The light of heaven: glass and its colours

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Archaeology provides an ideal opportunity to pursue applied science at an advanced level, or to study the humanities, or to employ any combination of those disciplines. For example only last week I was trying to interpret various geophysical studies of a series of coal pits in the light of work from a group who are researching industrial and social history. What material exemplifies the fusion of science and art better than glass? Since Roman times workers in glass have produced some truly astonishing artefacts, yet for centuries the true nature of the their material was unknown. The ways by which you might add colour were simply ‘rules of thumb’ known to skilled glass-makers. I’d like to write about glass but, since I am interested in archaeology not antiques, I’ll illustrate what I have to say with shards found in my brother in law’s vegetable bed starting with a piece marked ‘Hyde & Co. London’. The company made ink and ink-wells; this fragment will be late 19th or early 20th century in date.

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I remember first being astonished by the beauty of ecclesiastical stained glass in Kings College Chapel, Cambridge, the last great English medieval building. By the time I attended the university myself the chapel’s internal stonework had been cleaned and restored but on my earlier visit the stones were still darkened with centuries of candle smoke. The contrast between the coloured patterns of the great windows and the dim nave was breath-taking. The word nave originates from the Latin navis, a ship. Surely the masons intended the worshipers to sit, or perhaps stand, in a great enclosed stone ship with the light of heaven pouring through the stained glass panes. The chapel glass is Tudor and must be the most magnificent survival from that era in England. I now live nearer the medieval glass of York Minster but in both buildings the confidence and creativity of the masons and glaziers. who originally constructed the windows, inspires awe even today.

So, what exactly is glass? It is basically silica (silicon dioxide), which was originally sourced as quartz sand. The so-called ‘silica-oxygen tetrahedrons’ are formed by strong covalent bonds existing between four oxygen atoms and a central silicon atom. There are many possible arrangements of tetrahedrons in nature. In silica all four oxygen atoms are shared with neighbours, and oxygen bridge bonds give silica its unique properties. Tetrahedra must touch at the corners but can do so at varying angles, giving multiple configurations; I understand that there are 35 different permitted structures. The so called α-quartz is the most stable type but a number of interchanges can occur at various temperatures. All quartz configurations are liquid above 1700°C and on cooling will change irreversibly to glass.

Quartz sand alone can be fused to make a glass but only at a temperature which was inaccessible to the ancient world. The addition of a ‘modifier’ like sodium carbonate (either as the mineral natron or as soda containing plant ash) considerably lowers the fusion temperature of quartz. This discovery was the stroke of genius that made ancient glass-making possible. A 75% silica and 25% soda mixture fuses at only 800ºC but unfortunately such a glass subsequently dissolves in water! To modify glass in the direction of stability calcium or magnesium must be added. ‘Added’ may be a misleading word. It could well be that the ancient makers simple found that using a sand rich in sea shell fragments made a more durable glass. If a portion of cullet, as scrap glass is called, is added to the basic ingredients then it will help the components fuse and mix. Modern green glass may contain as much as 95% recycled cullet.

Ancient glassware that has never been buried is called historic glass; buried material is archaeological glass. Glass resists acid attack fairly readily, except for hydrofluoric acid which is consequently used for etching it. Prolonged water contact removes sodium from the glass by hydrolysis. Such corrosion is quicker in alkaline environments. Glass is a solid in which the molecules are not arranged in a regular repeating pattern, like a crystal, but are arranged randomly as they would be in a liquid. The network of molecules that makes up a glass consists of three basic groups of oxides:

Network former: silica

Network modifiers: sodium, potassium, calcium, magnesium

Intermediates: aluminium, titanium

‘Intermediates’ are metal oxides that can enter the network but, cannot form glasses themselves. The current view is that glass is not a uniformly homogenous network but does have some chemical inhomogeneities. Sodium ion distribution is not uniform but is associated with non-bridging oxygen atoms to create silica-rich and alkali-rich regions.

In the later medieval period there was huge need for ecclesiastical ‘stained glass’ (although much early material was, strictly speaking, painted not stained). The demand resulted in glass being manufactured in wooded country where fuel was readily available. This so-called ‘forest glass’ used beech wood ash, which is rich in potassium, as the alkali metal flux instead of sodium compounds. A rapid shift to this glass type occurred towards the end of the first millennium AD. Perhaps the change was the result of economic factors, or some politically caused disruption in alkali supply. Forest glass degrades much more quickly when buried. It is consequently not surprising that the earliest glazing in the great Gothic cathedrals is also deteriorating in our modern polluted atmosphere. The great east window at York Minster is being restored at present for this very reason. Eventually a change from wood ash to sea weed ash resulted in a return to a more stable soda-lime glass.

On the basis of the presence of a few metallic oxides – magnesium, potassium, manganese, antimony and lead – five compositional groups of glass were understood decades ago. A common type is LMLK, or low magnesia, low potash, soda-lime glass. Most of the glass found in Britain between 800 BC and AD 1000, including Roman, is of this type. Glass used in India, China and the Islamic world differs from European types. Recently modern analytical techniques have resulted in many new glass groups being recognised. It seems highly likely that the apparent switch from one group to another reflects changes in the raw materials used. The use of maritime plants, like the Egyptian Salicornia and Salsola species, produces a high magnesium and potassium glass. It seems that using a mineral source of alkali (trona) produces LMLK glass. It is usually not understood exactly where and why such changes occurred.

Glass blowing was first been discovered around 100 BC. To blow glass a ‘gather’ of molten material is attached the end of a hollow iron tube. The gather is allowed to cool and is rolled on an iron or marble slab (marvered) to form a parison. By reheating and further blowing the desired shape is obtained. Glass can also be shaped by being blown into a wet mould. If you ever get the opportunity to watch a modern glass-blower at work do take it. The way in which beautiful and complex shapes can be produced using a pipe, shears, and pads of newspaper is almost unbelievable, even if you understand the chemistry and physics involved.

No glass in Britain, except beads, pre-dates the Roman Iron Age. Glass used here in the post-Roman period was imported from Germany but there may have been an indigenous Anglo-Saxon glass industry around Jarrow and Wearmouth as early as the seventh century AD. By the 13th century the industry had probably been reintroduced from France or the Low Countries. The focus of the British glass making, which then required massive amounts of wood fuel and wood ash, had moved to the forests of Surrey and Sussex. In the post-medieval period George Ravenscroft created lead glass with its brilliance and high refractive index. Lead crystal contains potash and lead oxide instead of calcium oxide. It has a high refractive index and is easily cut. It was highly suitable for the lenses of optical instruments. Another early use of glass was for enamelling. The famous 11th and 12th century Limoges industry enamelled copper objects, and may have used recycled Roman material.

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Glass represents a molecular mixture, not a chemical compound. In follows that there are no crystal cleavage planes in glass, hence the characteristic conchoidal fracture patterns as shown above. All chemical bond energies vary slightly when compared with the fixed bond energies in an ideal crystal. Glass, unlike water for example, does not have a fixed melting point but gradually changes from a solid to a liquid as its temperature is raised. This property is called variable viscosity. In fact an important distinguishing feature of a glass is that it shows no discontinuous change of any measurable property on cooling from the liquid to the vitreous state. Hence it is usual to define certain reference points in terms of temperature or viscosity: for soda-lime glass the working point is at 1000°C and the softening point at 700°C. Soda-lime glass objects are annealed at 510°C to remove the internal strains introduced during manufacture.

Colourless glass was prized in the Roman empire but producing it was a severe test for the ancient makers. Iron containing ‘green glass’ must always have been much commoner.

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Necessary raw materials to achieve truly colourless glass included low iron containing white sand, and the mineral trona which was crystalline and of high purity. An alternative method was to add a decolourizing agent. Antimony was long used in the East as a decolourant; Roman glass-makers also used manganese, possibly by adding the mineral pyrolusite (managnese dioxide) to the mix.

When considering coloured glass you have to distinguish between painted glass (a surface colour) and true stained glass (a body colour). There are at least three ways of introducing colour. Glass-makers could add various transitional metal elements to the glass. They could also create a solid colloidal suspension of finely divided immiscible material, like gold, silver or copper, which would give rise to colours by scattering light. Finally a surface layer effect could be produced resulting in interference colours. I don’t think this last method was done deliberately in the ancient world but interference colours are produced by lamination at the surface as glass deteriorates after burial.

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A stained glass window is essentially an assemblage of variously coloured pieces or quarrels of glass held together by lead ‘cames’ and fixed in a frame. The frames of a window are made of iron armatures called saddle bars. The windows are prevented from distortion by iron bracing rods or vergettes but need to be re-leaded every century or so. Stained glass does not usually consist of a solid body colour since this would not permit the transmission of sufficient light. It is possible to coat or ‘flash’ plain window glass with a layer of coloured glass but more usually the colour was painted on. In early times details were painted with ‘grisaille’ which was iron oxide powder, crushed glass, lead flux and vinegar. The glass was then fired to make the colour permanent. The famous Five Sisters windows at York Minster consist of grisaille glass.

Traditional stained glass techniques have hardly changed since the medieval period. Among the colours used were: blue (cobalt oxide with manganese oxide for dark blue), red (manganese oxide or copper oxide), green (copper or iron oxides) and yellow (iron III oxide or manganese dioxide). The glass, which was about 2-3 mm thick, was cut, painted, fired (at 650°C) or treated with a blowpipe to fix the colour, and then assembled. There must have been a great deal of colour variability in earlier times. Cutting was initially done with red hot iron. Diamond glass cutters became available in the 16th century. The transitional metals include for colouring glass included iron, manganese, chromium and copper. Uranium was employed in a 19th century red glass. Chemically the final colour effect depends on: the oxidative state of the metal, the composition of the base glass and the position that the metal ion occupies in the glass structure.

What is the chemistry behind the colouring of glass? I think I partially understand this but I’d welcome more information from chemically literate readers. If molecules in coloured glass strongly absorb in the red and green parts of the spectrum the glass will appear blue. If green and blue are strongly absorbed the glass will appear red, and so forth. I think that is clear, but why should absorption occur at all?

Evidently the addition of transition metals to the glass is the crucial step in producing colour. To understand the colours produced by transition metals think back to growing copper sulphate crystals in school science. Pure anhydrous copper sulphate is actually colourless but transition metal ions are able to form ‘co-ordination complexes’, in the case of copper sulphate with water or ammonium molecules, which are strongly coloured.  To understand the next step you have to remember that atomic electrons are found in positions of probability which are called orbitals. By convention these are known as s, p, d, and f. Within each orbital the electrons can occupy a number of energy levels. Under the correct circumstances an electron can absorb a photon of electromagnetic radiation and be ‘pushed’ to a higher energy level as a result. Eventually a photon of a somewhat longer wavelength is emitted and the once excited electron returns to its ground state. For example some minerals contain electrons that can be excited by invisible ultra-violet radiation. When the excited electrons return to a lower energy level they produce photons in the visible spectrum. Such minerals when exposed to ‘black-light’ (ultra-violet) glow blue or pink in consequence.

In the case of transition metals their empty d orbitals accept lone pairs of electrons from other molecules called ‘ligands’ and form these co-ordination complexes. When copper salts are put in water you get complexes like Cu[(H2O)6]2+. Six water molecules act as ligands and it is the interaction between the outer orbital electron of the metal atom and the electric field created by the co-ordinating ligands that produce the colour.  This is also the reason why pure silica-soda glass is colourless. The electrons of silicates don’t get excited by photons of light. Some transitional metal ions can substitute for silicon, or act as interstitial network modifiers like sodium. As interstitial ions they attract 6 or more oxygen atoms as a co-ordination sphere in an approximately octahedral arrangement. If Fe3+ replaces silicon there is an approximately tetrahedral arrangement. Adding any transition metal to glass provides electrons that will get excited’. Now, electrons may ‘jump’ between the d orbitals of the metal atoms. Due to the energy gap some energy is absorbed and released in the form of photons, and it just happens to fall within the visible spectrum. Only certain wavelengths of light are absorbed, those matching exactly the energy difference needed to move the electron from one energy level to another. If this happens to be red light then this will be absorbed, and later re-emitted as infra-red, undetectable to the naked eye. The glass will appear blue-green since these wavelengths of light are not absorbed.

Well I think this is correct. But if you don’t find the explanation intelligible ignore it and simply concentrate on the beauty of glass.

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