Islamic glass, the term ‘mosaic glass’ is preferable to ‘millefiori’.

Natron is a naturally-occurring salt, mainly from Wadi el-Natrun, northwest of Cairo. Commonly used by Roman glassmakers as the alkali in a batch.

Omphalos (Greek, ‘navel’) is a circular element with a raised boss at the centre.

Overlay is a layer of glass covering a different colour of glass, often as the result of casing, a similar result of cameo glass.

Pattern-moulded glass has been blown into a raised-pattern mould; the object shows the pattern but it does not affect the final form of the object.

Plaques are ornamental plates made as wall decoration or inlaid in furniture.

Plastic is a descriptive word for a material which is able to be shaped and moulded. Molten glass can be described as plastic.

Polishing is the smoothing of a glass object when cold by holding it against a rotating wheel fed with a fine abrasive or with hand-held tools or, when hot, through the heat of the furnace.

Pontil, or punty, is a solid metal rod used to hold a vessel during manufacture, stuck onto the base with a small gob of hot glass. It often leaves a scar on the base when removed. This is called the ‘pontil mark’.

Potash is the alkali potassium carbonate, an alternative to soda in the manufacture of glass.

Potash-lime or Potassium-lime glass contains silica (about 60-75 per cent), potash (12-18 per cent), and lime (5-12 per cent). Forest glass is a common type of potassium-lime glass. It is slightly denser than soda-lime glass and quickly becomes rigid, and so is more difficult to manipulate into elaborate forms. Since it is harder and more brilliant,

it is highly suitable for techniques such as facet cutting and engraving.

Pressed glass is created by placing a blob of molten glass in a mould and pressing it to form a single-sided decoration. Mould-pressed is a different process than mould-blown glass.

Relief cutting is a type of cut-glass decoration where the negative space is cut away to leave the ornament in high relief.

Rock crystal is a colourless hardstone, quartz. Many glasses and decorative techniques used on glass were attempts to imitate rock crystal.

Silica refers to the mineral silicon dioxide, the main ingredient of glass. Sand is the most common form of silica used in glassmaking.

Soda is the mineral sodium carbonate. Soda (or alternatively potash) serves as a flux to reduce the melting point of the silica.

Soda-lime glass is the most common in the history of glassmaking, usually containing silica (about 60-75 per cent), soda (12-18 per cent) and lime (5-12 per cent). This type of glass, when molten, stays plastic and flexible for an extended amount of time under a wide range of temperatures. Soda-lime glass, therefore, is highly suited to elaborate production techniques.

Tessera (from Latin, ‘small square tablet or block’) is a small piece of glass or other material, used in mosaics.

Threading is the decorative method of winding a molten, thin trail of glass around an object to make lines and patterns.

Tool refers to any instrument used by glassworkers to create and shape an object. Some of the glassworkers’ tools include the blowpipe, pontil, jacks and shears.

Tooling is the action of using a tool or tools on the molten glass.

Trailing is the method of applying molten trails of glass as decoration on a vessel.

Trails are strands of glass, drawn out from a gather of molten glass.

Vitreous describes relating to, resembling or having the nature of glass.

Weathering is for describing the changes on the surface of glass caused by an environmental, chemical reaction.

Wheel engraving is a method of decorating the surface of glass by the grinding action of a wheel. Lines, grooves and facets are made by using disks of various sizes, rim profiles and materials (such as copper or stone) and abrasive slurry. See also Cutting.

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'I wanted to show everyone what I was seeing, to share an appreciation of the varied nature and beauty of the glass in the museum, a collection which has gone unseen by

most, both in the literal and figurative sense.'- Michelle Walton, author